Bug 1417264 - Write .mozconfig.json from Python; r=nalexander

In order to determine if we need to re-run configure, we write
a JSON file representing the evaluated mozconfig. If this JSON
file changes, configure (and config.status for that matter) is
out of data and it is re-executed.

This commit moves the generation of that JSON file to Python.

MozReview-Commit-ID: 636rpSY7gOm

client.mk

@@ -141,15 +141,6 @@ configure-preqs = \
   $(OBJDIR)/.mozconfig.json \
   $(NULL)
 
-CREATE_MOZCONFIG_JSON = $(shell $(TOPSRCDIR)/mach environment --format=json -o $(OBJDIR)/.mozconfig.json)
-# Force CREATE_MOZCONFIG_JSON above to be resolved, without side effects in
-# case the result is non empty, and allowing an override on the make command
-# line not running the command (using := $(shell) still runs the shell command).
-ifneq (,$(CREATE_MOZCONFIG_JSON))
-endif
-
-$(OBJDIR)/.mozconfig.json: ;
-
 configure:: $(configure-preqs)
 	$(call BUILDSTATUS,TIERS configure)
 	$(call BUILDSTATUS,TIER_START configure)
@@ -168,7 +159,7 @@ $(OBJDIR)/config.status: $(CONFIG_STATUS_DEPS)
 else
 $(OBJDIR)/Makefile: $(CONFIG_STATUS_DEPS)
 endif
-	@$(MAKE) -f $(TOPSRCDIR)/client.mk configure CREATE_MOZCONFIG_JSON=
+	@$(MAKE) -f $(TOPSRCDIR)/client.mk configure
 
 ####################################
 # Build it

python/mozbuild/mozbuild/controller/building.py

@@ -1372,6 +1372,14 @@ def _run_client_mk(self, target=None, line_handler=None, jobs=0,
         with FileAvoidWrite(mozconfig_mk) as fh:
             fh.write(b'\n'.join(mozconfig_filtered_lines))
 
+        mozconfig_json = os.path.join(self.topobjdir, '.mozconfig.json')
+        with FileAvoidWrite(mozconfig_json) as fh:
+            json.dump({
+                'topsrcdir': self.topsrcdir,
+                'topobjdir': self.topobjdir,
+                'mozconfig': mozconfig,
+            }, fh, sort_keys=True, indent=2)
+
         # Copy the original mozconfig to the objdir.
         mozconfig_objdir = os.path.join(self.topobjdir, '.mozconfig')
         if mozconfig['path']: