net: reduce LOCK(cs_main) scope in GETBLOCKTXN

Also adds a static assertion that MAX_BLOCKTXN_DEPTH <= MIN_BLOCKS_TO_KEEP

src/net_processing.cpp

@@ -118,6 +118,7 @@ static const unsigned int MAX_HEADERS_RESULTS = 2000;
 static const int MAX_CMPCTBLOCK_DEPTH = 5;
 /** Maximum depth of blocks we're willing to respond to GETBLOCKTXN requests for. */
 static const int MAX_BLOCKTXN_DEPTH = 10;
+static_assert(MAX_BLOCKTXN_DEPTH <= MIN_BLOCKS_TO_KEEP, "MAX_BLOCKTXN_DEPTH too high");
 /** Size of the "block download window": how far ahead of our current height do we fetch?
  *  Larger windows tolerate larger download speed differences between peer, but increase the potential
  *  degree of disordering of blocks on disk (which make reindexing and pruning harder). We'll probably
@@ -4366,6 +4367,7 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
             return;
         }
 
+        FlatFilePos block_pos{};
         {
             LOCK(cs_main);
 
@@ -4376,14 +4378,20 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
             }
 
             if (pindex->nHeight >= m_chainman.ActiveChain().Height() - MAX_BLOCKTXN_DEPTH) {
+                block_pos = pindex->GetBlockPos();
+            }
+        }
+
+        if (!block_pos.IsNull()) {
             CBlock block;
-                const bool ret{m_chainman.m_blockman.ReadBlockFromDisk(block, *pindex)};
+            const bool ret{m_chainman.m_blockman.ReadBlockFromDisk(block, block_pos)};
+            // If height is above MAX_BLOCKTXN_DEPTH then this block cannot get
+            // pruned after we release cs_main above, so this read should never fail.
             assert(ret);
 
             SendBlockTransactions(pfrom, *peer, block, req);
             return;
         }
-        }
 
         // If an older block is requested (should never happen in practice,
         // but can happen in tests) send a block response instead of a