rpc: Remove submitblock duplicate pre-check

The duplicate checks are repeated early in the contextual checks of
ProcessNewBlock. If duplicate blocks are detected much of their
validation is skipped. Depending on the constitution of the block,
validating the merkle root of the block is part of the more intensive
workload when validating a block. This could be an argument for moving
the pre-checks into block processing. In net_processing this would have
a smaller effect however, since the block mutation check, which also
validates the merkle root, is done before.

A side effect of this change is that a duplicate block is persisted
again on disk even when pruning is activated. This is similar to the
behaviour with getblockfrompeer. Add a release note for this change in
behaviour.

Testing spamming a node with valid, but duplicate unrequested blocks
seems to exhaust a CPU thread, but does not seem to significantly impact
keeping up with the tip. The benefits of adding these checks to
net_processing are questionable, especially since there are other ways
to trigger the more CPU-intensive checks without submitting a duplicate
block. Since these DOS concerns apply even less to the RPC interface,
which does not have banning mechanics built in, remove them too.

---

With the introduction of a mining ipc interface and the potential future
introduction of a kernel library API it becomes increasingly important
to offer common behaviour between them. An example of this is
ProcessNewBlock, which is used by ipc, rpc, net_processing and
(potentially) the kernel library. Having divergent behaviour on
suggested pre-checks and checks for these functions is confusing to both
developers and users and is a maintenance burden.

The rpc interface for ProcessNewBlock (submitblock) currently pre-checks
if the block has a coinbase transaction and whether it has been
processed before. While the current example binary for how to use the
kernel library, bitcoin-chainstate, imitates these checks, the other
interfaces do not.

Author: TheCharlatan

doc/release-notes-31175.md

@@ -0,0 +1,8 @@
+RPC
+---
+
+Duplicate blocks submitted with `submitblock` will now persist their block data
+even if it was previously pruned. If pruning is activated, the data will be
+pruned again eventually once the block file it is persisted in is selected for
+pruning. This is consistent with the behaviour of `getblockfrompeer` where the
+block is persisted as well even when pruning.

src/rpc/mining.cpp

@@ -1016,17 +1016,6 @@ static RPCHelpMan submitblock()
     }
 
     ChainstateManager& chainman = EnsureAnyChainman(request.context);
-    uint256 hash = block.GetHash();
-    {
-        LOCK(cs_main);
-        const CBlockIndex* pindex = chainman.m_blockman.LookupBlockIndex(hash);
-        if (pindex) {
-            if (pindex->IsValid(BLOCK_VALID_SCRIPTS)) {
-                return "duplicate";
-            }
-        }
-    }
-
     {
         LOCK(cs_main);
         const CBlockIndex* pindex = chainman.m_blockman.LookupBlockIndex(block.hashPrevBlock);