Add comment explaining forced processing of compact blocks

Github-Pull: #11458
Rebased-From: 01b52cedd4

src/net_processing.cpp

@@ -2144,7 +2144,16 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
                 mapBlockSource.emplace(pblock->GetHash(), std::make_pair(pfrom->GetId(), false));
             }
             bool fNewBlock = false;
-            ProcessNewBlock(chainparams, pblock, true, &fNewBlock);
+            // Setting fForceProcessing to true means that we bypass some of
+            // our anti-DoS protections in AcceptBlock, which filters
+            // unrequested blocks that might be trying to waste our resources
+            // (eg disk space). Because we only try to reconstruct blocks when
+            // we're close to caught up (via the CanDirectFetch() requirement
+            // above, combined with the behavior of not requesting blocks until
+            // we have a chain with at least nMinimumChainWork), and we ignore
+            // compact blocks with less work than our tip, it is safe to treat
+            // reconstructed compact blocks as having been requested.
+            ProcessNewBlock(chainparams, pblock, /*fForceProcessing=*/true, &fNewBlock);
             if (fNewBlock) {
                 pfrom->nLastBlockTime = GetTime();
             } else {
@@ -2224,7 +2233,11 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
             bool fNewBlock = false;
             // Since we requested this block (it was in mapBlocksInFlight), force it to be processed,
             // even if it would not be a candidate for new tip (missing previous block, chain not long enough, etc)
-            ProcessNewBlock(chainparams, pblock, true, &fNewBlock);
+            // This bypasses some anti-DoS logic in AcceptBlock (eg to prevent
+            // disk-space attacks), but this should be safe due to the
+            // protections in the compact block handler -- see related comment
+            // in compact block optimistic reconstruction handling.
+            ProcessNewBlock(chainparams, pblock, /*fForceProcessing=*/true, &fNewBlock);
             if (fNewBlock) {
                 pfrom->nLastBlockTime = GetTime();
             } else {