Fix goma regex detection and add optional nice launching

Here we add slightly improved detection for the goma flag.
The detection of goma in autoninja does not work for the use_goma flag
when it does not simply appear at the beginning of the line.

Below are examples of multi-arg lines we now support:
is_debug=false use_goma=true is_official_build=false
use_goma=false# use_goma=true This comment is ignored


Also adding linux nice process launch feature to improve the user
experience when running a large build. This feature is disabled by
default like other autoninja features.



R=brucedawson@chromium.org

Change-Id: I8a04e7480c5e8d37d57fb71143c6066d46abb004
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2479683
Commit-Queue: Peter McNeeley <petermcneeley@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>

autoninja.py

@@ -66,7 +66,15 @@ if os.path.exists(os.path.join(output_dir, 'args.gn')):
   with open(os.path.join(output_dir, 'args.gn')) as file_handle:
     for line in file_handle:
       # Either use_goma or use_rbe activate build acceleration.
-      if re.match(r'^\s*(use_goma|use_rbe)\s*=\s*true(\s*$|\s*#.*$)', line):
+      #
+      # This test can match multi-argument lines. Examples of this are:
+      # is_debug=false use_goma=true is_official_build=false
+      # use_goma=false# use_goma=true This comment is ignored
+      #
+      # Anything after a comment is not consider a valid argument.
+      line_without_comment = line.split('#')[0]
+      if re.search(r'(^|\s)(use_goma|use_rbe)\s*=\s*true($|\s)',
+                   line_without_comment):
         use_remote_build = True
         continue
 elif os.path.exists(os.path.join(output_dir, 'rules.ninja')):
@@ -92,9 +100,19 @@ if goma_disabled_env in ['true', 't', 'yes', 'y', '1']:
 ninja_exe = 'ninja.exe' if sys.platform.startswith('win') else 'ninja'
 ninja_exe_path = os.path.join(SCRIPT_DIR, ninja_exe)
 
+# A large build (with or without goma) tends to hog all system resources.
+# Launching the ninja process with 'nice' priorities improves this situation.
+prefix_args = []
+if (sys.platform.startswith('linux')
+    and os.environ.get('NINJA_BUILD_IN_BACKGROUND', '0') == '1'):
+  # nice -10 is process priority 10 lower than default 0
+  # ionice -c 3 is IO priority IDLE
+  prefix_args = ['nice'] + ['-10']
+
+
 # Use absolute path for ninja path,
 # or fail to execute ninja if depot_tools is not in PATH.
-args = [ninja_exe_path] + input_args[1:]
+args = prefix_args + [ninja_exe_path] + input_args[1:]
 
 num_cores = psutil.cpu_count()
 if not j_specified and not t_specified: