pkg-merge: Always submit a new MR, even with conflicts

Create a MR even when the local attempt to merge failed. In that case let the proposed updates branch point to the upstream branch and create a MR to submit it to the downstream branch.

This makes the failed updates more visbile and easier to track than the failed pipelines.

Task: T7797

package-source-builder/overlay/usr/bin/apertis-pkg-merge-upstream-to-downstreams

@@ -12,6 +12,7 @@ from sh.contrib import git
 from sh import apertis_pkg_merge_updates
 import re
 from pathlib import Path
+from functools import partial
 
 def parse_ref(ref: str) -> str:
     return git('rev-parse', '-q', '--verify', ref + '^{commit}', _ok_code=[0, 1]).strip('\n')
@@ -74,41 +75,50 @@ def main():
   local_suffix = args.local_suffix
 
   for upstream_branch in existing_upstream_branches(upstream):
-   # Mapping of downstream reference to proposed updates
-   known_refs = {}
    for downstream in downstreams:
     downstream_branch = get_matching_downstream_branch(downstream, upstream_branch)
 
-    # Skip branch not set
     if not downstream_branch:
-        continue
+      print(f"No branch for downstream {downstream}, skipping")
+      continue
 
     print(f"Looking at {downstream_branch} <- {upstream_branch}")
-    git.checkout(downstream_branch)
-    ref = git("rev-parse", "HEAD").rstrip()
-    known = known_refs.get(ref)
-
-    if known == None:
-      print("New target, doing merge")
-      proposed_branch = f"proposed-updates/{upstream_branch}/{ref[0:8]}"
-      git.checkout("-B", proposed_branch)
-      apertis_pkg_merge_updates(f"--downstream={downstream_branch}", f"--upstream={upstream_branch}", f"--local-version-suffix={local_suffix}", _fg=True)
-      o = git('diff', '--quiet', f"HEAD..{downstream_branch}", _ok_code=[0,1])
-      if o.exit_code == 1:
-        print("Merge done, pushing")
-        push_merge_request(args.project_url, proposed_branch, upstream_branch, downstream_branch)
-        known_refs[ref] = proposed_branch
-      else:
-        print("No merge required, skipping")
-        known_refs[ref] = ""
-    else:
-      if known == "":
-        print("Known target ref, Nothing required, skipping")
-        continue
 
-      # Check out known pre-pushed merge, and push that as an MR to the target
-      print("Known target, pushing merge request")
-      push_merge_request(args.project_url, known, upstream_branch, downstream_branch)
+    if git("merge-base", "--is-ancestor", upstream_branch, downstream_branch, _ok_code=[0,1]).exit_code == 0:
+      print(f"Upstream branch {upstream_branch} already merged into {downstream_branch}, no merge required")
+      continue
+
+    ref = git("rev-parse", downstream_branch).rstrip()
+    proposed_branch = f"proposed-updates/{upstream_branch}/{ref[0:8]}"
+
+    push_this_merge_request = partial(push_merge_request, args.project_url, proposed_branch, upstream_branch, downstream_branch)
+
+    if git("rev-parse", proposed_branch, _ok_code=[0, 128]).strip():
+      # handle the case where two downstreams are at the same commit and have
+      # the same upstream, so they should share the proposed branch
+      # for instance, if both v2021 and v2020 are at rev badf00d and are based
+      # on buster:
+      # 1. process v2021 and create proposed-updates/debian/buster/badf00d,
+      #    submit a MR from proposed-updates/debian/buster/badf00d
+      #    to apertis/v2021
+      # 1. process v2020 and re-use proposed-updates/debian/buster/badf00d to
+      #    submit a MR from proposed-updates/debian/buster/badf00d
+      #    to apertis/v2020
+      print(f"Reusing branch {proposed_branch} to be merged into {downstream_branch}")
+      push_this_merge_request()
+      continue
+
+    print(f"Attempt merging {upstream_branch} into {downstream_branch} via {proposed_branch}")
+    git("checkout", "--force", "-B", "tmp", downstream_branch)
+    apertis_pkg_merge_updates(f"--downstream={downstream_branch}", f"--upstream={upstream_branch}", f"--local-version-suffix={local_suffix}", _fg=True)
+    o = git('diff', '--quiet', f"HEAD..{downstream_branch}", _ok_code=[0,1])
+    if o.exit_code == 0:
+      print(f"No merge required for {downstream_branch}, skipping ")
+      continue
+
+    print("Merge done, pushing {proposed_branch} to update the existing MR")
+    git("branch", "--force", proposed_branch, "HEAD")
+    push_this_merge_request()
 
 if __name__ == '__main__':
   main()