Rollup merge of #89655 - tlyu:find-non-merge-commits, r=jyn514

bootstrap: don't use `--merges` to look for commit hashes for downloading artifacts

Shallow clones (and possibly worktrees, though I can't seem to reproduce the problem there) can cause `git rev-list --merges` to falsely return no results, even if a merge commit is present. Stop using the `--merges` option when looking for commit hashes that have build artifacts. `--first-parent` and `--author=bors@rust-lang.org` should be sufficient.

Also exit with an error if the configuration asks for artifacts to be downloaded and we can't determine an appropriate commit hash to use to download artifacts.

Fixes #87890.

r? ``@jyn514``
``@rustbot`` label +A-rustbuild +A-contributor-roadblock

1 files changed, 15 insertions, 3 deletions

diff --git a/src/bootstrap/bootstrap.py b/src/bootstrap/bootstrap.py
index 05d7b0f611f..817850b1ec7 100644
--- a/src/bootstrap/bootstrap.py
+++ b/src/bootstrap/bootstrap.py
@@ -460,7 +460,7 @@ class RustBuild(object):
             # LLVM more often than necessary.
             #
             # This git command finds that commit SHA, looking for bors-authored
-            # merges that modified src/llvm-project or other relevant version
+            # commits that modified src/llvm-project or other relevant version
             # stamp files.
             #
             # This works even in a repository that has not yet initialized
@@ -470,7 +470,7 @@ class RustBuild(object):
             ]).decode(sys.getdefaultencoding()).strip()
             llvm_sha = subprocess.check_output([
                 "git", "rev-list", "--author=bors@rust-lang.org", "-n1",
-                "--merges", "--first-parent", "HEAD",
+                "--first-parent", "HEAD",
                 "--",
                 "{}/src/llvm-project".format(top_level),
                 "{}/src/bootstrap/download-ci-llvm-stamp".format(top_level),
@@ -540,6 +540,12 @@ class RustBuild(object):
         unpack(tarball, tarball_suffix, self.bin_root(stage0), match=pattern, verbose=self.verbose)
 
     def _download_ci_llvm(self, llvm_sha, llvm_assertions):
+        if not llvm_sha:
+            print("error: could not find commit hash for downloading LLVM")
+            print("help: maybe your repository history is too shallow?")
+            print("help: consider disabling `download-ci-llvm`")
+            print("help: or fetch enough history to include one upstream commit")
+            exit(1)
         cache_prefix = "llvm-{}-{}".format(llvm_sha, llvm_assertions)
         cache_dst = os.path.join(self.build_dir, "cache")
         rustc_cache = os.path.join(cache_dst, cache_prefix)
@@ -685,9 +691,15 @@ class RustBuild(object):
         # Only commits merged by bors will have CI artifacts.
         merge_base = [
             "git", "rev-list", "--author=bors@rust-lang.org", "-n1",
-            "--merges", "--first-parent", "HEAD"
+            "--first-parent", "HEAD"
         ]
         commit = subprocess.check_output(merge_base, universal_newlines=True).strip()
+        if not commit:
+            print("error: could not find commit hash for downloading rustc")
+            print("help: maybe your repository history is too shallow?")
+            print("help: consider disabling `download-rustc`")
+            print("help: or fetch enough history to include one upstream commit")
+            exit(1)
 
         # Warn if there were changes to the compiler or standard library since the ancestor commit.
         status = subprocess.call(["git", "diff-index", "--quiet", commit, "--", compiler, library])