Fix [behind-upstream] when dealing with subtrees

src/github.rs

@@ -320,6 +320,18 @@ pub struct FileDiff {
     pub diff: String,
 }
 
+/// The return from GitHub compare API
+#[derive(Debug, serde::Deserialize)]
+pub struct GithubCompare {
+    /// The base commit of the PR
+    pub base_commit: GithubCommit,
+    /// The merge base commit
+    ///
+    /// See <https://git-scm.com/docs/git-merge-base> for more details
+    pub merge_base_commit: GithubCommit,
+    // FIXME: Also retrieve and use the files list (see our diff function)
+}
+
 impl PullRequestDetails {
     pub fn new() -> PullRequestDetails {
         PullRequestDetails {
@@ -994,6 +1006,23 @@ impl Issue {
         Ok(Some(diff))
     }
 
+    /// Returns the comparison of this event.
+    ///
+    /// Returns `None` if the issue is not a PR.
+    pub async fn compare(&self, client: &GithubClient) -> anyhow::Result<Option<GithubCompare>> {
+        let (before, after) = if let (Some(base), Some(head)) = (&self.base, &self.head) {
+            (&base.sha, &head.sha)
+        } else {
+            return Ok(None);
+        };
+
+        let req = client.get(&format!(
+            "{}/compare/{before}...{after}",
+            self.repository().url(client)
+        ));
+        Ok(Some(client.json(req).await?))
+    }
+
     /// Returns the commits from this pull request (no commits are returned if this `Issue` is not
     /// a pull request).
     pub async fn commits(&self, client: &GithubClient) -> anyhow::Result<Vec<GithubCommit>> {

src/handlers/check_commits.rs

@@ -58,6 +58,12 @@ pub(super) async fn handle(ctx: &Context, event: &Event, config: &Config) -> any
             event.issue.number
         )
     };
+    let Some(compare) = event.issue.compare(&ctx.github).await? else {
+        bail!(
+            "expected issue {} to be a PR, but the compare could not be determined",
+            event.issue.number
+        )
+    };
     let commits = event.issue.commits(&ctx.github).await?;
 
     let mut warnings = Vec::new();
@@ -101,7 +107,7 @@ pub(super) async fn handle(ctx: &Context, event: &Event, config: &Config) -> any
             .unwrap_or(behind_upstream::DEFAULT_DAYS_THRESHOLD);
 
         if let Some(warning) =
-            behind_upstream::behind_upstream(age_threshold, event, &ctx.github, &commits).await
+            behind_upstream::behind_upstream(age_threshold, event, &compare).await
         {
             warnings.push(warning);
         }

src/handlers/check_commits/behind_upstream.rs

@@ -1,4 +1,4 @@
-use crate::github::{GithubClient, GithubCommit, IssuesEvent, Repository};
+use crate::github::{GithubCompare, IssuesEvent};
 use tracing as log;
 
 /// Default threshold for parent commit age in days to trigger a warning
@@ -8,95 +8,32 @@ pub(super) const DEFAULT_DAYS_THRESHOLD: usize = 7;
 pub(super) async fn behind_upstream(
     age_threshold: usize,
     event: &IssuesEvent,
-    client: &GithubClient,
-    commits: &Vec<GithubCommit>,
+    compare: &GithubCompare,
 ) -> Option<String> {
     log::debug!("Checking if PR #{} is behind upstream", event.issue.number);
 
-    let Some(head_commit) = commits.first() else {
-        return None;
-    };
+    // Compute the number of days old the merge base commit is
+    let commit_date = compare.merge_base_commit.commit.author.date;
+    let now = chrono::Utc::now().with_timezone(&commit_date.timezone());
+    let days_old = (now - commit_date).num_days() as usize;
 
     // First try the parent commit age check as it's more accurate
-    match is_parent_commit_too_old(head_commit, &event.repository, client, age_threshold).await {
-        Ok(Some(days_old)) => {
-            log::info!(
-                "PR #{} has a parent commit that is {} days old",
-                event.issue.number,
-                days_old
-            );
-
-            return Some(format!(
-                r"This PR is based on an upstream commit that is {} days old.
-
-*It's recommended to update your branch according to the [rustc_dev_guide](https://rustc-dev-guide.rust-lang.org/contributing.html#keeping-your-branch-up-to-date).*",
-                days_old
-            ));
-        }
-        Ok(None) => {
-            // Parent commit is not too old, log and do nothing
-            log::debug!("PR #{} parent commit is not too old", event.issue.number);
-        }
-        Err(e) => {
-            // Error checking parent commit age, log and do nothing
-            log::error!(
-                "Error checking parent commit age for PR #{}: {}",
-                event.issue.number,
-                e
-            );
-        }
-    }
-
-    None
-}
-
-/// Checks if the PR's parent commit is too old.
-///
-/// This determines if a PR needs updating by examining the first parent of the PR's head commit,
-/// which typically represents the base branch commit that the PR is based on.
-///
-/// If this parent commit is older than the specified threshold, it suggests the PR
-/// should be updated/rebased to a more recent version of the base branch.
-///
-/// Returns:
-/// - Ok(Some(days_old)) - If parent commit is older than the threshold
-/// - Ok(None)
-///     - If there is no parent commit
-///     - If parent is within threshold
-/// - Err(...) - If an error occurred during processing
-pub(super) async fn is_parent_commit_too_old(
-    commit: &GithubCommit,
-    repo: &Repository,
-    client: &GithubClient,
-    max_days_old: usize,
-) -> anyhow::Result<Option<usize>> {
-    // Get the first parent (it should be from the base branch)
-    let Some(parent_sha) = commit.parents.get(0).map(|c| c.sha.clone()) else {
-        return Ok(None);
-    };
-
-    let days_old = commit_days_old(&parent_sha, repo, client).await?;
-
-    if days_old > max_days_old {
-        Ok(Some(days_old))
+    if days_old > age_threshold {
+        log::info!(
+            "PR #{} has a parent commit that is {} days old",
+            event.issue.number,
+            days_old
+        );
+
+        Some(format!(
+            r"This PR is based on an upstream commit that is {} days old.
+
+*It's recommended to update your branch according to the [rustc-dev-guide](https://rustc-dev-guide.rust-lang.org/contributing.html#keeping-your-branch-up-to-date).*",
+            days_old
+        ))
     } else {
-        Ok(None)
+        // Parent commit is not too old, log and do nothing
+        log::debug!("PR #{} parent commit is not too old", event.issue.number);
+        None
     }
 }
-
-/// Returns the number of days old the commit is
-pub(super) async fn commit_days_old(
-    sha: &str,
-    repo: &Repository,
-    client: &GithubClient,
-) -> anyhow::Result<usize> {
-    // Get the commit details to check its date
-    let commit: GithubCommit = repo.github_commit(client, &sha).await?;
-
-    // compute the number of days old the commit is
-    let commit_date = commit.commit.author.date;
-    let now = chrono::Utc::now().with_timezone(&commit_date.timezone());
-    let days_old = (now - commit_date).num_days() as usize;
-
-    Ok(days_old)
-}