Merge bitcoin/bitcoin#22215: refactor: Add FoundBlock.found member

5c5d0b6 Add FoundBlock.found member (Russell Yanofsky)

Pull request description:

  This change lets IPC serialization code handle FoundBlock arguments more simply and efficiently. Without this change there was no way to determine from a FoundBlock object whether a block was found or not. So in order to correctly implement behavior of leaving FoundBlock output variables unmodified when a block was not found, IPC code would have to read preexisting output variable values from the local process, send them to the remote process, receive output values back from the remote process, and save them to output variables unconditionally. With FoundBlock.found method, the process is simpler. There's no need to read or send preexisting local output variable values, just to read final output values from the remote process and set them conditionally if the block was found.

  ---

  This PR is part of the [process separation project](https://github.com/bitcoin/bitcoin/projects/10). The commit was first part of larger PR #10102.

ACKs for top commit:
  fjahr:
    Code review ACK 5c5d0b6
  theStack:
    Concept and code review ACK 5c5d0b6
  jamesob:
    ACK 5c5d0b6 ([`jamesob/ackr/22215.1.ryanofsky.refactor_add_foundblock`](https://github.com/jamesob/bitcoin/tree/ackr/22215.1.ryanofsky.refactor_add_foundblock))
  Zero-1729:
    crACK 5c5d0b6

Tree-SHA512: d906e1b7100ff72c3aa06d80bd77673887b2db670ebd52dce7c4f6f557a23a1744c6109308228a37fda6c6ea74f05ba0efecff0ef235ab06ea8acd861fbb8675

Author: fanquake

src/interfaces/chain.h

@@ -35,7 +35,9 @@ namespace interfaces {
 class Handler;
 class Wallet;
 
-//! Helper for findBlock to selectively return pieces of block data.
+//! Helper for findBlock to selectively return pieces of block data. If block is
+//! found, data will be returned by setting specified output variables. If block
+//! is not found, output variables will keep their previous values.
 class FoundBlock
 {
 public:
@@ -60,6 +62,7 @@ class FoundBlock
     bool* m_in_active_chain = nullptr;
     const FoundBlock* m_next_block = nullptr;
     CBlock* m_data = nullptr;
+    mutable bool found = false;
 };
 
 //! Interface giving clients (wallet processes, maybe other analysis tools in

src/node/interfaces.cpp

@@ -334,6 +334,7 @@ bool FillBlock(const CBlockIndex* index, const FoundBlock& block, UniqueLock<Rec
         REVERSE_LOCK(lock);
         if (!ReadBlockFromDisk(*block.m_data, index, Params().GetConsensus())) block.m_data->SetNull();
     }
+    block.found = true;
     return true;
 }