Bug 1849901: allow setting the maximum number of workers to use for taskgraph operations r=taskgraph-reviewers,jcristau

Differential Revision: https://phabricator.services.mozilla.com/D186726

taskcluster/gecko_taskgraph/main.py

@@ -203,7 +203,7 @@ def generate_taskgraph(options, parameters, logdir):
         return
 
     futures = {}
-    with ProcessPoolExecutor() as executor:
+    with ProcessPoolExecutor(max_workers=options["max_workers"]) as executor:
         for spec in parameters:
             f = executor.submit(format_taskgraph, options, spec, logfile(spec))
             futures[f] = spec
@@ -347,6 +347,15 @@ def generate_taskgraph(options, parameters, logdir):
     "Without args the base revision will be used. A revision specifier such as "
     "the hash or `.~1` (hg) or `HEAD~1` (git) can be used as well.",
 )
+@argument(
+    "-j",
+    "--max-workers",
+    dest="max_workers",
+    default=None,
+    type=int,
+    help="The maximum number of workers to use for parallel operations such as"
+    "when multiple parameters files are passed.",
+)
 def show_taskgraph(options):
     from mozversioncontrol import get_repository_object as get_repository
     from taskgraph.parameters import Parameters, parameters_loader