handlers: add no merge policy notifications

Add a handler for issue events that checks whether a merge commit has
been added to the pull request and informs the user of the project's no
merge policy.

Signed-off-by: David Wood <david.wood@huawei.com>

src/config.rs

@@ -33,6 +33,7 @@ pub(crate) struct Config {
     pub(crate) shortcut: Option<ShortcutConfig>,
     pub(crate) note: Option<NoteConfig>,
     pub(crate) mentions: Option<MentionsConfig>,
+    pub(crate) no_merges: Option<NoMergesConfig>,
 }
 
 #[derive(PartialEq, Eq, Debug, serde::Deserialize)]
@@ -79,6 +80,12 @@ pub(crate) struct AssignConfig {
     _empty: (),
 }
 
+#[derive(PartialEq, Eq, Debug, serde::Deserialize)]
+pub(crate) struct NoMergesConfig {
+    #[serde(default)]
+    _empty: (),
+}
+
 #[derive(PartialEq, Eq, Debug, serde::Deserialize)]
 pub(crate) struct NoteConfig {
     #[serde(default)]
@@ -365,6 +372,7 @@ mod tests {
                 github_releases: None,
                 review_submitted: None,
                 mentions: None,
+                no_merges: None,
             }
         );
     }

src/github.rs

@@ -749,6 +749,33 @@ impl Issue {
         Ok(Some(String::from(String::from_utf8_lossy(&diff))))
     }
 
+    /// 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>> {
+        if !self.is_pr() {
+            return Ok(vec![]);
+        }
+
+        let mut commits = Vec::new();
+        let mut page = 1;
+        loop {
+            let req = client.get(&format!(
+                "{}/pulls/{}/commits?page={page}&per_page=100",
+                self.repository().url(),
+                self.number
+            ));
+
+            let new: Vec<_> = client.json(req).await?;
+            if new.is_empty() {
+                break;
+            }
+            commits.extend(new);
+
+            page += 1;
+        }
+        Ok(commits)
+    }
+
     pub async fn files(&self, client: &GithubClient) -> anyhow::Result<Vec<PullRequestFile>> {
         if !self.is_pr() {
             return Ok(vec![]);

src/handlers.rs

@@ -31,6 +31,7 @@ mod glacier;
 mod major_change;
 mod mentions;
 mod milestone_prs;
+mod no_merges;
 mod nominate;
 mod note;
 mod notification;
@@ -155,6 +156,7 @@ issue_handlers! {
     autolabel,
     major_change,
     mentions,
+    no_merges,
     notify_zulip,
 }
 

src/handlers/no_merges.rs

@@ -0,0 +1,129 @@
+//! Purpose: When opening a PR, or pushing new changes, check for merge commits
+//! and notify the user of our no-merge policy.
+
+use crate::{
+    config::NoMergesConfig,
+    db::issue_data::IssueData,
+    github::{IssuesAction, IssuesEvent},
+    handlers::Context,
+};
+use anyhow::Context as _;
+use serde::{Deserialize, Serialize};
+use std::collections::HashSet;
+use std::fmt::Write;
+use tracing as log;
+
+const NO_MERGES_KEY: &str = "no_merges";
+
+pub(super) struct NoMergesInput {
+    /// Hashes of merge commits in the pull request.
+    merge_commits: HashSet<String>,
+}
+
+#[derive(Debug, Default, Deserialize, Serialize)]
+struct NoMergesState {
+    /// Hashes of merge commits that have already been mentioned by triagebot in a comment.
+    mentioned_merge_commits: HashSet<String>,
+}
+
+pub(super) async fn parse_input(
+    ctx: &Context,
+    event: &IssuesEvent,
+    config: Option<&NoMergesConfig>,
+) -> Result<Option<NoMergesInput>, String> {
+    if !matches!(
+        event.action,
+        IssuesAction::Opened | IssuesAction::Synchronize | IssuesAction::ReadyForReview
+    ) {
+        return Ok(None);
+    }
+
+    // Require an empty configuration block to enable no-merges notifications.
+    if config.is_none() {
+        return Ok(None);
+    }
+
+    // Don't ping on rollups or draft PRs.
+    if event.issue.title.starts_with("Rollup of") || event.issue.draft {
+        return Ok(None);
+    }
+
+    let mut merge_commits = HashSet::new();
+    let commits = event
+        .issue
+        .commits(&ctx.github)
+        .await
+        .map_err(|e| {
+            log::error!("failed to fetch commits: {:?}", e);
+        })
+        .unwrap_or_default();
+    for commit in commits {
+        if commit.parents.len() > 1 {
+            merge_commits.insert(commit.sha.clone());
+        }
+    }
+
+    let input = NoMergesInput { merge_commits };
+    Ok(if input.merge_commits.is_empty() {
+        None
+    } else {
+        Some(input)
+    })
+}
+
+pub(super) async fn handle_input(
+    ctx: &Context,
+    _config: &NoMergesConfig,
+    event: &IssuesEvent,
+    input: NoMergesInput,
+) -> anyhow::Result<()> {
+    let mut client = ctx.db.get().await;
+    let mut state: IssueData<'_, NoMergesState> =
+        IssueData::load(&mut client, &event.issue, NO_MERGES_KEY).await?;
+
+    let since_last_posted = if state.data.mentioned_merge_commits.is_empty() {
+        ""
+    } else {
+        " (since this message was last posted)"
+    };
+
+    let mut should_send = false;
+    let mut message = format!(
+        "
+        There are merge commits (commits with multiple parents) in your changes. We have a
+        [no merge policy](https://rustc-dev-guide.rust-lang.org/git.html#no-merge-policy) so
+        these commits will need to be removed for this pull request to be merged.
+
+        You can start a rebase with the following commands:
+
+        ```shell-session
+        $ # rebase
+        $ git rebase -i master
+        $ # delete any merge commits in the editor that appears
+        $ git push --force-with-lease
+        ```
+
+        The following commits are merge commits{since_last_posted}:
+
+    "
+    );
+    for commit in &input.merge_commits {
+        if state.data.mentioned_merge_commits.contains(commit) {
+            continue;
+        }
+
+        should_send = true;
+        state.data.mentioned_merge_commits.insert((*commit).clone());
+        write!(message, "- {commit}").unwrap();
+    }
+
+    if should_send {
+        event
+            .issue
+            .post_comment(&ctx.github, &message)
+            .await
+            .context("failed to post no_merges comment")?;
+        state.save().await?;
+    }
+    Ok(())
+}