Merge bitcoin/bitcoin#26533: prune: scan and unlink already pruned block files on startup

3141eab9c669488a2e7fef5f60d356ac92294922 test: add functional test for ScanAndUnlinkAlreadyPrunedFiles (Andrew Toth)
e252909e561e47d75cb3a892657662a139f6532c test: add unit test for ScanAndUnlinkAlreadyPrunedFiles (Andrew Toth)
77557dda4a123515d0fa2a545ee21d7c43a66988 prune: scan and unlink already pruned block files on startup (Andrew Toth)

Pull request description:

  There are a few cases where we can mark a block and undo file as pruned in our block index, but not actually remove the files from disk.
  1. If we call `FindFilesToPrune` or `FindFilesToPruneManual` and crash before `UnlinkPrunedFiles`.
  2. If on Windows there is an open file handle to the file somewhere else when calling `fs::remove` in `UnlinkPrunedFiles` (https://en.cppreference.com/w/cpp/filesystem/remove, https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-deletefilew#remarks). This could be from another process, or if we are calling `ReadBlockFromDisk`/`ReadRawBlockFromDisk` without having a lock on `cs_main` (which has been allowed since https://github.com/bitcoin/bitcoin/commit/ccd8ef65f93ed82a87cee634660bed3ac17d9eb5).

  This PR mitigates this by scanning all pruned block files on startup after `LoadBlockIndexDB` and unlinking them again.

ACKs for top commit:
  achow101:
    ACK 3141eab9c669488a2e7fef5f60d356ac92294922
  pablomartin4btc:
    re-ACK with added functional test 3141eab9c669488a2e7fef5f60d356ac92294922.
  furszy:
    Code review ACK 3141eab9
  theStack:
    Code-review ACK 3141eab9c669488a2e7fef5f60d356ac92294922

Tree-SHA512: 6c73bc57838ad1b7e5d441af3c4d6bf4c61c4382e2b86485e57fbb74a61240710c0ceeceb8b4834e610ecfa3175c6955c81ea4b2285fee11ca6383f472979d8d

2 files changed, 55 insertions, 0 deletions

diff --git a/test/functional/feature_remove_pruned_files_on_startup.py b/test/functional/feature_remove_pruned_files_on_startup.py
new file mode 100755
index 0000000000..ca0e5ace9f
--- /dev/null
+++ b/test/functional/feature_remove_pruned_files_on_startup.py
@@ -0,0 +1,54 @@
+#!/usr/bin/env python3
+# Copyright (c) 2022 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 removing undeleted pruned blk files on startup."""
+
+import os
+from test_framework.test_framework import BitcoinTestFramework
+
+class FeatureRemovePrunedFilesOnStartupTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 1
+        self.extra_args = [["-fastprune", "-prune=1"]]
+
+    def mine_batches(self, blocks):
+        n = blocks // 250
+        for _ in range(n):
+            self.generate(self.nodes[0], 250)
+        self.generate(self.nodes[0], blocks % 250)
+        self.sync_blocks()
+
+    def run_test(self):
+        blk0 = os.path.join(self.nodes[0].datadir, self.nodes[0].chain, 'blocks', 'blk00000.dat')
+        rev0 = os.path.join(self.nodes[0].datadir, self.nodes[0].chain, 'blocks', 'rev00000.dat')
+        blk1 = os.path.join(self.nodes[0].datadir, self.nodes[0].chain, 'blocks', 'blk00001.dat')
+        rev1 = os.path.join(self.nodes[0].datadir, self.nodes[0].chain, 'blocks', 'rev00001.dat')
+        self.mine_batches(800)
+        fo1 = os.open(blk0, os.O_RDONLY)
+        fo2 = os.open(rev1, os.O_RDONLY)
+        fd1 = os.fdopen(fo1)
+        fd2 = os.fdopen(fo2)
+        self.nodes[0].pruneblockchain(600)
+
+        # Windows systems will not remove files with an open fd
+        if os.name != 'nt':
+            assert not os.path.exists(blk0)
+            assert not os.path.exists(rev0)
+            assert not os.path.exists(blk1)
+            assert not os.path.exists(rev1)
+        else:
+            assert os.path.exists(blk0)
+            assert not os.path.exists(rev0)
+            assert not os.path.exists(blk1)
+            assert os.path.exists(rev1)
+
+        # Check that the files are removed on restart once the fds are closed
+        fd1.close()
+        fd2.close()
+        self.restart_node(0)
+        assert not os.path.exists(blk0)
+        assert not os.path.exists(rev1)
+
+if __name__ == '__main__':
+    FeatureRemovePrunedFilesOnStartupTest().main()
diff --git a/test/functional/test_runner.py b/test/functional/test_runner.py
index 26ebce039b..1d560706c2 100755
--- a/test/functional/test_runner.py
+++ b/test/functional/test_runner.py
@@ -342,6 +342,7 @@ BASE_SCRIPTS = [
     'p2p_permissions.py',
     'feature_blocksdir.py',
     'wallet_startup.py',
+    'feature_remove_pruned_files_on_startup.py',
     'p2p_i2p_ports.py',
     'p2p_i2p_sessions.py',
     'feature_config_args.py',