GitSCM: raise PatchConflict from get_diff_from_patches and handle in LandingWorker (bug 2000268) r=zeid

Includes the following changes:
 * GitSCM: add @detect_patch_conflict to transform SCMExceptions as needed
 * GitSCM: raise PatchConflict as needed in get_diff_from_patches
 * BaseWorker: allow handle_new_commit_failures to return something
 * landing_worker: handle PatchConflict from scm.get_diff_from_patches

Pull request: #742

Author: shtrom

src/lando/api/legacy/workers/base.py

@@ -6,7 +6,7 @@
 import subprocess
 from abc import ABC, abstractmethod
 from time import sleep
-from typing import Callable
+from typing import Callable, TypeVar
 
 from celery import Task
 from django.db import transaction
@@ -36,6 +36,8 @@
 
 logger = logging.getLogger(__name__)
 
+T = TypeVar("T")
+
 
 class Worker(ABC):
     """A base class for repository workers."""
@@ -314,15 +316,15 @@ def update_repo(
 
     def handle_new_commit_failures(
         self,
-        create_revision_callable: Callable[[Revision], None],
+        create_revision_callable: Callable[[Revision], T],
         repo: Repo,
         job: BaseJob,
         scm: AbstractSCM,
         revision: Revision,
-    ) -> None:
+    ) -> T:
         """Create revisions with job status handling."""
         try:
-            create_revision_callable(revision)
+            return create_revision_callable(revision)
         except NoDiffStartLine as exc:
             message = (
                 "Lando encountered a malformed patch, please try again. "

src/lando/api/legacy/workers/landing_worker.py

@@ -178,6 +178,10 @@ def convert_patches_to_diff(self, scm: AbstractSCM, job: LandingJob):
         # at this time. In theory this would work for any provided patches in a
         # standard format.
 
+        def get_diff_from_patches(revision: Revision) -> str:
+            logger.debug(f"Converting paches to single diff for {revision} ...")
+            return scm.get_diff_from_patches(revision.patches)
+
         # NOTE: this is only supported for jobs with a single revision at this time.
         # See bug 2001185.
 
@@ -192,7 +196,14 @@ def convert_patches_to_diff(self, scm: AbstractSCM, job: LandingJob):
         if not revision.patches:
             raise ValueError("Revision is missing patches.")
 
-        diff = scm.get_diff_from_patches(revision.patches)
+        diff = self.handle_new_commit_failures(
+            get_diff_from_patches,
+            job.target_repo,
+            job,
+            scm,
+            revision,
+        )
+
         revision.set_patch(diff)
         revision.save()
 

src/lando/main/scm/git.py

@@ -8,7 +8,7 @@
 from contextlib import contextmanager
 from datetime import datetime
 from pathlib import Path
-from typing import Any
+from typing import Any, Callable, TypeVar
 
 from typing_extensions import override
 
@@ -36,6 +36,24 @@
 ENV_COMMITTER_EMAIL = "GIT_COMMITTER_EMAIL"
 
 
+T = TypeVar("T")
+
+
+def detect_patch_conflict(fn: Callable[..., T]) -> Callable[..., T]:
+    """Decorator transforming SCMExceptions to PatchConflict as appropriate."""
+
+    def wrapper(*args, **kwargs) -> T:
+        try:
+            return fn(*args, **kwargs)
+        except SCMException as exc:
+            if "error: patch" in exc.err:
+                raise PatchConflict(exc.err) from exc
+
+            raise exc
+
+    return wrapper
+
+
 class GitSCM(AbstractSCM):
     """An implementation of the AbstractVCS for Git, for use by the Repo and LandingWorkers."""
 
@@ -122,6 +140,7 @@ def last_commit_for_path(self, path: str) -> str:
         return self._git_run(*command, cwd=self.path)
 
     @override
+    @detect_patch_conflict
     def apply_patch(
         self, diff: str, commit_description: str, commit_author: str, commit_date: str
     ):
@@ -152,13 +171,7 @@ def apply_patch(
             ]
 
             for c in cmds:
-                try:
-                    self._git_run(*c, cwd=self.path)
-                except SCMException as exc:
-                    if "error: patch" in exc.err:
-                        raise PatchConflict(exc.err) from exc
-
-                    raise exc
+                self._git_run(*c, cwd=self.path)
 
     @override
     def apply_patch_git(self, patch_bytes: bytes):
@@ -189,6 +202,7 @@ def apply_patch_git(self, patch_bytes: bytes):
                 # Re-raise the exception from the failed `git am`.
                 raise exc
 
+    @detect_patch_conflict
     def get_diff_from_patches(self, patches: str) -> str:
         """Apply multiple patches and return the diff output."""
         # TODO: add error handling so that if something goes wrong here,
@@ -202,6 +216,7 @@ def get_diff_from_patches(self, patches: str) -> str:
             patch_file.flush()
 
             self._git_run("apply", "--reject", patch_file.name, cwd=self.path)
+
             self._git_run("add", "-A", "-f", cwd=self.path)
             return self._git_run(
                 "diff", "--staged", "--binary", cwd=self.path, rstrip=False