sync: Warn about excessive job counts

Warn users if the effective job count specified via `-j`,
`--jobs-network`, or `--jobs-checkout` exceeds a threshold
(currently 100). This encourages users to use more reasonable
values.

Bug: 406868778
Bug: 254914814
Change-Id: I116e2bbaf3dc824c04d1b2fbe52cf9ca5be77b9a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/466801
Reviewed-by: Mike Frysinger <[email protected]>
Commit-Queue: Gavin Mak <[email protected]>
Tested-by: Gavin Mak <[email protected]>

Author: Gavin Mak

subcmds/sync.py

@@ -350,6 +350,8 @@ class Sync(Command, MirrorSafeCommand):
     # value later on.
     PARALLEL_JOBS = 0
 
+    _JOBS_WARN_THRESHOLD = 100
+
     def _Options(self, p, show_smart=True):
         p.add_option(
             "--jobs-network",
@@ -1728,6 +1730,24 @@ def _ValidateOptionsWithManifest(self, opt, mp):
         opt.jobs_network = min(opt.jobs_network, jobs_soft_limit)
         opt.jobs_checkout = min(opt.jobs_checkout, jobs_soft_limit)
 
+        # Warn once if effective job counts seem excessively high.
+        # Prioritize --jobs, then --jobs-network, then --jobs-checkout.
+        job_options_to_check = (
+            ("--jobs", opt.jobs),
+            ("--jobs-network", opt.jobs_network),
+            ("--jobs-checkout", opt.jobs_checkout),
+        )
+        for name, value in job_options_to_check:
+            if value > self._JOBS_WARN_THRESHOLD:
+                logger.warning(
+                    "High job count (%d > %d) specified for %s; this may "
+                    "lead to excessive resource usage or diminishing returns.",
+                    value,
+                    self._JOBS_WARN_THRESHOLD,
+                    name,
+                )
+                break
+
     def Execute(self, opt, args):
         errors = []
         try: