Implement adding of Gerrit trailers

Author: krlvi

Cargo.lock

@@ -89,6 +89,7 @@ but-api-macros = { path = "crates/but-api-macros" }
 but-claude = { path = "crates/but-claude" }
 but-cursor = { path = "crates/but-cursor" }
 but-broadcaster = { path = "crates/but-broadcaster" }
+but-gerrit = { path = "crates/but-gerrit" }
 git2-hooks = { version = "0.5.0" }
 itertools = "0.14.0"
 dirs = "6.0.0"

Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "but-gerrit"
+version = "0.0.0"
+edition = "2024"
+authors = ["GitButler <[email protected]>"]
+publish = false
+
+[lib]
+doctest = false
+test = false
+
+[dependencies]
+sha-1 = "0.10.1"
+uuid.workspace = true
+anyhow.workspace = true
+bstr.workspace = true
+but-core.workspace = true
+gix = { workspace = true }

crates/but-gerrit/Cargo.toml

@@ -0,0 +1,105 @@
+use bstr::{BString, ByteSlice};
+use but_core::commit::HeadersV2;
+use sha1::{Digest, Sha1};
+use std::fmt::Display;
+use uuid::Uuid;
+
+#[derive(Clone, Debug)]
+pub struct GerritChangeId(String);
+
+impl From<Uuid> for GerritChangeId {
+    fn from(value: Uuid) -> Self {
+        let mut hasher = Sha1::new();
+        hasher.update(value);
+        Self(format!("I{:x}", hasher.finalize()))
+    }
+}
+impl Display for GerritChangeId {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+pub fn set_trailers(commit: &mut gix::objs::Commit) {
+    if let Some(headers) = HeadersV2::try_from_commit(commit) {
+        commit.message = with_change_id_trailer(commit.message.clone(), headers.change_id.into());
+    }
+}
+
+fn with_change_id_trailer(msg: BString, change_id: Uuid) -> BString {
+    let change_id = GerritChangeId::from(change_id);
+    let change_id_line = format!("\nChange-Id: {change_id}\n");
+    let msg_bytes = msg.as_slice();
+
+    if msg_bytes.find(b"\nChange-Id:").is_some() {
+        return msg;
+    }
+
+    let lines: Vec<&[u8]> = msg_bytes.lines().collect();
+    let mut insert_pos = lines.len();
+
+    for (i, line) in lines.iter().enumerate().rev() {
+        if line.starts_with(b"Signed-off-by:") {
+            insert_pos = i;
+        }
+    }
+
+    let mut result = BString::from(Vec::new());
+    for (i, line) in lines.iter().enumerate() {
+        if i == insert_pos {
+            result.extend_from_slice(change_id_line.as_bytes());
+        }
+        result.extend_from_slice(line);
+        result.push(b'\n');
+    }
+
+    if insert_pos == lines.len() {
+        result.extend_from_slice(change_id_line.as_bytes());
+    }
+
+    result
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn output_is_41_characters_long() {
+        let uuid = Uuid::new_v4();
+        let change_id = GerritChangeId::from(uuid);
+        let output = format!("{change_id}");
+        assert_eq!(output.len(), 41); // "I" + 40 hex chars
+        assert!(output.starts_with('I'));
+    }
+
+    #[test]
+    fn test_add_trailers() {
+        let uuid = Uuid::new_v4();
+        let change_id = GerritChangeId::from(uuid);
+        let change_id_line = format!("Change-Id: {change_id}\n");
+
+        // Case 1: No trailers
+        let msg = BString::from("Initial commit\n");
+        let updated_msg = with_change_id_trailer(msg.clone(), uuid);
+        assert!(
+            updated_msg
+                .as_slice()
+                .windows(change_id_line.len())
+                .any(|w| w == change_id_line.as_bytes())
+        );
+
+        // Case 2: Already has Change-Id
+        let msg_with_change_id = BString::from(format!("Initial commit\n{change_id_line}"));
+        let updated_msg = with_change_id_trailer(msg_with_change_id.clone(), uuid);
+        assert_eq!(updated_msg, msg_with_change_id);
+
+        // Case 3: Has Signed-off-by trailer
+        let msg_with_signed_off =
+            BString::from("Initial commit\nSigned-off-by: User <[email protected]>\n");
+        let updated_msg = with_change_id_trailer(msg_with_signed_off.clone(), uuid);
+        let updated_msg_str = updated_msg.as_bstr();
+        let change_id_index = updated_msg_str.find(&change_id_line).unwrap();
+        let signed_off_index = updated_msg_str.find("Signed-off-by:").unwrap();
+        assert!(change_id_index < signed_off_index);
+    }
+}

crates/but-gerrit/src/lib.rs

@@ -19,6 +19,7 @@ bstr.workspace = true
 tempfile.workspace = true
 serde = { version = "1.0.217", features = ["derive"] }
 toml.workspace = true
+but-gerrit.workspace = true
 
 [dev-dependencies]
 but-testsupport.workspace = true

crates/but-rebase/Cargo.toml

@@ -110,6 +110,7 @@ pub fn create(
     {
         commit.extra_headers.remove(pos);
     }
+    but_gerrit::set_trailers(&mut commit);
     if repo.git_settings()?.gitbutler_sign_commits.unwrap_or(false) {
         let mut buf = Vec::new();
         commit.write_to(&mut buf)?;