rpc: add `descriptorprocesspsbt` rpc #25796

This PR implements an RPC called descriptorprocesspsbt. This RPC is based off of walletprocesspsbt, but instead of interacting with the wallet to update, sign and finalize a psbt, it instead accepts an array of output descriptors and uses that information along with information from the mempool, txindex, and the utxo set to do so. utxoupdatepsbt also updates a psbt in this manner, but doesn't sign or finalize it. Because of this overlap, a helper function that is added in this PR is called by both utxoupdatepsbt and descriptorprocesspsbt. Whether or not the helper function signs a psbt is dictated by if the HidingSigningProvider passed to it contains any private information. There is also a test added in this PR for this new RPC that uses p2wsh, p2wpkh, and legacy outputs. Edit: see #25796 (comment)

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #22838 (descriptors: Be able to specify change and receiving in a single descriptor string by achow101)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

This PR implements the last suggestion in #14739 (with a slightly modified name)

Concept ACK

<details><summary>A few suggestions for your consideration</summary><p>

diff --git a/src/rpc/rawtransaction.cpp b/src/rpc/rawtransaction.cpp
index b6715f7f20..1ccc267ec1 100644
--- a/src/rpc/rawtransaction.cpp
+++ b/src/rpc/rawtransaction.cpp
@@ -164,7 +164,7 @@ static std::vector<RPCArg> CreateTxDoc()
 
 // Update PSBT with information from the mempool, the UTXO set, and the provided descriptors.
 // Optionally sign the inputs which we can using information from the descriptors.
-PartiallySignedTransaction ProcessPSBT(const JSONRPCRequest& request, const HidingSigningProvider provider, int sighash_type = 1, bool finalize = false)
+PartiallySignedTransaction ProcessPSBT(const JSONRPCRequest& request, const HidingSigningProvider& provider, int sighash_type, bool finalize)
 {
     // Unserialize the transactions
     PartiallySignedTransaction psbtx;
@@ -179,11 +179,9 @@ PartiallySignedTransaction ProcessPSBT(const JSONRPCRequest& request, const Hidi
     {
-        NodeContext& node = EnsureAnyNodeContext(request.context);
+        const NodeContext& node = EnsureAnyNodeContext(request.context);
         const CTxMemPool& mempool = EnsureMemPool(node);
-        ChainstateManager& chainman = EnsureChainman(node);
-        LOCK2(cs_main, mempool.cs);
-        CCoinsViewCache &viewChain = chainman.ActiveChainstate().CoinsTip();
-        CCoinsViewMemPool viewMempool(&viewChain, mempool);
-        view.SetBackend(viewMempool); // temporarily switch cache backend to db+mempool view
+        LOCK2(::cs_main, mempool.cs);
+        CCoinsViewMemPool view_mempool{&EnsureChainman(node).ActiveChainstate().CoinsTip(), mempool};
+        view.SetBackend(view_mempool); // temporarily switch cache backend to db+mempool view
 
         for (const CTxIn& txin : psbtx.tx->vin) {
             view.AccessCoin(txin.prevout); // Load entries from viewChain into view; can fail.
@@ -211,7 +209,7 @@ PartiallySignedTransaction ProcessPSBT(const JSONRPCRequest& request, const Hidi
         }
     }
 
-    const PrecomputedTransactionData txdata = PrecomputePSBTData(psbtx);
+    const PrecomputedTransactionData& txdata = PrecomputePSBTData(psbtx);
 
     for (unsigned int i = 0; i < psbtx.tx->vin.size(); ++i) {
         if (PSBTInputSigned(psbtx.inputs.at(i))) {
@@ -220,9 +218,9 @@ PartiallySignedTransaction ProcessPSBT(const JSONRPCRequest& request, const Hidi
 
         // Update script/keypath information using descriptor data.
         // Note that SignPSBTInput does a lot more than just constructing ECDSA signatures.
-        // We only actually care about those if our signing provider doesn't hide private 
+        // We only actually care about those if our signing provider doesn't hide private
         // information, as is the case with `descriptorprocesspsbt`
-        SignPSBTInput(provider, psbtx, i, &txdata, sighash_type, nullptr, finalize);
+        SignPSBTInput(provider, psbtx, /*index=*/i, &txdata, sighash_type, /*out_sigdata=*/nullptr, finalize);
     }
 
     // Update script/keypath information using descriptor data.
@@ -1649,7 +1647,11 @@ static RPCHelpMan utxoupdatepsbt()
     }
 
     // We don't actually need private keys further on; hide them as a precaution.
-    PartiallySignedTransaction psbtx = ProcessPSBT(request, HidingSigningProvider(&provider, /*hide_secret=*/true, /*hide_origin=*/false));
+    const PartiallySignedTransaction& psbtx = ProcessPSBT(
+        request,
+        HidingSigningProvider(&provider, /*hide_secret=*/true, /*hide_origin=*/false),
+        /*sighash_type=*/1,
+        /*finalize=*/false);
 
     CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
     ssTx << psbtx;
diff --git a/src/rpc/util.cpp b/src/rpc/util.cpp
index d5693dc555..86311adfed 100644
--- a/src/rpc/util.cpp
+++ b/src/rpc/util.cpp
@@ -1087,7 +1087,7 @@ std::vector<CScript> EvalDescriptorStringOrObject(const UniValue& scanobject, Fl
             throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("Cannot derive script without private keys: '%s'", desc_str));
         }
         if (expand_priv) {
-            desc->ExpandPrivate(i, provider, provider);
+            desc->ExpandPrivate(/*pos=*/i, provider, /*out=*/provider);
         }
         std::move(scripts.begin(), scripts.end(), std::back_inserter(ret));
     }
diff --git a/test/functional/rpc_psbt.py b/test/functional/rpc_psbt.py
index d6db6fbfc6..b7a2a4628f 100755
--- a/test/functional/rpc_psbt.py
+++ b/test/functional/rpc_psbt.py
@@ -581,7 +581,7 @@ class PSBTTest(BitcoinTestFramework):
                 break
         assert shuffled
 
-        # Test that we can update and sign a psbt with descriptors
+        self.log.info("Test descriptorprocesspsbt updates and signs a psbt with descriptors")
         key_info = get_generate_key()
         key1 = key_info[0]
         wpkh_addr = key_info[5]

</p></details>

Thanks for the review @jonatack, I've implemented you're suggestions

Concept ACK

The first commit is a useful refactor in any case.

While working on this PR, I noticed that it might be useful for utxoupdatepbst to also look in the txindex (if enabled) for the full tx that an input is spending from, which would allow legacy inputs to be updated as well. I have opened a separate PR for this, #25939, and this PR builds on top of that one so that descriptorprocesspsbt can do this as well.

Approach ACK

Rebased

Now that #25939 has been merged, this is now ready for review.

ACK 9bf2d303295594b8327ce0d67b9c1de98701f80a

concept ACK, would appreciate test tighten-up for maintainability

ACK https://github.com/bitcoin/bitcoin/pull/25796/commits/80f8bdfeb3d9301e0aec4b797689cbe569eff5a6

re-ACK 1bce12acd3e271a7c88d9400b4e3a5645bc8a911

reACK https://github.com/bitcoin/bitcoin/pull/25796/commits/1bce12acd3e271a7c88d9400b4e3a5645bc8a911