Add bool PSBTInputSigned

Refactor out a "PSBTInputSigned" function to check if a PSBT is signed,
for use in subsequent commits.

Also improve a related comment.

GitHub-Pull: #14588
Rebased-From: 53e6fffb8f

src/rpc/rawtransaction.cpp

@@ -1309,6 +1309,9 @@ UniValue finalizepsbt(const JSONRPCRequest& request)
     }
 
     bool extract = request.params[1].isNull() || (!request.params[1].isNull() && request.params[1].get_bool());
+    //   signature, but have not combined them yet (e.g. because the combiner that created this
+    //   PartiallySignedTransaction did not understand them), this will combine them into a final
+    //   script.
 
     CMutableTransaction mtx;
     bool complete = FinalizeAndExtractPSBT(psbtx, mtx);

src/script/sign.cpp

@@ -245,6 +245,10 @@ bool ProduceSignature(const SigningProvider& provider, const BaseSignatureCreato
 }
 
 namespace {
+{
+    return !input.final_script_sig.empty() || !input.final_script_witness.IsNull();
+}
+
 class SignatureExtractorChecker final : public BaseSignatureChecker
 {
 private:

src/script/sign.h

@@ -156,6 +156,9 @@ bool ProduceSignature(const SigningProvider& provider, const BaseSignatureCreato
 bool SignSignature(const SigningProvider &provider, const CScript& fromPubKey, CMutableTransaction& txTo, unsigned int nIn, const CAmount& amount, int nHashType);
 bool SignSignature(const SigningProvider &provider, const CTransaction& txFrom, CMutableTransaction& txTo, unsigned int nIn, int nHashType);
 
+/** Checks whether a PSBTInput is already signed. */
+bool PSBTInputSigned(PSBTInput& input);
+
 /** Extract signature data from a transaction input, and insert it. */
 SignatureData DataFromTransaction(const CMutableTransaction& tx, unsigned int nIn, const CTxOut& txout);
 void UpdateInput(CTxIn& input, const SignatureData& data);