Bug 1543982 - Part 2: Don't invoke mach build ... recursively. r=emilio

This commit avoids moz.build tasks when we're already within `mach
build`.

This is belt-and-braces: from within `mach build`, we want the main
moz.build dependency graph to arrange for the Gradle invocations to be
in the right state.  It's only in other situations, like `mach android
...` or invocation from Android Studio, that we want Gradle to arrange
to be in the right state vis. moz.build.

Differential Revision: https://phabricator.services.mozilla.com/D30426

--HG--
extra : moz-landing-system : lando

build.gradle

@@ -107,6 +107,12 @@ class TaggedLogOutputStream extends org.apache.commons.exec.LogOutputStream {
 }
 
 ext.geckoBinariesOnlyIf = { task ->
+    // Never when Gradle was invoked within `mach build`.
+    if ('1' == System.env.GRADLE_INVOKED_WITHIN_MACH_BUILD) {
+        rootProject.logger.lifecycle("Skipping task ${task.path} because: within `mach build`")
+        return false
+    }
+
     // Never for official builds.
     if (mozconfig.substs.MOZILLA_OFFICIAL) {
         rootProject.logger.lifecycle("Skipping task ${task.path} because: MOZILLA_OFFICIAL")

mobile/android/gradle.py

@@ -5,6 +5,7 @@
 from __future__ import print_function
 
 import buildconfig
+import os
 import subprocess
 import sys
 
@@ -30,7 +31,11 @@ def android(verb, *args):
             verb,
         ]
         cmd.extend(args)
-        subprocess.check_call(cmd)
+        env = dict(os.environ)
+        # Confusingly, `MACH` is set only within `mach build`.
+        if env.get('MACH'):
+            env['GRADLE_INVOKED_WITHIN_MACH_BUILD'] = '1'
+        subprocess.check_call(cmd, env=env)
 
         return 0
     finally: