Parse try build CI job name from commit message

.github/workflows/ci.yml

@@ -52,6 +52,8 @@ jobs:
       - name: Checkout the source code
         uses: actions/checkout@v4
       - name: Calculate the CI job matrix
+        env:
+          COMMIT_MESSAGE: ${{ github.event.head_commit.message }}
         run: python3 src/ci/github-actions/calculate-job-matrix.py >> $GITHUB_OUTPUT
         id: jobs
   job:

src/ci/github-actions/calculate-job-matrix.py

@@ -11,6 +11,7 @@
 import json
 import logging
 import os
+import re
 import typing
 from pathlib import Path
 from typing import List, Dict, Any, Optional
@@ -67,6 +68,23 @@ class GitHubCtx:
     event_name: str
     ref: str
     repository: str
+    commit_message: Optional[str]
+
+
+def get_job_from_commit(ctx: GitHubCtx) -> Optional[str]:
+    """
+    Tries to parse a name of a CI job that should be executed in the form of
+    ci-job: <job-name>
+    from the commit message of the passed GitHub context.
+    """
+    if ctx.commit_message is None:
+        return None
+
+    regex = re.compile(r"ci-job: (.*)")
+    match = regex.search(ctx.commit_message)
+    if match is None:
+        return None
+    return match.group(1)
 
 
 def find_run_type(ctx: GitHubCtx) -> Optional[WorkflowRunType]:
@@ -84,7 +102,8 @@ def find_run_type(ctx: GitHubCtx) -> Optional[WorkflowRunType]:
         try_build = old_bors_try_build or new_bors_try_build
 
         if try_build:
-            return TryRunType()
+            job_name = get_job_from_commit(ctx)
+            return TryRunType(job=job_name)
 
         if ctx.ref == "refs/heads/auto" and ctx.repository == "rust-lang-ci/rust":
             return AutoRunType()
@@ -96,8 +115,16 @@ def calculate_jobs(run_type: WorkflowRunType, job_data: Dict[str, Any]) -> List[
     if isinstance(run_type, PRRunType):
         return add_base_env(name_jobs(job_data["pr"], "PR"), job_data["envs"]["pr"])
     elif isinstance(run_type, TryRunType):
-        return add_base_env(name_jobs(job_data["try"], "try"), job_data["envs"]["try"])
-    elif isinstance(run_type, AutoRunType):
+        jobs = job_data["try"]
+        if run_type.job is not None:
+            jobs = [job for job in job_data["auto"] if job["image"] == run_type.job]
+            if not jobs:
+                raise Exception(
+                    f"CI job `{run_type.job}` asked for in the try build does not exist"
+                )
+
+        return add_base_env(name_jobs(jobs, "try"), job_data["envs"]["try"])
+    elif run_type is AutoRunType:
         return add_base_env(name_jobs(job_data["auto"], "auto"), job_data["envs"]["auto"])
 
     return []
@@ -111,10 +138,16 @@ def skip_jobs(jobs: List[Dict[str, Any]], channel: str) -> List[Job]:
 
 
 def get_github_ctx() -> GitHubCtx:
+    event_name = os.environ["GITHUB_EVENT_NAME"]
+
+    commit_message = None
+    if event_name == "push":
+        commit_message = os.environ["COMMIT_MESSAGE"]
     return GitHubCtx(
-        event_name=os.environ["GITHUB_EVENT_NAME"],
+        event_name=event_name,
         ref=os.environ["GITHUB_REF"],
-        repository=os.environ["GITHUB_REPOSITORY"]
+        repository=os.environ["GITHUB_REPOSITORY"],
+        commit_message=commit_message
     )