Merge rust-bitcoin/rust-bitcoin#1999: Fix witness display bug

84614d9997 Unit test debug print of witness with empty instruction (Tobin C. Harding)
e96be5ee6e Fix Witness debug display bug (Tobin C. Harding)

Pull request description:

  When we introduce a custom `Debug` implementation for the `Witness` we introduced a bug that causes code to panic if the witness contains an empty instruction.

  The bug can be verified by putting patch 2 first or by running `cargo run --example sighash` on master.

ACKs for top commit:
  apoelstra:
    ACK 84614d9997
  RCasatta:
    ACK 84614d9997

Tree-SHA512: d51891206ab15f74dda07eb29ff3f6c69dc3f983a5a5abb55685688548481a19f7c1d33aa1183a89c553ff2bc86cf41057c2bae33d75e8a7f3b801056775bf9e

bitcoin/src/blockdata/witness.rs

@@ -68,21 +68,27 @@ fn fmt_debug(w: &Witness, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error>
     f.write_str("witnesses: [")?;
 
     let instructions = w.iter();
+    if instructions.len() > 0 {
         let last_instruction = instructions.len() - 1;
-
         for (i, instruction) in instructions.enumerate() {
             let bytes = instruction.iter();
+            if bytes.len() > 0 {
                 let last_byte = bytes.len() - 1;
-
                 f.write_str("[")?;
-
                 for (j, byte) in bytes.enumerate() {
                     write!(f, "{:#04x}", byte)?;
                     f.write_str(comma_or_close(j, last_byte))?;
                 }
-
+            } else {
+                // This is possible because the varint is not part of the instruction (see Iter).
+                write!(f, "[]")?;
+            }
             f.write_str(comma_or_close(i, last_instruction))?;
         }
+    } else {
+        // Witnesses can be empty because the 0x00 var int is not stored in content.
+        write!(f, "]")?;
+    }
 
     f.write_str(" }")
 }
@@ -529,6 +535,16 @@ mod test {
         v
     }
 
+    #[test]
+    fn witness_debug_can_display_empty_instruction() {
+        let witness = Witness {
+            witness_elements: 1,
+            content: append_u32_vec(vec![], &[0]),
+            indices_start: 2,
+        };
+        println!("{:?}", witness);
+    }
+
     #[test]
     fn test_push() {
         let mut witness = Witness::default();