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.

src/rpc/rawtransaction.cpp

@@ -1536,12 +1536,15 @@ UniValue finalizepsbt(const JSONRPCRequest& request)
         throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed %s", error));
     }
 
-    // Get all of the previous transactions
+    // Finalize input signatures -- in case we have partial signatures that add up to a complete
+    //   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.
     bool complete = true;
     for (unsigned int i = 0; i < psbtx.tx->vin.size(); ++i) {
         PSBTInput& input = psbtx.inputs.at(i);
 
-        complete &= SignPSBTInput(DUMMY_SIGNING_PROVIDER, *psbtx.tx, input, i, 1);
+        complete &= SignPSBTInput(DUMMY_SIGNING_PROVIDER, *psbtx.tx, input, i, SIGHASH_ALL);
     }
 
     UniValue result(UniValue::VOBJ);

src/script/sign.cpp

@@ -239,10 +239,14 @@ bool ProduceSignature(const SigningProvider& provider, const BaseSignatureCreato
     return sigdata.complete;
 }
 
+bool PSBTInputSigned(PSBTInput& input)
+{
+    return !input.final_script_sig.empty() || !input.final_script_witness.IsNull();
+}
+
 bool SignPSBTInput(const SigningProvider& provider, const CMutableTransaction& tx, PSBTInput& input, int index, int sighash)
 {
-    // if this input has a final scriptsig or scriptwitness, don't do anything with it
-    if (!input.final_script_sig.empty() || !input.final_script_witness.IsNull()) {
+    if (PSBTInputSigned(input)) {
         return true;
     }
 

src/script/sign.h

@@ -730,6 +730,9 @@ 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);
+
 /** Signs a PSBTInput, verifying that all provided data matches what is being signed. */
 bool SignPSBTInput(const SigningProvider& provider, const CMutableTransaction& tx, PSBTInput& input, int index, int sighash = SIGHASH_ALL);