From c4e937c0f936b6a7b08e507426620655c0531e1d Mon Sep 17 00:00:00 2001
From: Paul Schaub <vanitasvitae@fsfe.org>
Date: Wed, 26 Oct 2022 18:39:20 +0200
Subject: [PATCH] Improve syntax error reporting

---
 .../decryption_verification/syntax_check/PDA.java     | 11 +++++++++--
 .../exception/MalformedOpenPgpMessageException.java   |  6 +++++-
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/pgpainless-core/src/main/java/org/pgpainless/decryption_verification/syntax_check/PDA.java b/pgpainless-core/src/main/java/org/pgpainless/decryption_verification/syntax_check/PDA.java
index 40021734..550cb855 100644
--- a/pgpainless-core/src/main/java/org/pgpainless/decryption_verification/syntax_check/PDA.java
+++ b/pgpainless-core/src/main/java/org/pgpainless/decryption_verification/syntax_check/PDA.java
@@ -8,6 +8,9 @@ import org.pgpainless.exception.MalformedOpenPgpMessageException;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
 import java.util.Stack;
 
 import static org.pgpainless.decryption_verification.syntax_check.StackAlphabet.msg;
@@ -169,6 +172,7 @@ public class PDA {
     }
 
     private final Stack<StackAlphabet> stack = new Stack<>();
+    private final List<InputAlphabet> inputs = new ArrayList<>(); // keep track of inputs for debugging / error reporting
     private State state;
 
     public PDA() {
@@ -180,9 +184,12 @@ public class PDA {
     public void next(InputAlphabet input) throws MalformedOpenPgpMessageException {
         try {
             state = state.transition(input, this);
+            inputs.add(input);
         } catch (MalformedOpenPgpMessageException e) {
-            LOGGER.debug("Unexpected Packet or Token '" + input + "' encountered. Message is malformed.", e);
-            throw e;
+            MalformedOpenPgpMessageException wrapped = new MalformedOpenPgpMessageException("Malformed message: After reading stream " + Arrays.toString(inputs.toArray()) +
+                    ", token '" + input + "' is unexpected and illegal.", e);
+            LOGGER.debug("Invalid input '" + input + "'", wrapped);
+            throw wrapped;
         }
     }
 
diff --git a/pgpainless-core/src/main/java/org/pgpainless/exception/MalformedOpenPgpMessageException.java b/pgpainless-core/src/main/java/org/pgpainless/exception/MalformedOpenPgpMessageException.java
index cbe78c05..aa158746 100644
--- a/pgpainless-core/src/main/java/org/pgpainless/exception/MalformedOpenPgpMessageException.java
+++ b/pgpainless-core/src/main/java/org/pgpainless/exception/MalformedOpenPgpMessageException.java
@@ -21,6 +21,10 @@ public class MalformedOpenPgpMessageException extends RuntimeException {
     }
 
     public MalformedOpenPgpMessageException(PDA.State state, InputAlphabet input, StackAlphabet stackItem) {
-        this("Invalid input: There is no legal transition from state '" + state + "' for input '" + input + "' when '" + stackItem + "' is on top of the stack.");
+        this("There is no legal transition from state '" + state + "' for input '" + input + "' when '" + stackItem + "' is on top of the stack.");
+    }
+
+    public MalformedOpenPgpMessageException(String s, MalformedOpenPgpMessageException e) {
+        super(s, e);
     }
 }