backout: abort backout of subtree commits

Summary:
Subtree commits (copy/merge) carry metadata that would be lost if backed out. This adds a check in `_dobackout` that aborts early when the target commit has subtree metadata.

The hint message is configurable via the `subtree.backout-hint` config option, allowing teams to customize the guidance shown to users.

Differential Revision: D103686530

fbshipit-source-id: b6c76bd6da4b265a97e880c16c31adca1b2ed80a

Author: zzl0

eden/scm/sapling/commands/__init__.py

@@ -854,6 +854,8 @@ def _dobackout(ui, repo, node=None, rev=None, **opts):
             raise error.Abort(_("cannot use --parent on non-merge changeset"))
         parent = p1
 
+    subtreeutil.check_commit_backoutable(repo, node)
+
     rctx = scmutil.revsingle(repo, hex(parent))
     if not opts.get("merge") and op1 != node:
         dsguard = dirstateguard.dirstateguard(repo, "backout")

eden/scm/sapling/utils/subtreeutil.py

@@ -794,6 +794,26 @@ def check_commit_splitability(repo, node):
             raise error.Abort(_("cannot split subtree copy/merge commits"))
 
 
+def check_commit_backoutable(repo, node):
+    """Check if the given commit can be backed out.
+
+    Subtree commits (copy/merge) cannot be backed out because the backout
+    would lose subtree metadata.
+    """
+    extra = repo[node].extra()
+    if get_subtree_metadata(extra):
+        hint = _(
+            "use '@prog@ subtree copy -r <good-commit>' to overwrite the path,"
+            " then graft changes on top if needed"
+        )
+        if extra_hint := repo.ui.config("subtree", "backout-hint"):
+            hint = f"{hint}. {extra_hint}"
+        raise error.Abort(
+            _("cannot backout subtree copy/merge commits"),
+            hint=hint,
+        )
+
+
 def get_or_clone_git_repo(ui, url, from_rev=None):
     def try_reuse_git_repo(git_repo_dir):
         """try to reuse an existing git repo, otherwise return None"""

eden/scm/tests/test-subtree-backout.t

@@ -0,0 +1,83 @@
+  $ setconfig diff.git=True
+  $ setconfig subtree.allow-any-source-commit=True
+  $ setconfig subtree.min-path-depth=1
+
+test backout of subtree commit is aborted
+  $ newclientrepo
+  $ drawdag <<'EOS'
+  > B  # B/foo/x = 1a\n2\n3\n
+  > |
+  > A  # A/foo/x = 1\n2\n3\n
+  > EOS
+  $ sl go -q $B
+  $ sl subtree copy -r $A --from-path foo --to-path foo2 -m "subtree copy foo to foo2"
+  copying foo to foo2
+  $ sl backout -r .
+  abort: cannot backout subtree copy/merge commits
+  (use 'sl subtree copy -r <good-commit>' to overwrite the path, then graft changes on top if needed)
+  [255]
+
+test backout of subtree commit shows configurable hint
+  $ sl backout -r . --config subtree.backout-hint="see https://example.com for details"
+  abort: cannot backout subtree copy/merge commits
+  (use 'sl subtree copy -r <good-commit>' to overwrite the path, then graft changes on top if needed. see https://example.com for details)
+  [255]
+
+test backout subtree merge: backout fails, subtree copy overwrites, future merge works
+  $ newclientrepo
+  $ drawdag <<'EOS'
+  > B  # B/foo/x = 1a\n2\n3\n
+  > |
+  > A  # A/foo/x = 1\n2\n3\n
+  > EOS
+  $ sl go -q $B
+  $ sl subtree copy -r $A --from-path foo --to-path bar -m "subtree copy foo to bar"
+  copying foo to bar
+
+step 1: create a subtree merge commit
+  $ echo "change1" >> foo/x && sl ci -m "update foo with change1"
+  $ sl subtree merge --from-path foo --to-path bar | grep "merge base:"
+  merge base: b4cb27eee4e2
+  $ sl ci -m "subtree merge foo to bar"
+  $ cat bar/x
+  1a
+  2
+  3
+  change1
+
+step 2: backout the subtree merge commit does not work
+  $ sl backout -r .
+  abort: cannot backout subtree copy/merge commits
+  (use 'sl subtree copy -r <good-commit>' to overwrite the path, then graft changes on top if needed)
+  [255]
+
+step 3: subtree copy overwrites the dest path to a good state
+  $ sl log -G -T '{node|short} {desc|firstline}'
+  @  26d65d502329 subtree merge foo to bar
+  │
+  o  49d9ca68c22d update foo with change1
+  │
+  o  ee6785824a72 subtree copy foo to bar
+  │
+  o  c4fbbcdf676b B
+  │
+  o  b4cb27eee4e2 A
+  $ sl subtree copy -r .^ --from-path bar --to-path bar --force -m "overwrite bar to pre-merge state"
+  removing bar/x
+  copying bar to bar
+  $ cat bar/x
+  1
+  2
+  3
+
+step 4: future subtree merge finds correct merge base and merges the changes back
+  $ echo "change2" >> foo/x && sl ci -m "update foo with change2"
+  $ sl subtree merge --from-path foo --to-path bar | grep "merge base:"
+  merge base: b4cb27eee4e2
+  $ sl ci -m "subtree merge foo to bar again"
+  $ cat bar/x
+  1a
+  2
+  3
+  change1
+  change2