Bug 1461795 - Use FileAvoidWrite when writing mozinfo.json; r=chmanchester

Since MozbuildObject.from_environment() reads from mozinfo.json, tup
picks up that file as a dependency for anything that imports buildconfig
(eg: all generated files). Using FileAvoidWrite when creating
mozinfo.json will help avoid unnecessary work after re-running configure
in the tup backend.

MozReview-Commit-ID: EEOPQYJA1MV

configure.py

@@ -108,7 +108,6 @@ def config_status(config):
     # executable permissions.
     os.chmod('config.status', 0o755)
     if config.get('MOZ_BUILD_APP') != 'js' or config.get('JS_STANDALONE'):
-        os.environ[b'WRITE_MOZINFO'] = b'1'
         from mozbuild.config_status import config_status
 
         # Some values in sanitized_config also have more complex types, such as

python/mozbuild/mozbuild/config_status.py

@@ -22,6 +22,7 @@ from mozbuild.base import MachCommandConditions
 from mozbuild.frontend.emitter import TreeMetadataEmitter
 from mozbuild.frontend.reader import BuildReader
 from mozbuild.mozinfo import write_mozinfo
+from mozbuild.util import FileAvoidWrite
 from itertools import chain
 
 from mozbuild.backend import (
@@ -114,10 +115,8 @@ def config_status(topobjdir='.', topsrcdir='.', defines=None,
             non_global_defines=non_global_defines, substs=substs,
             source=source, mozconfig=mozconfig)
 
-    # mozinfo.json only needs written if configure changes and configure always
-    # passes this environment variable.
-    if 'WRITE_MOZINFO' in os.environ:
-        write_mozinfo(os.path.join(topobjdir, 'mozinfo.json'), env, os.environ)
+    with FileAvoidWrite(os.path.join(topobjdir, 'mozinfo.json')) as f:
+        write_mozinfo(f, env, os.environ)
 
     cpu_start = time.clock()
     time_start = time.time()