From e96be5ee6e141b5d8a9163dc4066301d81957fa9 Mon Sep 17 00:00:00 2001
From: "Tobin C. Harding" <me@tobin.cc>
Date: Thu, 17 Aug 2023 09:34:46 +1000
Subject: [PATCH 1/2] Fix Witness debug display bug

Currently if the witness has zero elements or any of the individual
witnesses is empty we panic. Panic is caused by subtracting 1 from a
zero length.

Check the length is non-zero before subtracting 1, print `[]` if empty.
---
 bitcoin/src/blockdata/witness.rs | 32 +++++++++++++++++++-------------
 1 file changed, 19 insertions(+), 13 deletions(-)

diff --git a/bitcoin/src/blockdata/witness.rs b/bitcoin/src/blockdata/witness.rs
index 390ff0b0..3d10a0a0 100644
--- a/bitcoin/src/blockdata/witness.rs
+++ b/bitcoin/src/blockdata/witness.rs
@@ -68,20 +68,26 @@ fn fmt_debug(w: &Witness, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error>
     f.write_str("witnesses: [")?;
 
     let instructions = w.iter();
-    let last_instruction = instructions.len() - 1;
-
-    for (i, instruction) in instructions.enumerate() {
-        let bytes = instruction.iter();
-        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))?;
+    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))?;
         }
-
-        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(" }")

From 84614d9997a69bac249f3e8ac3b90240b38ca747 Mon Sep 17 00:00:00 2001
From: "Tobin C. Harding" <me@tobin.cc>
Date: Wed, 16 Aug 2023 16:03:33 +1000
Subject: [PATCH 2/2] Unit test debug print of witness with empty instruction

We recently fixed a bug that causes a panic if a `Witness` contains an
empty instruction. Add a unit test to verify it.
---
 bitcoin/src/blockdata/witness.rs | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/bitcoin/src/blockdata/witness.rs b/bitcoin/src/blockdata/witness.rs
index 3d10a0a0..43e69d69 100644
--- a/bitcoin/src/blockdata/witness.rs
+++ b/bitcoin/src/blockdata/witness.rs
@@ -535,6 +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();