Add new isolation mode "prepare".

In that mode isolate_driver.py will collect all arguments needed for
isolate.py invocation into *.isolated.gen.json file, but won't actually
run the isolation itself.

It's a feature of isolate_driver.py. Isolate client itself knows nothing about
this mode.

BUG=389227
R=maruel@chromium.org

Review URL: https://codereview.chromium.org/555553002

Cr-Commit-Position: refs/heads/master@{#293885}

build/isolate.gypi

@@ -63,14 +63,12 @@
         # the switch-over to running tests on Swarm is completed.
         #'<@(isolate_dependency_tracked)',
       ],
-      'outputs': [
-        '<(PRODUCT_DIR)/<(RULE_INPUT_ROOT).isolated',
-      ],
+      'outputs': [],
       'action': [
         'python',
         '<(DEPTH)/tools/isolate_driver.py',
         '<(test_isolation_mode)',
-        '--isolated', '<@(_outputs)',
+        '--isolated', '<(PRODUCT_DIR)/<(RULE_INPUT_ROOT).isolated',
         '--isolate', '<(RULE_INPUT_PATH)',
 
         # Variables should use the -V FOO=<(FOO) form so frequent values,
@@ -118,6 +116,15 @@
         ['test_isolation_fail_on_missing == 0', {
           'action': ['--ignore_broken_items'],
         }],
+        ["test_isolation_mode == 'prepare'", {
+          'outputs': [
+            '<(PRODUCT_DIR)/<(RULE_INPUT_ROOT).isolated.gen.json',
+          ],
+        }, {
+          'outputs': [
+            '<(PRODUCT_DIR)/<(RULE_INPUT_ROOT).isolated',
+          ],
+        }],
       ],
     },
   ],

tools/isolate_driver.py

@@ -19,11 +19,12 @@ WARNING: The target to use for build.ninja analysis is the base name of the
 'foo_test_run' analysed.
 """
 
-import StringIO
 import glob
+import json
 import logging
 import os
 import posixpath
+import StringIO
 import subprocess
 import sys
 import time
@@ -248,9 +249,23 @@ def create_wrapper(args, isolate_index, isolated_index):
   args[isolate_index] = temp_isolate
 
 
+def prepare_isolate_call(args, output):
+  """Gathers all information required to run isolate.py later.
+
+  Dumps it as JSON to |output| file.
+  """
+  with open(output, 'wb') as f:
+    json.dump({
+      'args': args,
+      'dir': os.getcwd(),
+      'version': 1,
+    }, f, indent=2, sort_keys=True)
+
+
 def main():
   logging.basicConfig(level=logging.ERROR, format='%(levelname)7s %(message)s')
   args = sys.argv[1:]
+  mode = args[0] if args else None
   isolate = None
   isolated = None
   is_component = False
@@ -261,13 +276,19 @@ def main():
       isolated = i + 1
     if arg == 'component=shared_library':
       is_component = True
-  if isolate is None or isolated is None:
+  if isolate is None or isolated is None or not mode:
     print >> sys.stderr, 'Internal failure'
     return 1
 
   if is_component:
     create_wrapper(args, isolate, isolated)
 
+  # In 'prepare' mode just collect all required information for postponed
+  # isolated.py invocation later, store it in *.isolated.gen.json file.
+  if mode == 'prepare':
+    prepare_isolate_call(args[1:], args[isolated] + '.gen.json')
+    return 0
+
   swarming_client = os.path.join(SRC_DIR, 'tools', 'swarming_client')
   sys.stdout.flush()
   result = subprocess.call(