Merge bitcoin/bitcoin#27039: blockstorage: do not flush block to disk if it is already there

dfcef536d0e6c40e98dce35ae7af6e3e4a2595cd blockstorage: do not flush block to disk if it is already there (Matthew Zipkin)

Pull request description:

  Closes https://github.com/bitcoin/bitcoin/issues/2039

  When reindexing from flat-file block storage there is no need to write anything back to disk, since the block data is already there. This PR skips flushing to disk those blocks that already have a known position in the datastore. Skipping this means that users can write-protect the `blk` files on disk which may be useful for security or even safely sharing that data between multiple bitcoind instances.

  `FindBlockPos()` may also flush the undo data file, but again this is skipped if the corresponding block position is known, like during the initial stage of a reindex when block data is being indexed. Once the block index is complete the validation mechanism will call `ConnectBlock()` which will save undo data at that time.

  The call stack looks like this:

  ```
  init()
  ThreadImport() <-- process fReindex flag
  LoadExternalBlockFile()
  AcceptBlock()
  SaveBlockToDisk()
  FindBlockPos()
  FlushBlockFile() <-- unnecessary if block is already on disk
  ```

  A larger refactor of this part of the code was started by mzumsande here:  https://github.com/mzumsande/bitcoin/tree/202207_refactor_findblockpos including this fix, reviewers can let me know if the changes should be combined.

ACKs for top commit:
  sipa:
    utACK dfcef536d0e6c40e98dce35ae7af6e3e4a2595cd
  mzumsande:
    re-ACK dfcef536d0e6c40e98dce35ae7af6e3e4a2595cd
  achow101:
    ACK dfcef536d0e6c40e98dce35ae7af6e3e4a2595cd
  furszy:
    Rebase diff ACK dfcef53.

Tree-SHA512: 385c5ac1288b325135398d0ddd3ab788fa98cc0ca19bd2474c74039f2ce70d5088c1d1c9d4dd10aefcbd4c757767ec5805d07ba8cee9289a66f96e6f9eaa5279

1 files changed, 5 insertions, 4 deletions

diff --git a/test/functional/feature_reindex_readonly.py b/test/functional/feature_reindex_readonly.py
index dd99c3c4fa..25cff87a3b 100755
--- a/test/functional/feature_reindex_readonly.py
+++ b/test/functional/feature_reindex_readonly.py
@@ -24,6 +24,7 @@ class BlockstoreReindexTest(BitcoinTestFramework):
         opreturn = "6a"
         nulldata = fastprune_blockfile_size * "ff"
         self.generateblock(self.nodes[0], output=f"raw({opreturn}{nulldata})", transactions=[])
+        block_count = self.nodes[0].getblockcount()
         self.stop_node(0)
 
         assert (self.nodes[0].chain_path / "blocks" / "blk00000.dat").exists()
@@ -73,10 +74,10 @@ class BlockstoreReindexTest(BitcoinTestFramework):
                 pass
 
         if undo_immutable:
-            self.log.info("Attempt to restart and reindex the node with the unwritable block file")
-            with self.nodes[0].assert_debug_log(expected_msgs=['FlushStateToDisk', 'failed to open file'], unexpected_msgs=[]):
-                self.nodes[0].assert_start_raises_init_error(extra_args=['-reindex', '-fastprune'],
-                    expected_msg="Error: A fatal internal error occurred, see debug.log for details")
+            self.log.debug("Attempt to restart and reindex the node with the unwritable block file")
+            with self.nodes[0].wait_for_debug_log([b"Reindexing finished"]):
+                self.start_node(0, extra_args=['-reindex', '-fastprune'])
+            assert block_count == self.nodes[0].getblockcount()
             undo_immutable()
 
         filename.chmod(0o777)