Avoid annotating unsubmitted suffix commits

src/ghstack/submit.py

@@ -581,8 +581,9 @@ async def run(self) -> List[DiffMeta]:
         # we always have to generate orig commits for submitted diffs.)
         # However, commits_to_submit does not necessarily contain
         # diffs_to_rebase.  If you ask to submit only a prefix of your current
-        # stack, the suffix is not to be submitted, but it needs to be rebased
-        # (to, e.g., update the ghstack-source-id)
+        # stack, the suffix is not to be submitted, but it needs to be rebased.
+        # Existing ghstack PRs in the suffix still need their source IDs kept
+        # up-to-date, but local-only suffix commits should stay local-only.
 
         commit_count = len(commits_to_submit)
 
@@ -883,7 +884,8 @@ async def prepare_updates(
             pr_info_cache = await self._prefetch_pr_info(commits_to_rebase)
 
         # Phase 1: Process all commits (oldest first) to determine what
-        # needs updating, create head/base commits, and identify new PRs.
+        # needs updating, create head/base commits, and identify submitted
+        # new PRs.
         # New PRs are NOT pushed/created yet — deferred to batch operation.
         submit_set = set(h.commit_id for h in commits_to_submit)
         diff_meta_index: Dict[GitCommitHash, DiffMeta] = {}
@@ -1214,6 +1216,12 @@ async def process_commit(
                 )
             return None
 
+        # This commit has no existing PR and is not part of the requested
+        # submit set.  It may still be rebased later if an earlier commit was
+        # rewritten, but it should not get ghstack trailers or a new PR.
+        if elab_diff is None and not submit:
+            return None
+
         # Edge case: check if the commit is empty; if so skip submitting
         if base.tree == diff.tree:
             self._warn_empty(diff, elab_diff)

test/submit/prefix_only_new_suffix_no_metadata.py.test

@@ -0,0 +1,26 @@
+from ghstack.test_prelude import *
+
+await init_test()
+
+await commit("A")
+await commit("B")
+await commit("C")
+
+# Submit only A. B and C are local-only suffix commits and should not be
+# annotated as ghstack PRs.
+(A,) = await gh_submit("Submit bottom only", revs=["HEAD~2"], stack=False)
+
+bottom_msg = await git("show", "-s", "--pretty=%B", "HEAD~2")
+middle_msg = await git("show", "-s", "--pretty=%B", "HEAD~")
+top_msg = await git("show", "-s", "--pretty=%B", "HEAD")
+
+assert "ghstack-source-id:" in bottom_msg
+assert "Pull-Request:" in bottom_msg
+
+assert "ghstack-source-id:" not in middle_msg, middle_msg
+assert "Pull-Request:" not in middle_msg, middle_msg
+
+assert "ghstack-source-id:" not in top_msg, top_msg
+assert "Pull-Request:" not in top_msg, top_msg
+
+ok()