subcmds: reduce multiprocessing serialization overhead

Follow the same approach as 39ffd9977e to reduce serialization overhead.

Below benchmarks are tested with 2.7k projects on my workstation
(warm cache). git tracing is disabled for benchmark.

(seconds)              | v2.48 | v2.48 | this CL | this CL
	               |       |  -j32 |         |    -j32
-----------------------------------------------------------
with clean tree state:
branches (none)        |   5.6 |   5.9 |    1.0  |    0.9
status (clean)         |  21.3 |   9.4 |   19.4  |    4.7
diff (none)            |   7.6 |   7.2 |    5.7  |    2.2
prune (none)           |   5.7 |   6.1 |    1.3  |    1.2
abandon (none)         |  19.4 |  18.6 |    0.9  |    0.8
upload (none)          |  19.7 |  18.7 |    0.9  |    0.8
forall -c true         |   7.5 |   7.6 |    0.6  |    0.6
forall -c "git log -1" |  11.3 |  11.1 |    0.6  |    0.6

with branches:
start BRANCH --all     |  21.9 |  20.3 |   13.6  |    2.6
checkout BRANCH        |  29.1 |  27.8 |    1.1  |    1.0
branches (2)           |  28.0 |  28.6 |    1.5  |    1.3
abandon BRANCH         |  29.2 |  27.5 |    9.7  |    2.2

Bug: b/371638995
Change-Id: I53989a3d1e43063587b3f52f852b1c2c56b49412
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/440221
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Tested-by: Kuang-che Wu <kcwu@google.com>
Commit-Queue: Kuang-che Wu <kcwu@google.com>

1 files changed, 14 insertions, 10 deletions

diff --git a/subcmds/upload.py b/subcmds/upload.py
index 8039a1cd..6344ee31 100644
--- a/subcmds/upload.py
+++ b/subcmds/upload.py
@@ -713,16 +713,17 @@ Gerrit Code Review:  https://www.gerritcodereview.com/
         merge_branch = p.stdout.strip()
         return merge_branch
 
-    @staticmethod
-    def _GatherOne(opt, project):
+    @classmethod
+    def _GatherOne(cls, opt, project_idx):
         """Figure out the upload status for |project|."""
+        project = cls.get_parallel_context()["projects"][project_idx]
         if opt.current_branch:
             cbr = project.CurrentBranch
             up_branch = project.GetUploadableBranch(cbr)
             avail = [up_branch] if up_branch else None
         else:
             avail = project.GetUploadableBranches(opt.branch)
-        return (project, avail)
+        return (project_idx, avail)
 
     def Execute(self, opt, args):
         projects = self.GetProjects(
@@ -732,8 +733,9 @@ Gerrit Code Review:  https://www.gerritcodereview.com/
         def _ProcessResults(_pool, _out, results):
             pending = []
             for result in results:
-                project, avail = result
+                project_idx, avail = result
                 if avail is None:
+                    project = projects[project_idx]
                     logger.error(
                         'repo: error: %s: Unable to upload branch "%s". '
                         "You might be able to fix the branch by running:\n"
@@ -746,12 +748,14 @@ Gerrit Code Review:  https://www.gerritcodereview.com/
                     pending.append(result)
             return pending
 
-        pending = self.ExecuteInParallel(
-            opt.jobs,
-            functools.partial(self._GatherOne, opt),
-            projects,
-            callback=_ProcessResults,
-        )
+        with self.ParallelContext():
+            self.get_parallel_context()["projects"] = projects
+            pending = self.ExecuteInParallel(
+                opt.jobs,
+                functools.partial(self._GatherOne, opt),
+                range(len(projects)),
+                callback=_ProcessResults,
+            )
 
         if not pending:
             if opt.branch is None: