fix: return HTTP 200 with ERR pkt-line for git protocol errors

Previously, all git upload-pack/receive-pack failures returned HTTP 500,
but the git smart HTTP protocol requires protocol-level errors (like
"not our ref") to be returned as HTTP 200 OK with an ERR pkt-line in
the response body.

Changes:
- Add build_git_protocol_error_response() to create HTTP 200 responses
  with properly formatted ERR pkt-line ("ERR <message>\n")
- Add is_git_protocol_error() to detect protocol errors (exit code 128
  with stderr content) vs transport errors
- Update handle_upload_pack() and handle_receive_pack() to return
  protocol errors as HTTP 200 with ERR pkt-line
- Keep HTTP 500 for actual transport errors (spawn failures, I/O errors,
  signals)

This allows git clients to properly parse and display protocol error
messages instead of seeing generic HTTP 500 errors.

1 files changed, 66 insertions, 0 deletions

diff --git a/src/git/handlers.rs b/src/git/handlers.rs
index 017eee4..e3a6ad4 100644
--- a/src/git/handlers.rs
+++ b/src/git/handlers.rs
@@ -99,6 +99,42 @@ pub async fn handle_info_refs(
         .unwrap())
 }
 
+/// Build an HTTP 200 OK response with an ERR pkt-line for git protocol errors.
+///
+/// Per the git smart HTTP protocol spec, protocol-level errors (like "not our ref")
+/// should be returned as HTTP 200 OK with the error message in pkt-line format:
+/// `PKT-LINE("ERR" SP explanation-text)`
+///
+/// This allows git clients to properly parse and display the error message.
+fn build_git_protocol_error_response(
+    service: GitService,
+    error_message: &str,
+) -> Response<Full<Bytes>> {
+    // Format: "ERR <message>\n"
+    let err_content = format!("ERR {}\n", error_message.trim());
+    let err_pktline = PktLine::data(err_content.as_bytes()).encode();
+
+    Response::builder()
+        .status(StatusCode::OK)
+        .header("content-type", service.result_content_type())
+        .header("cache-control", "no-cache")
+        .body(Full::new(Bytes::from(err_pktline)))
+        .unwrap()
+}
+
+/// Check if a git process failure is a protocol error (vs transport error).
+///
+/// Protocol errors are communicated via stderr when git exits with code 128.
+/// These should be returned to the client as HTTP 200 with ERR pkt-line.
+///
+/// Transport errors (process spawn failures, I/O errors, signals) should
+/// remain as HTTP 500 errors.
+fn is_git_protocol_error(exit_code: Option<i32>, stderr: &[u8]) -> bool {
+    // Git uses exit code 128 for protocol/usage errors
+    // If there's stderr content, it's a protocol error message
+    exit_code == Some(128) && !stderr.is_empty()
+}
+
 /// Handle POST /git-upload-pack (clone/fetch)
 pub async fn handle_upload_pack(
     repo_path: PathBuf,
@@ -150,6 +186,21 @@ pub async fn handle_upload_pack(
 
     if !status.success() {
         let stderr_str = String::from_utf8_lossy(&stderr_output);
+
+        // Check if this is a git protocol error (exit code 128 with stderr)
+        // Protocol errors should be returned as HTTP 200 with ERR pkt-line
+        if is_git_protocol_error(status.code(), &stderr_output) {
+            warn!(
+                "Git upload-pack protocol error (returning ERR pkt-line): {}",
+                stderr_str
+            );
+            return Ok(build_git_protocol_error_response(
+                GitService::UploadPack,
+                &stderr_str,
+            ));
+        }
+
+        // Transport errors (spawn failures, signals, etc.) remain as HTTP 500
         error!("Git upload-pack failed: {}", stderr_str);
         return Err(GitError::GitFailed(status.code()));
     }
@@ -277,6 +328,21 @@ pub async fn handle_receive_pack(
 
     if !status.success() {
         let stderr_str = String::from_utf8_lossy(&stderr_output);
+
+        // Check if this is a git protocol error (exit code 128 with stderr)
+        // Protocol errors should be returned as HTTP 200 with ERR pkt-line
+        if is_git_protocol_error(status.code(), &stderr_output) {
+            warn!(
+                "Git receive-pack protocol error (returning ERR pkt-line): {}",
+                stderr_str
+            );
+            return Ok(build_git_protocol_error_response(
+                GitService::ReceivePack,
+                &stderr_str,
+            ));
+        }
+
+        // Transport errors (spawn failures, signals, etc.) remain as HTTP 500
         error!("Git receive-pack failed: {}", stderr_str);
         return Err(GitError::GitFailed(status.code()));
     }