Push branches atomically when squashing the index

Author: jtgeibel

src/git.rs

@@ -384,7 +384,7 @@ pub fn squash_index(env: &Environment) -> Result<(), PerformError> {
     println!("Squashing the index into a single commit.");
 
     let now = Utc::now().format("%Y-%m-%d");
-    let original_head = repo.head_oid()?;
+    let original_head = repo.head_oid()?.to_string();
     let msg = format!("Collapse index into one commit\n\n\
 
         Previous HEAD was {}, now on the `snapshot-{}` branch\n\n\
@@ -394,9 +394,6 @@ pub fn squash_index(env: &Environment) -> Result<(), PerformError> {
         [online]: https://internals.rust-lang.org/t/cargos-crate-index-upcoming-squash-into-one-commit/8440\n\
         [this issue]: https://github.com/rust-lang/crates-io-cargo-teams/issues/47", original_head, now);
 
-    // Create a snapshot branch of current `HEAD`.
-    repo.push(&format!("HEAD:refs/heads/snapshot-{}", now))?;
-
     repo.squash_to_single_commit(&msg)?;
 
     // Shell out to git because libgit2 does not currently support push leases
@@ -431,9 +428,15 @@ pub fn squash_index(env: &Environment) -> Result<(), PerformError> {
         )
         .args(&[
             "push",
+            // Both updates should succeed or fail together
+            "--atomic",
             "origin",
-            "HEAD:refs/heads/master",
+            // Overwrite master, but only if it server matches the expected value
             &format!("--force-with-lease=refs/heads/master:{}", original_head),
+            // The new squashed commit is pushed to master
+            "HEAD:refs/heads/master",
+            // The previous value of HEAD is pushed to a snapshot branch
+            &format!("{}:refs/heads/snapshot-{}", original_head, now),
         ])
         .output()?;