Merge bitcoin/bitcoin#22895: consensus: don't call GetBlockPos in ReadBlockFromDisk without cs_main lock

350e034 consensus: don't call GetBlockPos in ReadBlockFromDisk without lock (Jon Atack)

Pull request description:

  Commit ccd8ef6 "Reduce cs_main lock in ReadBlockFromDisk, only read GetBlockPos under the lock" in #11281 moved the cs_main lock from caller to `ReadBlockFromDisk()` for calling `CBlockIndex::GetBlockPos()`, but the second invocation doesn't have the lock, and IIUC there is no guarantee the compiler can know if state has changed.

  Use the `blockPos` local variable instead, rename it to `block_pos`, and make it const.

ACKs for top commit:
  laanwj:
    Code review ACK 350e034
  theStack:
    Code-review ACK 350e034
  promag:
    Code review ACK 350e034.

Tree-SHA512: 0df0614ab1876885c85f7b53c604a759a29008da8027e95503b4726d2b820ec6d27546020c613337ff954406e01cb5d191978ba4a12124052fed6e1b0e9a226f

Author: laanwj

src/node/blockstorage.cpp

@@ -394,18 +394,14 @@ bool ReadBlockFromDisk(CBlock& block, const FlatFilePos& pos, const Consensus::P
 
 bool ReadBlockFromDisk(CBlock& block, const CBlockIndex* pindex, const Consensus::Params& consensusParams)
 {
-    FlatFilePos blockPos;
-    {
-        LOCK(cs_main);
-        blockPos = pindex->GetBlockPos();
-    }
+    const FlatFilePos block_pos{WITH_LOCK(cs_main, return pindex->GetBlockPos())};
 
-    if (!ReadBlockFromDisk(block, blockPos, consensusParams)) {
+    if (!ReadBlockFromDisk(block, block_pos, consensusParams)) {
         return false;
     }
     if (block.GetHash() != pindex->GetBlockHash()) {
         return error("ReadBlockFromDisk(CBlock&, CBlockIndex*): GetHash() doesn't match index for %s at %s",
-                     pindex->ToString(), pindex->GetBlockPos().ToString());
+                     pindex->ToString(), block_pos.ToString());
     }
     return true;
 }