Merge #17524: psbt: handle unspendable psbts

773d4572a4 Mark PSBTs spending unspendable outputs as invalid in analysis (Andrew Chow)
638e40cb60 Have a PSBTAnalysis state that indicates invalid PSBT (Andrew Chow)

Pull request description:

  When analyzing an unspendable PSBT, report that it is unspendable and exit analysis early.

ACKs for top commit:
  Sjors:
    ACK 773d457
  instagibbs:
    After some thought ACK 773d4572a4

Tree-SHA512: 99b0cb2fa1ea37593fc65a20effe881639d69ddeeecf5197bc87bc7f2220cbeb40f1d429d517e4d27f2e9fb563a00cd845d2b4b1ce05246a75a6cb56fb9b0ba5

src/node/psbt.cpp

@@ -7,6 +7,7 @@
 #include <node/psbt.h>
 #include <policy/policy.h>
 #include <policy/settings.h>
+#include <tinyformat.h>
 
 #include <numeric>
 
@@ -39,6 +40,11 @@ PSBTAnalysis AnalyzePSBT(PartiallySignedTransaction psbtx)
             calc_fee = false;
         }
 
+        if (!utxo.IsNull() && utxo.scriptPubKey.IsUnspendable()) {
+            result.SetInvalid(strprintf("PSBT is not valid. Input %u spends unspendable output", i));
+            return result;
+        }
+
         // Check if it is final
         if (!utxo.IsNull() && !PSBTInputSigned(input)) {
             input_analysis.is_final = false;

src/node/psbt.h

@@ -30,6 +30,17 @@ struct PSBTAnalysis {
     Optional<CAmount> fee;                 //!< Amount of fee being paid by the transaction
     std::vector<PSBTInputAnalysis> inputs; //!< More information about the individual inputs of the transaction
     PSBTRole next;                         //!< Which of the BIP 174 roles needs to handle the transaction next
+    std::string error;                     //!< Error message
+
+    void SetInvalid(std::string err_msg)
+    {
+        estimated_vsize = nullopt;
+        estimated_feerate = nullopt;
+        fee = nullopt;
+        inputs.clear();
+        next = PSBTRole::CREATOR;
+        error = err_msg;
+    }
 };
 
 /**