git: Make fetch return the new head for the fetched branch

1 files changed, 35 insertions, 4 deletions

diff --git a/server/common/src/git.rs b/server/common/src/git.rs
index 053ff3f..8fe7863 100644
--- a/server/common/src/git.rs
+++ b/server/common/src/git.rs
@@ -122,6 +122,22 @@ fn parse_tree_entries(output: String) -> Vec<TreeEntry> {
     ret
 }
 
+fn branch_eq(a: impl AsRef<str>, b: impl AsRef<str>) -> bool {
+    let a = a.as_ref();
+    let b = b.as_ref();
+    if a.starts_with("refs/heads/") {
+        if b.starts_with("refs/heads/") {
+            return a[11..] == b[11..];
+        }
+        return a == format!("refs/heads/{b}");
+    } else {
+        if b.starts_with("refs/heads/") {
+            return format!("refs/heads/{a}") == b;
+        }
+        return a == b;
+    }
+}
+
 impl RepoData {
     fn new() -> Self {
         Self {
@@ -130,7 +146,7 @@ impl RepoData {
         }
     }
 
-    async fn fetch(&self, repo: &Repository, branch: String) -> Result<(), Error> {
+    async fn fetch(&self, repo: &Repository, branch: String) -> Result<String, Error> {
         if repo.remote.is_none() {
             return Err(Error::new("No remote set"));
         }
@@ -149,9 +165,24 @@ impl RepoData {
         // <+ force update><remote branch>:<local branch>
         cmd.arg(format!("+{branch}:{branch}"));
 
-        self.output(&mut cmd).await?;
+        let output = self.output(&mut cmd).await?;
 
-        Ok(())
+        // git fetch porcelain format:
+        // <flag> <old-object-id> <new-object-id> <local-reference>
+        for line in output.lines() {
+            let mut parts = line.split(' ');
+            parts.next(); // flag
+            parts.next(); // old-object-id
+            if let Some(new_object_id) = parts.next() {
+                let local_reference = parts.collect::<Vec<&str>>().join(" ");
+                if branch_eq(local_reference, &branch) {
+                    return Ok(new_object_id.to_string());
+                }
+            }
+        }
+
+        assert!(false);
+        Err(Error::new("Fetch succeeded but branch not found"))
     }
 
     async fn init(&mut self, repo: &Repository) -> Result<(), Error> {
@@ -590,7 +621,7 @@ impl Repository {
         Ok(())
     }
 
-    pub async fn fetch(&self, branch: impl Into<String>) -> Result<(), Error> {
+    pub async fn fetch(&self, branch: impl Into<String>) -> Result<String, Error> {
         let branch = branch.into();
         let data = self.lock.read().await;