Add FoundBlock.found member

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.

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 @@ public:
     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

@@ -345,6 +345,7 @@ 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;
 }