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

src/node/blockstorage.cpp

@@ -624,7 +624,18 @@ 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

test/functional/feature_fastprune.py

@@ -0,0 +1,43 @@
+#!/usr/bin/env python3
+# Copyright (c) 2023 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 fastprune mode."""
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import (
+    assert_equal
+)
+from test_framework.blocktools import (
+    create_block,
+    create_coinbase,
+)
+from test_framework.wallet import MiniWallet
+
+
+class FeatureFastpruneTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 1
+        self.extra_args = [["-fastprune"]]
+
+    def run_test(self):
+        self.log.info("ensure that large blocks don't crash or freeze in -fastprune")
+        wallet = MiniWallet(self.nodes[0])
+
+        # Create many transactions to make a large block (>64kb)
+        # Since Dash doesn't have witness data, we need to create many regular transactions
+        txs = []
+        for _ in range(500):  # Create enough transactions to exceed 64kb
+            tx = wallet.create_self_transfer()['tx']
+            txs.append(tx)
+
+        tip = int(self.nodes[0].getbestblockhash(), 16)
+        time = self.nodes[0].getblock(self.nodes[0].getbestblockhash())['time'] + 1
+        height = self.nodes[0].getblockcount() + 1
+        block = create_block(hashprev=tip, ntime=time, txlist=txs, coinbase=create_coinbase(height=height))
+        block.solve()
+        self.nodes[0].submitblock(block.serialize().hex())
+        assert_equal(int(self.nodes[0].getbestblockhash(), 16), block.sha256)
+
+
+if __name__ == '__main__':
+    FeatureFastpruneTest().main()

test/functional/test_runner.py

@@ -365,6 +365,7 @@
     'p2p_message_capture.py',
     'feature_addrman.py',
     'feature_asmap.py',
+    'feature_fastprune.py',
     'feature_includeconf.py',
     'mempool_unbroadcast.py',
     'mempool_compatibility.py',