Merge pull request #5863

f9ec3f0 Add block pruning functionality (mrbandrews)

7 files changed, 744 insertions, 37 deletions

diff --git a/qa/rpc-tests/pruning.py b/qa/rpc-tests/pruning.py
new file mode 100755
index 0000000000..f26cbee1e2
--- /dev/null
+++ b/qa/rpc-tests/pruning.py
@@ -0,0 +1,356 @@
+#!/usr/bin/env python2
+# Copyright (c) 2014 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#
+# Test pruning code
+# ********
+# WARNING:
+# This test uses 4GB of disk space and takes in excess of 30 mins to run
+# ********
+
+from test_framework import BitcoinTestFramework
+from bitcoinrpc.authproxy import AuthServiceProxy, JSONRPCException
+from util import *
+import os.path
+
+def calc_usage(blockdir):
+    return sum(os.path.getsize(blockdir+f) for f in os.listdir(blockdir) if os.path.isfile(blockdir+f))/(1024*1024)
+
+class PruneTest(BitcoinTestFramework):
+
+    def __init__(self):
+        self.utxo = []
+        self.address = ["",""]
+
+        # Some pre-processing to create a bunch of OP_RETURN txouts to insert into transactions we create
+        # So we have big transactions and full blocks to fill up our block files
+
+        # create one script_pubkey
+        script_pubkey = "6a4d0200" #OP_RETURN OP_PUSH2 512 bytes
+        for i in xrange (512):
+            script_pubkey = script_pubkey + "01"
+        # concatenate 128 txouts of above script_pubkey which we'll insert before the txout for change
+        self.txouts = "81"
+        for k in xrange(128):
+            # add txout value
+            self.txouts = self.txouts + "0000000000000000"
+            # add length of script_pubkey
+            self.txouts = self.txouts + "fd0402"
+            # add script_pubkey
+            self.txouts = self.txouts + script_pubkey
+
+
+    def setup_chain(self):
+        print("Initializing test directory "+self.options.tmpdir)
+        initialize_chain_clean(self.options.tmpdir, 3)
+
+    def setup_network(self):
+        self.nodes = []
+        self.is_network_split = False
+
+        # Create nodes 0 and 1 to mine
+        self.nodes.append(start_node(0, self.options.tmpdir, ["-debug","-maxreceivebuffer=20000","-blockmaxsize=999000", "-checkblocks=5"], timewait=300))
+        self.nodes.append(start_node(1, self.options.tmpdir, ["-debug","-maxreceivebuffer=20000","-blockmaxsize=999000", "-checkblocks=5"], timewait=300))
+
+        # Create node 2 to test pruning
+        self.nodes.append(start_node(2, self.options.tmpdir, ["-debug","-maxreceivebuffer=20000","-prune=550"], timewait=300))
+        self.prunedir = self.options.tmpdir+"/node2/regtest/blocks/"
+
+        self.address[0] = self.nodes[0].getnewaddress()
+        self.address[1] = self.nodes[1].getnewaddress()
+
+        connect_nodes(self.nodes[0], 1)
+        connect_nodes(self.nodes[1], 2)
+        connect_nodes(self.nodes[2], 0)
+        sync_blocks(self.nodes[0:3])
+
+    def create_big_chain(self):
+        # Start by creating some coinbases we can spend later
+        self.nodes[1].generate(200)
+        sync_blocks(self.nodes[0:2])
+        self.nodes[0].generate(150)
+        # Then mine enough full blocks to create more than 550MB of data
+        for i in xrange(645):
+            self.mine_full_block(self.nodes[0], self.address[0])
+
+        sync_blocks(self.nodes[0:3])
+
+    def test_height_min(self):
+        if not os.path.isfile(self.prunedir+"blk00000.dat"):
+            raise AssertionError("blk00000.dat is missing, pruning too early")
+        print "Success"
+        print "Though we're already using more than 550MB, current usage:", calc_usage(self.prunedir)
+        print "Mining 25 more blocks should cause the first block file to be pruned"
+        # Pruning doesn't run until we're allocating another chunk, 20 full blocks past the height cutoff will ensure this
+        for i in xrange(25):
+            self.mine_full_block(self.nodes[0],self.address[0])
+
+        waitstart = time.time()
+        while os.path.isfile(self.prunedir+"blk00000.dat"):
+            time.sleep(0.1)
+            if time.time() - waitstart > 10:
+                raise AssertionError("blk00000.dat not pruned when it should be")
+
+        print "Success"
+        usage = calc_usage(self.prunedir)
+        print "Usage should be below target:", usage
+        if (usage > 550):
+            raise AssertionError("Pruning target not being met")
+
+    def create_chain_with_staleblocks(self):
+        # Create stale blocks in manageable sized chunks
+        print "Mine 24 (stale) blocks on Node 1, followed by 25 (main chain) block reorg from Node 0, for 12 rounds"
+
+        for j in xrange(12):
+            # Disconnect node 0 so it can mine a longer reorg chain without knowing about node 1's soon-to-be-stale chain
+            # Node 2 stays connected, so it hears about the stale blocks and then reorg's when node0 reconnects
+            # Stopping node 0 also clears its mempool, so it doesn't have node1's transactions to accidentally mine
+            stop_node(self.nodes[0],0)
+            self.nodes[0]=start_node(0, self.options.tmpdir, ["-debug","-maxreceivebuffer=20000","-blockmaxsize=999000", "-checkblocks=5"], timewait=300)
+            # Mine 24 blocks in node 1
+            self.utxo = self.nodes[1].listunspent()
+            for i in xrange(24):
+                if j == 0:
+                    self.mine_full_block(self.nodes[1],self.address[1])
+                else:
+                    self.nodes[1].generate(1) #tx's already in mempool from previous disconnects
+
+            # Reorg back with 25 block chain from node 0
+            self.utxo = self.nodes[0].listunspent()
+            for i in xrange(25): 
+                self.mine_full_block(self.nodes[0],self.address[0])
+
+            # Create connections in the order so both nodes can see the reorg at the same time
+            connect_nodes(self.nodes[1], 0)
+            connect_nodes(self.nodes[2], 0)
+            sync_blocks(self.nodes[0:3])
+
+        print "Usage can be over target because of high stale rate:", calc_usage(self.prunedir)
+
+    def reorg_test(self):
+        # Node 1 will mine a 300 block chain starting 287 blocks back from Node 0 and Node 2's tip
+        # This will cause Node 2 to do a reorg requiring 288 blocks of undo data to the reorg_test chain
+        # Reboot node 1 to clear its mempool (hopefully make the invalidate faster)
+        # Lower the block max size so we don't keep mining all our big mempool transactions (from disconnected blocks)
+        stop_node(self.nodes[1],1)
+        self.nodes[1]=start_node(1, self.options.tmpdir, ["-debug","-maxreceivebuffer=20000","-blockmaxsize=5000", "-checkblocks=5", "-disablesafemode"], timewait=300)
+
+        height = self.nodes[1].getblockcount()
+        print "Current block height:", height
+
+        invalidheight = height-287
+        badhash = self.nodes[1].getblockhash(invalidheight)
+        print "Invalidating block at height:",invalidheight,badhash
+        self.nodes[1].invalidateblock(badhash)
+
+        # We've now switched to our previously mined-24 block fork on node 1, but thats not what we want
+        # So invalidate that fork as well, until we're on the same chain as node 0/2 (but at an ancestor 288 blocks ago)
+        mainchainhash = self.nodes[0].getblockhash(invalidheight - 1)
+        curhash = self.nodes[1].getblockhash(invalidheight - 1)
+        while curhash != mainchainhash:
+            self.nodes[1].invalidateblock(curhash)
+            curhash = self.nodes[1].getblockhash(invalidheight - 1)
+
+        assert(self.nodes[1].getblockcount() == invalidheight - 1)
+        print "New best height", self.nodes[1].getblockcount()
+
+        # Reboot node1 to clear those giant tx's from mempool
+        stop_node(self.nodes[1],1)
+        self.nodes[1]=start_node(1, self.options.tmpdir, ["-debug","-maxreceivebuffer=20000","-blockmaxsize=5000", "-checkblocks=5", "-disablesafemode"], timewait=300)
+
+        print "Generating new longer chain of 300 more blocks"
+        self.nodes[1].generate(300)
+
+        print "Reconnect nodes"
+        connect_nodes(self.nodes[0], 1)
+        connect_nodes(self.nodes[2], 1)
+        sync_blocks(self.nodes[0:3])
+
+        print "Verify height on node 2:",self.nodes[2].getblockcount()
+        print "Usage possibly still high bc of stale blocks in block files:", calc_usage(self.prunedir)
+
+        print "Mine 220 more blocks so we have requisite history (some blocks will be big and cause pruning of previous chain)"
+        self.nodes[0].generate(220) #node 0 has many large tx's in its mempool from the disconnects
+        sync_blocks(self.nodes[0:3])
+
+        usage = calc_usage(self.prunedir)
+        print "Usage should be below target:", usage
+        if (usage > 550):
+            raise AssertionError("Pruning target not being met")
+
+        return invalidheight,badhash
+
+    def reorg_back(self):
+        # Verify that a block on the old main chain fork has been pruned away
+        try:
+            self.nodes[2].getblock(self.forkhash)
+            raise AssertionError("Old block wasn't pruned so can't test redownload")
+        except JSONRPCException as e:
+            print "Will need to redownload block",self.forkheight
+
+        # Verify that we have enough history to reorg back to the fork point
+        # Although this is more than 288 blocks, because this chain was written more recently
+        # and only its other 299 small and 220 large block are in the block files after it,
+        # its expected to still be retained
+        self.nodes[2].getblock(self.nodes[2].getblockhash(self.forkheight))
+
+        first_reorg_height = self.nodes[2].getblockcount()
+        curchainhash = self.nodes[2].getblockhash(self.mainchainheight)
+        self.nodes[2].invalidateblock(curchainhash)
+        goalbestheight = self.mainchainheight
+        goalbesthash = self.mainchainhash2
+
+        # As of 0.10 the current block download logic is not able to reorg to the original chain created in
+        # create_chain_with_stale_blocks because it doesn't know of any peer thats on that chain from which to
+        # redownload its missing blocks.
+        # Invalidate the reorg_test chain in node 0 as well, it can successfully switch to the original chain
+        # because it has all the block data.
+        # However it must mine enough blocks to have a more work chain than the reorg_test chain in order
+        # to trigger node 2's block download logic.
+        # At this point node 2 is within 288 blocks of the fork point so it will preserve its ability to reorg
+        if self.nodes[2].getblockcount() < self.mainchainheight:
+            blocks_to_mine = first_reorg_height + 1 - self.mainchainheight
+            print "Rewind node 0 to prev main chain to mine longer chain to trigger redownload. Blocks needed:", blocks_to_mine
+            self.nodes[0].invalidateblock(curchainhash)
+            assert(self.nodes[0].getblockcount() == self.mainchainheight)
+            assert(self.nodes[0].getbestblockhash() == self.mainchainhash2)
+            goalbesthash = self.nodes[0].generate(blocks_to_mine)[-1]
+            goalbestheight = first_reorg_height + 1
+
+        print "Verify node 2 reorged back to the main chain, some blocks of which it had to redownload"
+        waitstart = time.time()
+        while self.nodes[2].getblockcount() < goalbestheight:
+            time.sleep(0.1)
+            if time.time() - waitstart > 300:
+                raise AssertionError("Node 2 didn't reorg to proper height")
+        assert(self.nodes[2].getbestblockhash() == goalbesthash)
+        # Verify we can now have the data for a block previously pruned
+        assert(self.nodes[2].getblock(self.forkhash)["height"] == self.forkheight)
+
+    def mine_full_block(self, node, address):
+        # Want to create a full block
+        # We'll generate a 66k transaction below, and 14 of them is close to the 1MB block limit
+        for j in xrange(14):
+            if len(self.utxo) < 14:
+                self.utxo = node.listunspent()
+            inputs=[]
+            outputs = {}
+            t = self.utxo.pop()
+            inputs.append({ "txid" : t["txid"], "vout" : t["vout"]})
+            remchange = t["amount"] - Decimal("0.001000")
+            outputs[address]=remchange
+            # Create a basic transaction that will send change back to ourself after account for a fee
+            # And then insert the 128 generated transaction outs in the middle rawtx[92] is where the #
+            # of txouts is stored and is the only thing we overwrite from the original transaction
+            rawtx = node.createrawtransaction(inputs, outputs)
+            newtx = rawtx[0:92]
+            newtx = newtx + self.txouts
+            newtx = newtx + rawtx[94:]
+            # Appears to be ever so slightly faster to sign with SIGHASH_NONE
+            signresult = node.signrawtransaction(newtx,None,None,"NONE")
+            txid = node.sendrawtransaction(signresult["hex"], True)
+        # Mine a full sized block which will be these transactions we just created
+        node.generate(1)
+
+
+    def run_test(self):
+        print "Warning! This test requires 4GB of disk space and takes over 30 mins"
+        print "Mining a big blockchain of 995 blocks"
+        self.create_big_chain()
+        # Chain diagram key:
+        # *   blocks on main chain
+        # +,&,$,@ blocks on other forks
+        # X   invalidated block
+        # N1  Node 1
+        #
+        # Start by mining a simple chain that all nodes have
+        # N0=N1=N2 **...*(995)
+
+        print "Check that we haven't started pruning yet because we're below PruneAfterHeight"
+        self.test_height_min()
+        # Extend this chain past the PruneAfterHeight
+        # N0=N1=N2 **...*(1020)
+
+        print "Check that we'll exceed disk space target if we have a very high stale block rate"
+        self.create_chain_with_staleblocks()
+        # Disconnect N0
+        # And mine a 24 block chain on N1 and a separate 25 block chain on N0
+        # N1=N2 **...*+...+(1044)
+        # N0    **...**...**(1045)
+        #
+        # reconnect nodes causing reorg on N1 and N2
+        # N1=N2 **...*(1020) *...**(1045)
+        #                   \
+        #                    +...+(1044)
+        #
+        # repeat this process until you have 12 stale forks hanging off the
+        # main chain on N1 and N2
+        # N0    *************************...***************************(1320)
+        #
+        # N1=N2 **...*(1020) *...**(1045) *..         ..**(1295) *...**(1320)
+        #                   \            \                      \
+        #                    +...+(1044)  &..                    $...$(1319)
+
+        # Save some current chain state for later use
+        self.mainchainheight = self.nodes[2].getblockcount()   #1320
+        self.mainchainhash2 = self.nodes[2].getblockhash(self.mainchainheight)
+
+        print "Check that we can survive a 288 block reorg still"
+        (self.forkheight,self.forkhash) = self.reorg_test() #(1033, )
+        # Now create a 288 block reorg by mining a longer chain on N1
+        # First disconnect N1
+        # Then invalidate 1033 on main chain and 1032 on fork so height is 1032 on main chain
+        # N1   **...*(1020) **...**(1032)X..
+        #                  \
+        #                   ++...+(1031)X..
+        #
+        # Now mine 300 more blocks on N1
+        # N1    **...*(1020) **...**(1032) @@...@(1332)
+        #                 \               \
+        #                  \               X...
+        #                   \                 \
+        #                    ++...+(1031)X..   ..
+        #
+        # Reconnect nodes and mine 220 more blocks on N1
+        # N1    **...*(1020) **...**(1032) @@...@@@(1552)
+        #                 \               \
+        #                  \               X...
+        #                   \                 \
+        #                    ++...+(1031)X..   ..
+        #
+        # N2    **...*(1020) **...**(1032) @@...@@@(1552)
+        #                 \               \
+        #                  \               *...**(1320)
+        #                   \                 \
+        #                    ++...++(1044)     ..
+        #
+        # N0    ********************(1032) @@...@@@(1552) 
+        #                                 \
+        #                                  *...**(1320)
+
+        print "Test that we can rerequest a block we previously pruned if needed for a reorg"
+        self.reorg_back()
+        # Verify that N2 still has block 1033 on current chain (@), but not on main chain (*)
+        # Invalidate 1033 on current chain (@) on N2 and we should be able to reorg to
+        # original main chain (*), but will require redownload of some blocks
+        # In order to have a peer we think we can download from, must also perform this invalidation
+        # on N0 and mine a new longest chain to trigger.
+        # Final result:
+        # N0    ********************(1032) **...****(1553)
+        #                                 \
+        #                                  X@...@@@(1552)
+        #
+        # N2    **...*(1020) **...**(1032) **...****(1553)
+        #                 \               \
+        #                  \               X@...@@@(1552)
+        #                   \
+        #                    +..
+        #
+        # N1 doesn't change because 1033 on main chain (*) is invalid
+
+        print "Done"
+
+if __name__ == '__main__':
+    PruneTest().main()
diff --git a/qa/rpc-tests/util.py b/qa/rpc-tests/util.py
index 9ecee31959..cf789f48e2 100644
--- a/qa/rpc-tests/util.py
+++ b/qa/rpc-tests/util.py
@@ -158,7 +158,7 @@ def _rpchost_to_args(rpchost):
         rv += ['-rpcport=' + rpcport]
     return rv
 
-def start_node(i, dirname, extra_args=None, rpchost=None):
+def start_node(i, dirname, extra_args=None, rpchost=None, timewait=None):
     """
     Start a bitcoind and return RPC connection to it
     """
@@ -172,7 +172,10 @@ def start_node(i, dirname, extra_args=None, rpchost=None):
                           ["-rpcwait", "getblockcount"], stdout=devnull)
     devnull.close()
     url = "http://rt:rt@%s:%d" % (rpchost or '127.0.0.1', rpc_port(i))
-    proxy = AuthServiceProxy(url)
+    if timewait is not None:
+        proxy = AuthServiceProxy(url, timeout=timewait)
+    else:
+        proxy = AuthServiceProxy(url)
     proxy.url = url # store URL on proxy for info
     return proxy
 
diff --git a/src/chainparams.cpp b/src/chainparams.cpp
index 2d4fde2476..589c7b5472 100644
--- a/src/chainparams.cpp
+++ b/src/chainparams.cpp
@@ -121,6 +121,7 @@ public:
         vAlertPubKey = ParseHex("04fc9702847840aaf195de8442ebecedf5b095cdbb9bc716bda9110971b28a49e0ead8564ff0db22209e0374782c093bb899692d524e9d6a6956e7c5ecbcd68284");
         nDefaultPort = 8333;
         nMinerThreads = 0;
+        nPruneAfterHeight = 100000;
 
         /**
          * Build the genesis block. Note that the output of the genesis coinbase cannot
@@ -198,6 +199,7 @@ public:
         vAlertPubKey = ParseHex("04302390343f91cc401d56d68b123028bf52e5fca1939df127f63c6467cdf9c8e2c14b61104cf817d0b780da337893ecc4aaff1309e536162dabbdb45200ca2b0a");
         nDefaultPort = 18333;
         nMinerThreads = 0;
+        nPruneAfterHeight = 1000;
 
         //! Modify the testnet genesis block so the timestamp is valid for a later start.
         genesis.nTime = 1296688602;
@@ -257,6 +259,7 @@ public:
         consensus.hashGenesisBlock = genesis.GetHash();
         nDefaultPort = 18444;
         assert(consensus.hashGenesisBlock == uint256S("0x0f9188f13cb7b2c71f2a335e3a4fc328bf5beb436012afca590b1a11466e2206"));
+        nPruneAfterHeight = 1000;
 
         vFixedSeeds.clear(); //! Regtest mode doesn't have any fixed seeds.
         vSeeds.clear();  //! Regtest mode doesn't have any DNS seeds.
diff --git a/src/chainparams.h b/src/chainparams.h
index 0692094478..5e974123dc 100644
--- a/src/chainparams.h
+++ b/src/chainparams.h
@@ -58,6 +58,7 @@ public:
     bool DefaultConsistencyChecks() const { return fDefaultConsistencyChecks; }
     /** Make standard checks */
     bool RequireStandard() const { return fRequireStandard; }
+    int64_t PruneAfterHeight() const { return nPruneAfterHeight; }
     /** Make miner stop after a block is found. In RPC, don't return until nGenProcLimit blocks are generated */
     bool MineBlocksOnDemand() const { return fMineBlocksOnDemand; }
     /** In the future use NetworkIDString() for RPC fields */
@@ -77,6 +78,7 @@ protected:
     std::vector<unsigned char> vAlertPubKey;
     int nDefaultPort;
     int nMinerThreads;
+    uint64_t nPruneAfterHeight;
     std::vector<CDNSSeedData> vSeeds;
     std::vector<unsigned char> base58Prefixes[MAX_BASE58_TYPES];
     std::string strNetworkID;
diff --git a/src/init.cpp b/src/init.cpp
index 665aebe1c5..b648a237bf 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -275,7 +275,12 @@ std::string HelpMessage(HelpMessageMode mode)
 #ifndef WIN32
     strUsage += HelpMessageOpt("-pid=<file>", strprintf(_("Specify pid file (default: %s)"), "bitcoind.pid"));
 #endif
-    strUsage += HelpMessageOpt("-reindex", _("Rebuild block chain index from current blk000??.dat files") + " " + _("on startup"));
+    strUsage += HelpMessageOpt("-prune=<n>", _("Reduce storage requirements by pruning (deleting) old blocks. This mode disables wallet support and is incompatible with -txindex.") + " " +
+            _("Warning: Reverting this setting requires re-downloading the entire blockchain.") + " " +
+            _("(default: 0 = disable pruning blocks,") + " " +
+            strprintf(_(">%u = target size in MiB to use for block files)"), MIN_DISK_SPACE_FOR_BLOCK_FILES / 1024 / 1024));
+strUsage += HelpMessageOpt("-reindex", _("Rebuild block chain index from current blk000??.dat files") + " " + _("on startup"));
+
 #if !defined(WIN32)
     strUsage += HelpMessageOpt("-sysperms", _("Create new files with system default permissions, instead of umask 077 (only effective with disabled wallet functionality)"));
 #endif
@@ -352,7 +357,7 @@ std::string HelpMessage(HelpMessageMode mode)
         strUsage += HelpMessageOpt("-flushwallet", strprintf(_("Run a thread to flush wallet periodically (default: %u)"), 1));
         strUsage += HelpMessageOpt("-stopafterblockimport", strprintf(_("Stop running after importing blocks from disk (default: %u)"), 0));
     }
-    string debugCategories = "addrman, alert, bench, coindb, db, lock, rand, rpc, selectcoins, mempool, net, proxy"; // Don't translate these and qt below
+    string debugCategories = "addrman, alert, bench, coindb, db, lock, rand, rpc, selectcoins, mempool, net, proxy, prune"; // Don't translate these and qt below
     if (mode == HMM_BITCOIN_QT)
         debugCategories += ", qt";
     strUsage += HelpMessageOpt("-debug=<category>", strprintf(_("Output debugging information (default: %u, supplying <category> is optional)"), 0) + ". " +
@@ -458,10 +463,33 @@ struct CImportingNow
     }
 };
 
+
+// If we're using -prune with -reindex, then delete block files that will be ignored by the
+// reindex.  Since reindexing works by starting at block file 0 and looping until a blockfile
+// is missing, and since pruning works by deleting the oldest block file first, just check
+// for block file 0, and if it doesn't exist, delete all the block files in the
+// directory (since they won't be read by the reindex but will take up disk space).
+void DeleteAllBlockFiles()
+{
+    if (boost::filesystem::exists(GetBlockPosFilename(CDiskBlockPos(0, 0), "blk")))
+        return;
+
+    LogPrintf("Removing all blk?????.dat and rev?????.dat files for -reindex with -prune\n");
+    boost::filesystem::path blocksdir = GetDataDir() / "blocks";
+    for (boost::filesystem::directory_iterator it(blocksdir); it != boost::filesystem::directory_iterator(); it++) {
+        if (is_regular_file(*it)) {
+            if ((it->path().filename().string().length() == 12) &&
+                (it->path().filename().string().substr(8,4) == ".dat") &&
+                ((it->path().filename().string().substr(0,3) == "blk") ||
+                 (it->path().filename().string().substr(0,3) == "rev")))
+                boost::filesystem::remove(it->path());
+        }
+    }
+}
+
 void ThreadImport(std::vector<boost::filesystem::path> vImportFiles)
 {
     RenameThread("bitcoin-loadblk");
-
     // -reindex
     if (fReindex) {
         CImportingNow imp;
@@ -674,6 +702,21 @@ bool AppInit2(boost::thread_group& threadGroup)
     if (nFD - MIN_CORE_FILEDESCRIPTORS < nMaxConnections)
         nMaxConnections = nFD - MIN_CORE_FILEDESCRIPTORS;
 
+    // if using block pruning, then disable txindex
+    // also disable the wallet (for now, until SPV support is implemented in wallet)
+    if (GetArg("-prune", 0)) {
+        if (GetBoolArg("-txindex", false))
+            return InitError(_("Prune mode is incompatible with -txindex."));
+#ifdef ENABLE_WALLET
+        if (!GetBoolArg("-disablewallet", false)) {
+            if (SoftSetBoolArg("-disablewallet", true))
+                LogPrintf("%s : parameter interaction: -prune -> setting -disablewallet=1\n", __func__);
+            else
+                return InitError(_("Can't run with a wallet in prune mode."));
+        }
+#endif
+    }
+
     // ********************************************************* Step 3: parameter-to-internal-flags
 
     fDebug = !mapMultiArgs["-debug"].empty();
@@ -710,6 +753,21 @@ bool AppInit2(boost::thread_group& threadGroup)
         nScriptCheckThreads = MAX_SCRIPTCHECK_THREADS;
 
     fServer = GetBoolArg("-server", false);
+
+    // block pruning; get the amount of disk space (in MB) to allot for block & undo files
+    int64_t nSignedPruneTarget = GetArg("-prune", 0) * 1024 * 1024;
+    if (nSignedPruneTarget < 0) {
+        return InitError(_("Prune cannot be configured with a negative value."));
+    }
+    nPruneTarget = (uint64_t) nSignedPruneTarget;
+    if (nPruneTarget) {
+        if (nPruneTarget < MIN_DISK_SPACE_FOR_BLOCK_FILES) {
+            return InitError(strprintf(_("Prune configured below the minimum of %d MB.  Please use a higher number."), MIN_DISK_SPACE_FOR_BLOCK_FILES / 1024 / 1024));
+        }
+        LogPrintf("Prune configured to target %uMiB on disk for block and undo files.\n", nPruneTarget / 1024 / 1024);
+        fPruneMode = true;
+    }
+
 #ifdef ENABLE_WALLET
     bool fDisableWallet = GetBoolArg("-disablewallet", false);
 #endif
@@ -1030,8 +1088,12 @@ bool AppInit2(boost::thread_group& threadGroup)
                 pcoinscatcher = new CCoinsViewErrorCatcher(pcoinsdbview);
                 pcoinsTip = new CCoinsViewCache(pcoinscatcher);
 
-                if (fReindex)
+                if (fReindex) {
                     pblocktree->WriteReindexing(true);
+                    //If we're reindexing in prune mode, wipe away all our block and undo data files
+                    if (fPruneMode)
+                        DeleteAllBlockFiles();
+                }
 
                 if (!LoadBlockIndex()) {
                     strLoadError = _("Error loading block database");
@@ -1055,7 +1117,18 @@ bool AppInit2(boost::thread_group& threadGroup)
                     break;
                 }
 
+                // Check for changed -prune state.  What we are concerned about is a user who has pruned blocks
+                // in the past, but is now trying to run unpruned.
+                if (fHavePruned && !fPruneMode) {
+                    strLoadError = _("You need to rebuild the database using -reindex to go back to unpruned mode.  This will redownload the entire blockchain");
+                    break;
+                }
+
                 uiInterface.InitMessage(_("Verifying blocks..."));
+                if (fHavePruned && GetArg("-checkblocks", 288) > MIN_BLOCKS_TO_KEEP) {
+                    LogPrintf("Prune: pruned datadir may not have more than %d blocks; -checkblocks=%d may fail\n",
+                        MIN_BLOCKS_TO_KEEP, GetArg("-checkblocks", 288));
+                }
                 if (!CVerifyDB().VerifyDB(pcoinsdbview, GetArg("-checklevel", 3),
                               GetArg("-checkblocks", 288))) {
                     strLoadError = _("Corrupted block database detected");
@@ -1106,6 +1179,15 @@ bool AppInit2(boost::thread_group& threadGroup)
         mempool.ReadFeeEstimates(est_filein);
     fFeeEstimatesInitialized = true;
 
+    // if prune mode, unset NODE_NETWORK and prune block files
+    if (fPruneMode) {
+        LogPrintf("Unsetting NODE_NETWORK on prune mode\n");
+        nLocalServices &= ~NODE_NETWORK;
+        if (!fReindex) {
+            PruneAndFlush();
+        }
+    }
+
     // ********************************************************* Step 8: load wallet
 #ifdef ENABLE_WALLET
     if (fDisableWallet) {
diff --git a/src/main.cpp b/src/main.cpp
index 4352c719a1..bf32ac91e7 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -52,9 +52,12 @@ int nScriptCheckThreads = 0;
 bool fImporting = false;
 bool fReindex = false;
 bool fTxIndex = false;
+bool fHavePruned = false;
+bool fPruneMode = false;
 bool fIsBareMultisigStd = true;
 bool fCheckBlockIndex = false;
 unsigned int nCoinCacheSize = 5000;
+uint64_t nPruneTarget = 0;
 
 /** Fees smaller than this (in satoshi) are considered zero fee (for relaying and mining) */
 CFeeRate minRelayTxFee = CFeeRate(1000);
@@ -110,17 +113,25 @@ namespace {
 
     /**
      * The set of all CBlockIndex entries with BLOCK_VALID_TRANSACTIONS (for itself and all ancestors) and
-     * as good as our current tip or better. Entries may be failed, though.
+     * as good as our current tip or better. Entries may be failed, though, and pruning nodes may be
+     * missing the data for the block.
      */
     set<CBlockIndex*, CBlockIndexWorkComparator> setBlockIndexCandidates;
     /** Number of nodes with fSyncStarted. */
     int nSyncStarted = 0;
-    /** All pairs A->B, where A (or one if its ancestors) misses transactions, but B has transactions. */
+    /** All pairs A->B, where A (or one if its ancestors) misses transactions, but B has transactions.
+      * Pruned nodes may have entries where B is missing data.
+      */
     multimap<CBlockIndex*, CBlockIndex*> mapBlocksUnlinked;
 
     CCriticalSection cs_LastBlockFile;
     std::vector<CBlockFileInfo> vinfoBlockFile;
     int nLastBlockFile = 0;
+    /** Global flag to indicate we should check to see if there are
+     *  block/undo files that should be deleted.  Set on startup
+     *  or if we allocate more file space when we're in prune mode
+     */
+    bool fCheckForPruning = false;
 
     /**
      * Every received block is assigned a unique and increasing identifier, so we
@@ -1849,6 +1860,7 @@ bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pin
 }
 
 enum FlushStateMode {
+    FLUSH_STATE_NONE,
     FLUSH_STATE_IF_NEEDED,
     FLUSH_STATE_PERIODIC,
     FLUSH_STATE_ALWAYS
@@ -1856,16 +1868,30 @@ enum FlushStateMode {
 
 /**
  * Update the on-disk chain state.
- * The caches and indexes are flushed if either they're too large, forceWrite is set, or
- * fast is not set and it's been a while since the last write.
+ * The caches and indexes are flushed depending on the mode we're called with
+ * if they're too large, if it's been a while since the last write,
+ * or always and in all cases if we're in prune mode and are deleting files.
  */
 bool static FlushStateToDisk(CValidationState &state, FlushStateMode mode) {
-    LOCK(cs_main);
+    LOCK2(cs_main, cs_LastBlockFile);
     static int64_t nLastWrite = 0;
+    std::set<int> setFilesToPrune;
+    bool fFlushForPrune = false;
     try {
+    if (fPruneMode && fCheckForPruning) {
+        FindFilesToPrune(setFilesToPrune);
+        if (!setFilesToPrune.empty()) {
+            fFlushForPrune = true;
+            if (!fHavePruned) {
+                pblocktree->WriteFlag("prunedblockfiles", true);
+                fHavePruned = true;
+            }
+        }
+    }
     if ((mode == FLUSH_STATE_ALWAYS) ||
         ((mode == FLUSH_STATE_PERIODIC || mode == FLUSH_STATE_IF_NEEDED) && pcoinsTip->GetCacheSize() > nCoinCacheSize) ||
-        (mode == FLUSH_STATE_PERIODIC && GetTimeMicros() > nLastWrite + DATABASE_WRITE_INTERVAL * 1000000)) {
+        (mode == FLUSH_STATE_PERIODIC && GetTimeMicros() > nLastWrite + DATABASE_WRITE_INTERVAL * 1000000) ||
+        fFlushForPrune) {
         // Typical CCoins structures on disk are around 100 bytes in size.
         // Pushing a new one to the database can cause it to be written
         // twice (once in the log, and once in the tables). This is already
@@ -1893,9 +1919,16 @@ bool static FlushStateToDisk(CValidationState &state, FlushStateMode mode) {
                 return state.Abort("Files to write to block index database");
             }
         }
-        // Finally flush the chainstate (which may refer to block index entries).
+        // Flush the chainstate (which may refer to block index entries).
         if (!pcoinsTip->Flush())
             return state.Abort("Failed to write to coin database");
+
+        // Finally remove any pruned files
+        if (fFlushForPrune) {
+            UnlinkPrunedFiles(setFilesToPrune);
+            fCheckForPruning = false;
+        }
+
         // Update best block in wallet (so we can detect restored wallets).
         if (mode != FLUSH_STATE_IF_NEEDED) {
             GetMainSignals().SetBestChain(chainActive.GetLocator());
@@ -1913,6 +1946,12 @@ void FlushStateToDisk() {
     FlushStateToDisk(state, FLUSH_STATE_ALWAYS);
 }
 
+void PruneAndFlush() {
+    CValidationState state;
+    fCheckForPruning = true;
+    FlushStateToDisk(state, FLUSH_STATE_NONE);
+}
+
 /** Update chainActive and related internal data structures. */
 void static UpdateTip(CBlockIndex *pindexNew) {
     chainActive.SetTip(pindexNew);
@@ -2083,15 +2122,29 @@ static CBlockIndex* FindMostWorkChain() {
         CBlockIndex *pindexTest = pindexNew;
         bool fInvalidAncestor = false;
         while (pindexTest && !chainActive.Contains(pindexTest)) {
-            assert(pindexTest->nStatus & BLOCK_HAVE_DATA);
             assert(pindexTest->nChainTx || pindexTest->nHeight == 0);
-            if (pindexTest->nStatus & BLOCK_FAILED_MASK) {
-                // Candidate has an invalid ancestor, remove entire chain from the set.
-                if (pindexBestInvalid == NULL || pindexNew->nChainWork > pindexBestInvalid->nChainWork)
+
+            // Pruned nodes may have entries in setBlockIndexCandidates for
+            // which block files have been deleted.  Remove those as candidates
+            // for the most work chain if we come across them; we can't switch
+            // to a chain unless we have all the non-active-chain parent blocks.
+            bool fFailedChain = pindexTest->nStatus & BLOCK_FAILED_MASK;
+            bool fMissingData = !(pindexTest->nStatus & BLOCK_HAVE_DATA);
+            if (fFailedChain || fMissingData) {
+                // Candidate chain is not usable (either invalid or missing data)
+                if (fFailedChain && (pindexBestInvalid == NULL || pindexNew->nChainWork > pindexBestInvalid->nChainWork))
                     pindexBestInvalid = pindexNew;
                 CBlockIndex *pindexFailed = pindexNew;
+                // Remove the entire chain from the set.
                 while (pindexTest != pindexFailed) {
-                    pindexFailed->nStatus |= BLOCK_FAILED_CHILD;
+                    if (fFailedChain) {
+                        pindexFailed->nStatus |= BLOCK_FAILED_CHILD;
+                    } else if (fMissingData) {
+                        // If we're missing data, then add back to mapBlocksUnlinked,
+                        // so that if the block arrives in the future we can try adding
+                        // to setBlockIndexCandidates again.
+                        mapBlocksUnlinked.insert(std::make_pair(pindexFailed->pprev, pindexFailed));
+                    }
                     setBlockIndexCandidates.erase(pindexFailed);
                     pindexFailed = pindexFailed->pprev;
                 }
@@ -2219,7 +2272,9 @@ bool ActivateBestChain(CValidationState &state, CBlock *pblock) {
             uint256 hashNewTip = pindexNewTip->GetBlockHash();
             // Relay inventory, but don't relay old inventory during initial block download.
             int nBlockEstimate = Checkpoints::GetTotalBlocksEstimate();
-            {
+            // Don't relay blocks if pruning -- could cause a peer to try to download, resulting
+            // in a stalled download if the block file is pruned before the request.
+            if (nLocalServices & NODE_NETWORK) {
                 LOCK(cs_vNodes);
                 BOOST_FOREACH(CNode* pnode, vNodes)
                     if (chainActive.Height() > (pnode->nStartingHeight != -1 ? pnode->nStartingHeight - 2000 : nBlockEstimate))
@@ -2419,6 +2474,8 @@ bool FindBlockPos(CValidationState &state, CDiskBlockPos &pos, unsigned int nAdd
         unsigned int nOldChunks = (pos.nPos + BLOCKFILE_CHUNK_SIZE - 1) / BLOCKFILE_CHUNK_SIZE;
         unsigned int nNewChunks = (vinfoBlockFile[nFile].nSize + BLOCKFILE_CHUNK_SIZE - 1) / BLOCKFILE_CHUNK_SIZE;
         if (nNewChunks > nOldChunks) {
+            if (fPruneMode)
+                fCheckForPruning = true;
             if (CheckDiskSpace(nNewChunks * BLOCKFILE_CHUNK_SIZE - pos.nPos)) {
                 FILE *file = OpenBlockFile(pos);
                 if (file) {
@@ -2450,6 +2507,8 @@ bool FindUndoPos(CValidationState &state, int nFile, CDiskBlockPos &pos, unsigne
     unsigned int nOldChunks = (pos.nPos + UNDOFILE_CHUNK_SIZE - 1) / UNDOFILE_CHUNK_SIZE;
     unsigned int nNewChunks = (nNewSize + UNDOFILE_CHUNK_SIZE - 1) / UNDOFILE_CHUNK_SIZE;
     if (nNewChunks > nOldChunks) {
+        if (fPruneMode)
+            fCheckForPruning = true;
         if (CheckDiskSpace(nNewChunks * UNDOFILE_CHUNK_SIZE - pos.nPos)) {
             FILE *file = OpenUndoFile(pos);
             if (file) {
@@ -2665,7 +2724,10 @@ bool AcceptBlock(CBlock& block, CValidationState& state, CBlockIndex** ppindex,
     if (!AcceptBlockHeader(block, state, &pindex))
         return false;
 
-    if (pindex->nStatus & BLOCK_HAVE_DATA) {
+    // If we're pruning, ensure that we don't allow a peer to dump a copy
+    // of old blocks.  But we might need blocks that are not on the main chain
+    // to handle a reorg, even if we've processed once.
+    if (pindex->nStatus & BLOCK_HAVE_DATA || chainActive.Contains(pindex)) {
         // TODO: deal better with duplicate blocks.
         // return state.DoS(20, error("AcceptBlock(): already have block %d %s", pindex->nHeight, pindex->GetBlockHash().ToString()), REJECT_DUPLICATE, "duplicate");
         return true;
@@ -2698,6 +2760,9 @@ bool AcceptBlock(CBlock& block, CValidationState& state, CBlockIndex** ppindex,
         return state.Abort(std::string("System error: ") + e.what());
     }
 
+    if (fCheckForPruning)
+        FlushStateToDisk(state, FLUSH_STATE_NONE); // we just allocated more disk space for block files
+
     return true;
 }
 
@@ -2785,6 +2850,112 @@ bool AbortNode(const std::string &strMessage, const std::string &userMessage) {
     return false;
 }
 
+
+/**
+ * BLOCK PRUNING CODE
+ */
+
+/* Calculate the amount of disk space the block & undo files currently use */
+uint64_t CalculateCurrentUsage()
+{
+    uint64_t retval = 0;
+    BOOST_FOREACH(const CBlockFileInfo &file, vinfoBlockFile) {
+        retval += file.nSize + file.nUndoSize;
+    }
+    return retval;
+}
+
+/* Prune a block file (modify associated database entries)*/
+void PruneOneBlockFile(const int fileNumber)
+{
+    for (BlockMap::iterator it = mapBlockIndex.begin(); it != mapBlockIndex.end(); ++it) {
+        CBlockIndex* pindex = it->second;
+        if (pindex->nFile == fileNumber) {
+            pindex->nStatus &= ~BLOCK_HAVE_DATA;
+            pindex->nStatus &= ~BLOCK_HAVE_UNDO;
+            pindex->nFile = 0;
+            pindex->nDataPos = 0;
+            pindex->nUndoPos = 0;
+            setDirtyBlockIndex.insert(pindex);
+
+            // Prune from mapBlocksUnlinked -- any block we prune would have
+            // to be downloaded again in order to consider its chain, at which
+            // point it would be considered as a candidate for
+            // mapBlocksUnlinked or setBlockIndexCandidates.
+            std::pair<std::multimap<CBlockIndex*, CBlockIndex*>::iterator, std::multimap<CBlockIndex*, CBlockIndex*>::iterator> range = mapBlocksUnlinked.equal_range(pindex->pprev);
+            while (range.first != range.second) {
+                std::multimap<CBlockIndex *, CBlockIndex *>::iterator it = range.first;
+                range.first++;
+                if (it->second == pindex) {
+                    mapBlocksUnlinked.erase(it);
+                }
+            }
+        }
+    }
+
+    vinfoBlockFile[fileNumber].SetNull();
+    setDirtyFileInfo.insert(fileNumber);
+}
+
+
+void UnlinkPrunedFiles(std::set<int>& setFilesToPrune)
+{
+    for (set<int>::iterator it = setFilesToPrune.begin(); it != setFilesToPrune.end(); ++it) {
+        CDiskBlockPos pos(*it, 0);
+        boost::filesystem::remove(GetBlockPosFilename(pos, "blk"));
+        boost::filesystem::remove(GetBlockPosFilename(pos, "rev"));
+        LogPrintf("Prune: %s deleted blk/rev (%05u)\n", __func__, *it);
+    }
+}
+
+/* Calculate the block/rev files that should be deleted to remain under target*/
+void FindFilesToPrune(std::set<int>& setFilesToPrune)
+{
+    LOCK2(cs_main, cs_LastBlockFile);
+    if (chainActive.Tip() == NULL || nPruneTarget == 0) {
+        return;
+    }
+    if (chainActive.Tip()->nHeight <= Params().PruneAfterHeight()) {
+        return;
+    }
+
+    unsigned int nLastBlockWeMustKeep = chainActive.Tip()->nHeight - MIN_BLOCKS_TO_KEEP;
+    uint64_t nCurrentUsage = CalculateCurrentUsage();
+    // We don't check to prune until after we've allocated new space for files
+    // So we should leave a buffer under our target to account for another allocation
+    // before the next pruning.
+    uint64_t nBuffer = BLOCKFILE_CHUNK_SIZE + UNDOFILE_CHUNK_SIZE;
+    uint64_t nBytesToPrune;
+    int count=0;
+
+    if (nCurrentUsage + nBuffer >= nPruneTarget) {
+        for (int fileNumber = 0; fileNumber < nLastBlockFile; fileNumber++) {
+            nBytesToPrune = vinfoBlockFile[fileNumber].nSize + vinfoBlockFile[fileNumber].nUndoSize;
+
+            if (vinfoBlockFile[fileNumber].nSize == 0)
+                continue;
+
+            if (nCurrentUsage + nBuffer < nPruneTarget)  // are we below our target?
+                break;
+
+            // don't prune files that could have a block within MIN_BLOCKS_TO_KEEP of the main chain's tip
+            if (vinfoBlockFile[fileNumber].nHeightLast > nLastBlockWeMustKeep)
+                break;
+
+            PruneOneBlockFile(fileNumber);
+            // Queue up the files for removal
+            setFilesToPrune.insert(fileNumber);
+            nCurrentUsage -= nBytesToPrune;
+            count++;
+        }
+    }
+
+    LogPrint("prune", "Prune: target=%dMiB actual=%dMiB diff=%dMiB min_must_keep=%d removed %d blk/rev pairs\n",
+           nPruneTarget/1024/1024, nCurrentUsage/1024/1024,
+           ((int64_t)nPruneTarget - (int64_t)nCurrentUsage)/1024/1024,
+           nLastBlockWeMustKeep, count);
+}
+
 bool CheckDiskSpace(uint64_t nAdditionalBytes)
 {
     uint64_t nFreeBytesAvailable = boost::filesystem::space(GetDataDir()).available;
@@ -2872,7 +3043,9 @@ bool static LoadBlockIndexDB()
     {
         CBlockIndex* pindex = item.second;
         pindex->nChainWork = (pindex->pprev ? pindex->pprev->nChainWork : 0) + GetBlockProof(*pindex);
-        if (pindex->nStatus & BLOCK_HAVE_DATA) {
+        // We can link the chain of blocks for which we've received transactions at some point.
+        // Pruned nodes may have deleted the block.
+        if (pindex->nTx > 0) {
             if (pindex->pprev) {
                 if (pindex->pprev->nChainTx) {
                     pindex->nChainTx = pindex->pprev->nChainTx + pindex->nTx;
@@ -2929,6 +3102,11 @@ bool static LoadBlockIndexDB()
         }
     }
 
+    // Check whether we have ever pruned block & undo files
+    pblocktree->ReadFlag("prunedblockfiles", fHavePruned);
+    if (fHavePruned)
+        LogPrintf("LoadBlockIndexDB(): Block files have previously been pruned\n");
+
     // Check whether we need to continue reindexing
     bool fReindexing = false;
     pblocktree->ReadReindexing(fReindexing);
@@ -3069,6 +3247,7 @@ void UnloadBlockIndex()
         delete entry.second;
     }
     mapBlockIndex.clear();
+    fHavePruned = false;
 }
 
 bool LoadBlockIndex()
@@ -3260,6 +3439,7 @@ void static CheckBlockIndex()
     int nHeight = 0;
     CBlockIndex* pindexFirstInvalid = NULL; // Oldest ancestor of pindex which is invalid.
     CBlockIndex* pindexFirstMissing = NULL; // Oldest ancestor of pindex which does not have BLOCK_HAVE_DATA.
+    CBlockIndex* pindexFirstNeverProcessed = NULL; // Oldest ancestor of pindex for which nTx == 0.
     CBlockIndex* pindexFirstNotTreeValid = NULL; // Oldest ancestor of pindex which does not have BLOCK_VALID_TREE (regardless of being valid or not).
     CBlockIndex* pindexFirstNotTransactionsValid = NULL; // Oldest ancestor of pindex which does not have BLOCK_VALID_TRANSACTIONS (regardless of being valid or not).
     CBlockIndex* pindexFirstNotChainValid = NULL; // Oldest ancestor of pindex which does not have BLOCK_VALID_CHAIN (regardless of being valid or not).
@@ -3268,6 +3448,7 @@ void static CheckBlockIndex()
         nNodes++;
         if (pindexFirstInvalid == NULL && pindex->nStatus & BLOCK_FAILED_VALID) pindexFirstInvalid = pindex;
         if (pindexFirstMissing == NULL && !(pindex->nStatus & BLOCK_HAVE_DATA)) pindexFirstMissing = pindex;
+        if (pindexFirstNeverProcessed == NULL && pindex->nTx == 0) pindexFirstNeverProcessed = pindex;
         if (pindex->pprev != NULL && pindexFirstNotTreeValid == NULL && (pindex->nStatus & BLOCK_VALID_MASK) < BLOCK_VALID_TREE) pindexFirstNotTreeValid = pindex;
         if (pindex->pprev != NULL && pindexFirstNotTransactionsValid == NULL && (pindex->nStatus & BLOCK_VALID_MASK) < BLOCK_VALID_TRANSACTIONS) pindexFirstNotTransactionsValid = pindex;
         if (pindex->pprev != NULL && pindexFirstNotChainValid == NULL && (pindex->nStatus & BLOCK_VALID_MASK) < BLOCK_VALID_CHAIN) pindexFirstNotChainValid = pindex;
@@ -3279,12 +3460,21 @@ void static CheckBlockIndex()
             assert(pindex->GetBlockHash() == consensusParams.hashGenesisBlock); // Genesis block's hash must match.
             assert(pindex == chainActive.Genesis()); // The current active chain's genesis block must be this block.
         }
-        // HAVE_DATA is equivalent to VALID_TRANSACTIONS and equivalent to nTx > 0 (we stored the number of transactions in the block)
-        assert(!(pindex->nStatus & BLOCK_HAVE_DATA) == (pindex->nTx == 0));
-        assert(((pindex->nStatus & BLOCK_VALID_MASK) >= BLOCK_VALID_TRANSACTIONS) == (pindex->nTx > 0));
         if (pindex->nChainTx == 0) assert(pindex->nSequenceId == 0);  // nSequenceId can't be set for blocks that aren't linked
-        // All parents having data is equivalent to all parents being VALID_TRANSACTIONS, which is equivalent to nChainTx being set.
-        assert((pindexFirstMissing != NULL) == (pindex->nChainTx == 0)); // nChainTx == 0 is used to signal that all parent block's transaction data is available.
+        // VALID_TRANSACTIONS is equivalent to nTx > 0 for all nodes (whether or not pruning has occurred).
+        // HAVE_DATA is only equivalent to nTx > 0 (or VALID_TRANSACTIONS) if no pruning has occurred.
+        if (!fHavePruned) {
+            // If we've never pruned, then HAVE_DATA should be equivalent to nTx > 0
+            assert(!(pindex->nStatus & BLOCK_HAVE_DATA) == (pindex->nTx == 0));
+            assert(pindexFirstMissing == pindexFirstNeverProcessed);
+        } else {
+            // If we have pruned, then we can only say that HAVE_DATA implies nTx > 0
+            if (pindex->nStatus & BLOCK_HAVE_DATA) assert(pindex->nTx > 0);
+        }
+        if (pindex->nStatus & BLOCK_HAVE_UNDO) assert(pindex->nStatus & BLOCK_HAVE_DATA);
+        assert(((pindex->nStatus & BLOCK_VALID_MASK) >= BLOCK_VALID_TRANSACTIONS) == (pindex->nTx > 0)); // This is pruning-independent.
+        // All parents having had data (at some point) is equivalent to all parents being VALID_TRANSACTIONS, which is equivalent to nChainTx being set.
+        assert((pindexFirstNeverProcessed != NULL) == (pindex->nChainTx == 0)); // nChainTx != 0 is used to signal that all parent blocks have been processed (but may have been pruned).
         assert((pindexFirstNotTransactionsValid != NULL) == (pindex->nChainTx == 0));
         assert(pindex->nHeight == nHeight); // nHeight must be consistent.
         assert(pindex->pprev == NULL || pindex->nChainWork >= pindex->pprev->nChainWork); // For every block except the genesis block, the chainwork must be larger than the parent's.
@@ -3297,11 +3487,20 @@ void static CheckBlockIndex()
             // Checks for not-invalid blocks.
             assert((pindex->nStatus & BLOCK_FAILED_MASK) == 0); // The failed mask cannot be set for blocks without invalid parents.
         }
-        if (!CBlockIndexWorkComparator()(pindex, chainActive.Tip()) && pindexFirstMissing == NULL) {
-            if (pindexFirstInvalid == NULL) { // If this block sorts at least as good as the current tip and is valid, it must be in setBlockIndexCandidates.
-                 assert(setBlockIndexCandidates.count(pindex));
+        if (!CBlockIndexWorkComparator()(pindex, chainActive.Tip()) && pindexFirstNeverProcessed == NULL) {
+            if (pindexFirstInvalid == NULL) {
+                // If this block sorts at least as good as the current tip and
+                // is valid and we have all data for its parents, it must be in
+                // setBlockIndexCandidates.  chainActive.Tip() must also be there
+                // even if some data has been pruned.
+                if (pindexFirstMissing == NULL || pindex == chainActive.Tip()) {
+                    assert(setBlockIndexCandidates.count(pindex));
+                }
+                // If some parent is missing, then it could be that this block was in
+                // setBlockIndexCandidates but had to be removed because of the missing data.
+                // In this case it must be in mapBlocksUnlinked -- see test below.
             }
-        } else { // If this block sorts worse than the current tip, it cannot be in setBlockIndexCandidates.
+        } else { // If this block sorts worse than the current tip or some ancestor's block has never been seen, it cannot be in setBlockIndexCandidates.
             assert(setBlockIndexCandidates.count(pindex) == 0);
         }
         // Check whether this block is in mapBlocksUnlinked.
@@ -3315,12 +3514,28 @@ void static CheckBlockIndex()
             }
             rangeUnlinked.first++;
         }
-        if (pindex->pprev && pindex->nStatus & BLOCK_HAVE_DATA && pindexFirstMissing != NULL) {
-            if (pindexFirstInvalid == NULL) { // If this block has block data available, some parent doesn't, and has no invalid parents, it must be in mapBlocksUnlinked.
-                assert(foundInUnlinked);
+        if (pindex->pprev && (pindex->nStatus & BLOCK_HAVE_DATA) && pindexFirstNeverProcessed != NULL && pindexFirstInvalid == NULL) {
+            // If this block has block data available, some parent was never received, and has no invalid parents, it must be in mapBlocksUnlinked.
+            assert(foundInUnlinked);
+        }
+        if (!(pindex->nStatus & BLOCK_HAVE_DATA)) assert(!foundInUnlinked); // Can't be in mapBlocksUnlinked if we don't HAVE_DATA
+        if (pindexFirstMissing == NULL) assert(!foundInUnlinked); // We aren't missing data for any parent -- cannot be in mapBlocksUnlinked.
+        if (pindex->pprev && (pindex->nStatus & BLOCK_HAVE_DATA) && pindexFirstNeverProcessed == NULL && pindexFirstMissing != NULL) {
+            // We HAVE_DATA for this block, have received data for all parents at some point, but we're currently missing data for some parent.
+            assert(fHavePruned); // We must have pruned.
+            // This block may have entered mapBlocksUnlinked if:
+            //  - it has a descendant that at some point had more work than the
+            //    tip, and
+            //  - we tried switching to that descendant but were missing
+            //    data for some intermediate block between chainActive and the
+            //    tip.
+            // So if this block is itself better than chainActive.Tip() and it wasn't in
+            // setBlockIndexCandidates, then it must be in mapBlocksUnlinked.
+            if (!CBlockIndexWorkComparator()(pindex, chainActive.Tip()) && setBlockIndexCandidates.count(pindex) == 0) {
+                if (pindexFirstInvalid == NULL) {
+                    assert(foundInUnlinked);
+                }
             }
-        } else { // If this block does not have block data available, or all parents do, it cannot be in mapBlocksUnlinked.
-            assert(!foundInUnlinked);
         }
         // assert(pindex->GetBlockHash() == pindex->GetBlockHeader().GetHash()); // Perhaps too slow
         // End: actual consistency checks.
@@ -3340,6 +3555,7 @@ void static CheckBlockIndex()
             // If pindex was the first with a certain property, unset the corresponding variable.
             if (pindex == pindexFirstInvalid) pindexFirstInvalid = NULL;
             if (pindex == pindexFirstMissing) pindexFirstMissing = NULL;
+            if (pindex == pindexFirstNeverProcessed) pindexFirstNeverProcessed = NULL;
             if (pindex == pindexFirstNotTreeValid) pindexFirstNotTreeValid = NULL;
             if (pindex == pindexFirstNotTransactionsValid) pindexFirstNotTransactionsValid = NULL;
             if (pindex == pindexFirstNotChainValid) pindexFirstNotChainValid = NULL;
@@ -3497,7 +3713,9 @@ void static ProcessGetData(CNode* pfrom)
                         }
                     }
                 }
-                if (send)
+                // Pruned nodes may have deleted the block, so check whether
+                // it's available before trying to send.
+                if (send && (mi->second->nStatus & BLOCK_HAVE_DATA))
                 {
                     // Send block from disk
                     CBlock block;
diff --git a/src/main.h b/src/main.h
index bb6fd6f204..d34f6c30d1 100644
--- a/src/main.h
+++ b/src/main.h
@@ -132,6 +132,26 @@ extern CBlockIndex *pindexBestHeader;
 /** Minimum disk space required - used in CheckDiskSpace() */
 static const uint64_t nMinDiskSpace = 52428800;
 
+/** Pruning-related variables and constants */
+/** True if any block files have ever been pruned. */
+extern bool fHavePruned;
+/** True if we're running in -prune mode. */
+extern bool fPruneMode;
+/** Number of MiB of block files that we're trying to stay below. */
+extern uint64_t nPruneTarget;
+/** Block files containing a block-height within MIN_BLOCKS_TO_KEEP of chainActive.Tip() will not be pruned. */
+static const signed int MIN_BLOCKS_TO_KEEP = 288;
+
+// Require that user allocate at least 550MB for block & undo files (blk???.dat and rev???.dat)
+// At 1MB per block, 288 blocks = 288MB.
+// Add 15% for Undo data = 331MB
+// Add 20% for Orphan block rate = 397MB
+// We want the low water mark after pruning to be at least 397 MB and since we prune in
+// full block file chunks, we need the high water mark which triggers the prune to be
+// one 128MB block file + added 15% undo data = 147MB greater for a total of 545MB
+// Setting the target to > than 550MB will make it likely we can respect the target.
+static const signed int MIN_DISK_SPACE_FOR_BLOCK_FILES = 550 * 1024 * 1024;
+
 /** Register with a network node to receive its signals */
 void RegisterNodeSignals(CNodeSignals& nodeSignals);
 /** Unregister a network node */
@@ -186,6 +206,28 @@ bool GetTransaction(const uint256 &hash, CTransaction &tx, uint256 &hashBlock, b
 bool ActivateBestChain(CValidationState &state, CBlock *pblock = NULL);
 CAmount GetBlockValue(int nHeight, const CAmount& nFees);
 
+/**
+ * Prune block and undo files (blk???.dat and undo???.dat) so that the disk space used is less than a user-defined target.
+ * The user sets the target (in MB) on the command line or in config file.  This will be run on startup and whenever new
+ * space is allocated in a block or undo file, staying below the target. Changing back to unpruned requires a reindex
+ * (which in this case means the blockchain must be re-downloaded.)
+ *
+ * Pruning functions are called from FlushStateToDisk when the global fCheckForPruning flag has been set.
+ * Block and undo files are deleted in lock-step (when blk00003.dat is deleted, so is rev00003.dat.)
+ * Pruning cannot take place until the longest chain is at least a certain length (100000 on mainnet, 1000 on testnet, 10 on regtest).
+ * Pruning will never delete a block within a defined distance (currently 288) from the active chain's tip.
+ * The block index is updated by unsetting HAVE_DATA and HAVE_UNDO for any blocks that were stored in the deleted files.
+ * A db flag records the fact that at least some block files have been pruned.
+ *
+ * @param[out]   setFilesToPrune   The set of file indices that can be unlinked will be returned
+ */
+void FindFilesToPrune(std::set<int>& setFilesToPrune);
+
+/**
+ *  Actually unlink the specified files
+ */
+void UnlinkPrunedFiles(std::set<int>& setFilesToPrune);
+
 /** Create a new block index entry for a given block hash */
 CBlockIndex * InsertBlockIndex(uint256 hash);
 /** Abort with a message */
@@ -196,7 +238,8 @@ bool GetNodeStateStats(NodeId nodeid, CNodeStateStats &stats);
 void Misbehaving(NodeId nodeid, int howmuch);
 /** Flush all state, indexes and buffers to disk. */
 void FlushStateToDisk();
-
+/** Prune block files and flush state to disk. */
+void PruneAndFlush();
 
 /** (try to) add transaction to memory pool **/
 bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransaction &tx, bool fLimitFree,