bootstrap: use git merge-base for LLVM CI download logic

src/bootstrap/llvm.rs

@@ -24,6 +24,7 @@ use crate::util::{self, exe, output, t, up_to_date};
 use crate::{CLang, GitRepo, Kind};
 
 use build_helper::ci::CiEnv;
+use build_helper::git::get_git_merge_base;
 
 #[derive(Clone)]
 pub struct LlvmResult {
@@ -128,13 +129,19 @@ pub fn prebuilt_llvm_config(
 /// This retrieves the LLVM sha we *want* to use, according to git history.
 pub(crate) fn detect_llvm_sha(config: &Config, is_git: bool) -> String {
     let llvm_sha = if is_git {
+        // We proceed in 2 steps. First we get the closest commit that is actually upstream. Then we
+        // walk back further to the last bors merge commit that actually changed LLVM. The first
+        // step will fail on CI because only the `auto` branch exists; we just fall back to `HEAD`
+        // in that case.
+        let closest_upstream =
+            get_git_merge_base(Some(&config.src)).unwrap_or_else(|_| "HEAD".into());
         let mut rev_list = config.git();
         rev_list.args(&[
             PathBuf::from("rev-list"),
             format!("--author={}", config.stage0_metadata.config.git_merge_commit_email).into(),
             "-n1".into(),
             "--first-parent".into(),
-            "HEAD".into(),
+            closest_upstream.into(),
             "--".into(),
             config.src.join("src/llvm-project"),
             config.src.join("src/bootstrap/download-ci-llvm-stamp"),

src/tools/build_helper/src/git.rs

@@ -78,13 +78,22 @@ pub fn rev_exists(rev: &str, git_dir: Option<&Path>) -> Result<bool, String> {
 /// We will then fall back to origin/master in the hope that at least this exists.
 pub fn updated_master_branch(git_dir: Option<&Path>) -> Result<String, String> {
     let upstream_remote = get_rust_lang_rust_remote(git_dir)?;
-    let upstream_master = format!("{upstream_remote}/master");
+    for upstream_master in [format!("{upstream_remote}/master"), format!("origin/master")] {
         if rev_exists(&upstream_master, git_dir)? {
             return Ok(upstream_master);
         }
+    }
 
-    // We could implement smarter logic here in the future.
+    Err(format!("Cannot find any suitable upstream master branch"))
-    Ok("origin/master".into())
+}
+
+pub fn get_git_merge_base(git_dir: Option<&Path>) -> Result<String, String> {
+    let updated_master = updated_master_branch(git_dir)?;
+    let mut git = Command::new("git");
+    if let Some(git_dir) = git_dir {
+        git.current_dir(git_dir);
+    }
+    Ok(output_result(git.arg("merge-base").arg(&updated_master).arg("HEAD"))?.trim().to_owned())
 }
 
 /// Returns the files that have been modified in the current branch compared to the master branch.
@@ -94,20 +103,13 @@ pub fn get_git_modified_files(
     git_dir: Option<&Path>,
     extensions: &Vec<&str>,
 ) -> Result<Option<Vec<String>>, String> {
-    let Ok(updated_master) = updated_master_branch(git_dir) else {
+    let merge_base = get_git_merge_base(git_dir)?;
-        return Ok(None);
-    };
 
-    let git = || {
     let mut git = Command::new("git");
     if let Some(git_dir) = git_dir {
         git.current_dir(git_dir);
     }
-        git
+    let files = output_result(git.args(["diff-index", "--name-only", merge_base.trim()]))?
-    };
-
-    let merge_base = output_result(git().arg("merge-base").arg(&updated_master).arg("HEAD"))?;
-    let files = output_result(git().arg("diff-index").arg("--name-only").arg(merge_base.trim()))?
         .lines()
         .map(|s| s.trim().to_owned())
         .filter(|f| {