pygit2: handle alternate merge strategies

Author: pmrowla

scmrepo/git/backend/pygit2.py

@@ -596,37 +596,86 @@ def merge(
         msg: Optional[str] = None,
         squash: bool = False,
     ) -> Optional[str]:
-        from pygit2 import GIT_RESET_MIXED, GitError
+        from pygit2 import (
+            GIT_MERGE_ANALYSIS_FASTFORWARD,
+            GIT_MERGE_ANALYSIS_NONE,
+            GIT_MERGE_ANALYSIS_UNBORN,
+            GIT_MERGE_ANALYSIS_UP_TO_DATE,
+            GIT_MERGE_PREFERENCE_FASTFORWARD_ONLY,
+            GIT_MERGE_PREFERENCE_NO_FASTFORWARD,
+            GitError,
+        )
 
         if commit and squash:
             raise SCMError("Cannot merge with 'squash' and 'commit'")
 
-        if commit and not msg:
-            raise SCMError("Merge commit message is required")
-
         with self.release_odb_handles():
+            self.repo.index.read(False)
+            obj, _ref = self.repo.resolve_refish(rev)
+            analysis, ff_pref = self.repo.merge_analysis(obj.id)
+
+            if analysis == GIT_MERGE_ANALYSIS_NONE:
+                raise SCMError(f"'{rev}' cannot be merged into HEAD")
+            if analysis & GIT_MERGE_ANALYSIS_UP_TO_DATE:
+                return None
+
             try:
-                self.repo.index.read(False)
-                self.repo.merge(rev)
+                self.repo.merge(obj.id)
                 self.repo.index.write()
             except GitError as exc:
                 raise SCMError("Merge failed") from exc
 
             if self.repo.index.conflicts:
                 raise MergeConflictError("Merge contained conflicts")
 
-            if commit:
-                user = self.default_signature
-                tree = self.repo.index.write_tree()
-                merge_commit = self.repo.create_commit(
-                    "HEAD", user, user, msg, tree, [self.repo.head.target, rev]
-                )
-                return str(merge_commit)
-            if squash:
-                self.repo.reset(self.repo.head.target, GIT_RESET_MIXED)
+            try:
+                if not (
+                    squash or ff_pref & GIT_MERGE_PREFERENCE_NO_FASTFORWARD
+                ):
+                    if analysis & GIT_MERGE_ANALYSIS_FASTFORWARD:
+                        return self._merge_ff(rev, obj)
+
+                    if analysis & GIT_MERGE_ANALYSIS_UNBORN:
+                        self.repo.set_head(obj.id)
+                        return str(obj.id)
+
+                if ff_pref & GIT_MERGE_PREFERENCE_FASTFORWARD_ONLY:
+                    raise SCMError("Cannot fast-forward HEAD to '{rev}'")
+
+                if commit:
+                    if not msg:
+                        raise SCMError("Merge commit message is required")
+                    user = self.default_signature
+                    tree = self.repo.index.write_tree()
+                    merge_commit = self.repo.create_commit(
+                        "HEAD",
+                        user,
+                        user,
+                        msg,
+                        tree,
+                        [self.repo.head.target, obj.id],
+                    )
+                    return str(merge_commit)
+
+                # --squash merge:
+                # HEAD is not moved and merge changes stay in index
+                return None
+            finally:
                 self.repo.state_cleanup()
                 self.repo.index.write()
-        return None
+
+    def _merge_ff(self, rev: str, obj) -> str:
+        if self.repo.head_is_detached:
+            self.repo.set_head(obj.id)
+        else:
+            branch = self.get_ref("HEAD", follow=False)
+            assert branch
+            self.set_ref(
+                branch,
+                str(obj.id),
+                message=f"merge {rev}: Fast-forward",
+            )
+        return str(obj.id)
 
     def validate_git_remote(self, url: str, **kwargs):
         raise NotImplementedError