Merge bitcoin/bitcoin#27191: blockstorage: Adjust fastprune limit if block exceeds blockfile size

8f14fc86225d8fe77353f61ebd6b0bdda8d13aa9 test: cover fastprune with excessive block size (Matthew Zipkin)
271c23e87f61276d7acab74e115b25a35144c8b4 blockstorage: Adjust fastprune limit if block exceeds blockfile size (Martin Zumsande)

Pull request description:

  The debug-only `-fastprune` option used in several tests is not always safe to use:
  If a `-fastprune` node receives a block larger than the maximum blockfile size of `64kb` bad things happen: The while loop in `BlockManager::FindBlockPos` never terminates, and the node runs oom because memory for `m_blockfile_info` is allocated in each iteration of the loop.
  The same would happen if a naive user used `-fastprune` on anything other than regtest (so this can be tested by syncing on signet for example, the first block that crashes the node is at height 2232).

  Change the approach by raising the blockfile size to the size of the block, if that block otherwise wouldn't fit (idea by TheCharlatan).

ACKs for top commit:
  ryanofsky:
    Code review ACK 8f14fc86225d8fe77353f61ebd6b0bdda8d13aa9. Added new assert, test, and comment since last review
  TheCharlatan:
    ACK 8f14fc86225d8fe77353f61ebd6b0bdda8d13aa9
  pinheadmz:
    ACK 8f14fc86225d8fe77353f61ebd6b0bdda8d13aa9

Tree-SHA512: df2fea30613ef9d40ebbc2416eacb574f6d7d96847db5c33dda22a29a2c61a8db831aa9552734ea4477e097f253dbcb6dcb1395d43d2a090cc0588c9ce66eac3

1 files changed, 12 insertions, 1 deletions

diff --git a/src/node/blockstorage.cpp b/src/node/blockstorage.cpp
index 38859f1b0d..abd71ca50b 100644
--- a/src/node/blockstorage.cpp
+++ b/src/node/blockstorage.cpp
@@ -620,7 +620,18 @@ bool BlockManager::FindBlockPos(FlatFilePos& pos, unsigned int nAddSize, unsigne
 
     bool finalize_undo = false;
     if (!fKnown) {
-        while (m_blockfile_info[nFile].nSize + nAddSize >= (gArgs.GetBoolArg("-fastprune", false) ? 0x10000 /* 64kb */ : MAX_BLOCKFILE_SIZE)) {
+        unsigned int max_blockfile_size{MAX_BLOCKFILE_SIZE};
+        // Use smaller blockfiles in test-only -fastprune mode - but avoid
+        // the possibility of having a block not fit into the block file.
+        if (gArgs.GetBoolArg("-fastprune", false)) {
+            max_blockfile_size = 0x10000; // 64kiB
+            if (nAddSize >= max_blockfile_size) {
+                // dynamically adjust the blockfile size to be larger than the added size
+                max_blockfile_size = nAddSize + 1;
+            }
+        }
+        assert(nAddSize < max_blockfile_size);
+        while (m_blockfile_info[nFile].nSize + nAddSize >= max_blockfile_size) {
             // when the undo file is keeping up with the block file, we want to flush it explicitly
             // when it is lagging behind (more blocks arrive than are being connected), we let the
             // undo block write case handle it