Bug 1695312 - Selectively load only the mach command modules needed for the command about to be run r=firefox-build-system-reviewers,glandium

This makes loading almost all commands faster, since only one module
file is loaded rather than all of them. There is one main exception,
dealing with 'help'. Running `./mach help` (or -h or --help) requires
the description text for every command, so every module file is still
loaded.

We could expand this improvement here to consolidate all commands and
their parameters in this `MACH_COMMANDS` dict, but the only two benefits
are improving help, and not having two places where the commands are
specified (their file, and this dict).

There's a lot of extra work needed to do that, especially for handling
sub commands, and it did not seem worth the cost for the benefit at this
time.

Depends on D180499

Differential Revision: https://phabricator.services.mozilla.com/D180500

build/mach_initialize.py

@@ -706,7 +706,25 @@ def initialize(topsrcdir, args=()):
         ) or os.path.exists(os.path.join(topsrcdir, "INSTALL"))
     else:
         missing_ok = ()
-    driver.load_commands_from_spec(MACH_COMMANDS, topsrcdir, missing_ok=missing_ok)
+
+    commands_that_need_all_modules_loaded = [
+        "mach-commands",
+        "mach-completion",
+        "mach-debug-commands",
+        "help",
+    ]
+
+    if (
+        command_name not in MACH_COMMANDS
+        or command_name in commands_that_need_all_modules_loaded
+    ):
+        command_modules_to_load = MACH_COMMANDS
+    else:
+        command_modules_to_load = {command_name: MACH_COMMANDS[command_name]}
+
+    driver.load_commands_from_spec(
+        command_modules_to_load, topsrcdir, missing_ok=missing_ok
+    )
     return driver
 
 

python/mozbuild/mozbuild/artifact_commands.py

@@ -12,6 +12,10 @@ import os
 import shutil
 from collections import OrderedDict
 
+# As a result of the selective module loading changes, this import has to be
+# done here. It is not explicitly used, but it has an implicit side-effect
+# (bringing in TASKCLUSTER_ROOT_URL) which is necessary.
+import gecko_taskgraph.main  # noqa: F401
 import mozversioncontrol
 import six
 from mach.decorators import Command, CommandArgument, SubCommand

python/mozbuild/mozbuild/mach_commands.py

@@ -21,7 +21,6 @@ from os import path
 from pathlib import Path
 
 import mozpack.path as mozpath
-import yaml
 from mach.decorators import (
     Command,
     CommandArgument,
@@ -206,6 +205,7 @@ def cargo(
     continue_on_error=False,
     subcommand_args=[],
 ):
+    import yaml
 
     from mozbuild.controller.building import BuildDriver