Run calculate_matrix job on the master branch

This allows us to reuse its cache on PR CI jobs.

.github/workflows/ci.yml

@@ -11,6 +11,10 @@ name: CI
 on:
   push:
     branches:
+      # CI on master only serves for caching citool builds for the `calculate_matrix` job.
+      # In order to use GHA cache on PR CI (and auto/try) jobs, we need to write to it
+      # from the default branch.
+      - master
       - auto
       - try
       - try-perf

src/ci/citool/src/jobs.rs

@@ -161,6 +161,8 @@ pub enum RunType {
     TryJob { job_patterns: Option<Vec<String>> },
     /// Merge attempt workflow
     AutoJob,
+    /// Fake job only used for sharing Github Actions cache.
+    MasterJob,
 }
 
 /// Maximum number of custom try jobs that can be requested in a single
@@ -210,6 +212,7 @@ fn calculate_jobs(
             (jobs, "try", &db.envs.try_env)
         }
         RunType::AutoJob => (db.auto_jobs.clone(), "auto", &db.envs.auto_env),
+        RunType::MasterJob => return Ok(vec![]),
     };
     let jobs = substitute_github_vars(jobs.clone())
         .context("Failed to substitute GitHub context variables in jobs")?;
@@ -262,7 +265,7 @@ pub fn calculate_job_matrix(
     eprintln!("Run type: {run_type:?}");
 
     let jobs = calculate_jobs(&run_type, &db, channel)?;
-    if jobs.is_empty() {
+    if jobs.is_empty() && !matches!(run_type, RunType::MasterJob) {
         return Err(anyhow::anyhow!("Computed job list is empty"));
     }
 
@@ -270,6 +273,7 @@ pub fn calculate_job_matrix(
         RunType::PullRequest => "pr",
         RunType::TryJob { .. } => "try",
         RunType::AutoJob => "auto",
+        RunType::MasterJob => "master",
     };
 
     eprintln!("Output");

src/ci/citool/src/main.rs

@@ -47,6 +47,7 @@ impl GitHubContext {
                 Some(RunType::TryJob { job_patterns: patterns })
             }
             ("push", "refs/heads/auto") => Some(RunType::AutoJob),
+            ("push", "refs/heads/master") => Some(RunType::MasterJob),
             _ => None,
         }
     }

src/ci/citool/tests/jobs.rs

@@ -45,6 +45,15 @@ fn pr_jobs() {
     "#);
 }
 
+#[test]
+fn master_jobs() {
+    let stdout = get_matrix("push", "commit", "refs/heads/master");
+    insta::assert_snapshot!(stdout, @r#"
+    jobs=[]
+    run_type=master
+    "#);
+}
+
 fn get_matrix(event_name: &str, commit_msg: &str, branch_ref: &str) -> String {
     let path = std::env::var("PATH");
     let mut cmd = Command::new("cargo");