Auto merge of #21759 - jdm:ndkgdb, r=paulrouget

Add mach command to setup remote debugging on Android devices.

This removes any need to fiddle with search paths for source files and makes the experience of remote debugging much less frustrating. I've tried this on a Pixel, Pixel 2, and emulator and successfully set breakpoints and investigated variables on all of them. The APP_ABI changes are necessary to prevent ndk-gdb from thinking that the build is arm64 when it's not.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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python/servo/devenv_commands.py

@@ -8,10 +8,12 @@
 # except according to those terms.
 
 from __future__ import print_function, unicode_literals
-from os import path, listdir
+from os import path, listdir, getcwd
 from time import time
 
+import signal
 import sys
+import tempfile
 import urllib2
 import json
 import subprocess
@@ -265,3 +267,57 @@ class MachCommands(CommandBase):
             "local",
             self.config["android"]["lib"])
         print(subprocess.check_output([ndk_stack, "-sym", sym_path, "-dump", logfile]))
+
+    @Command('ndk-gdb',
+             description='Invoke ndk-gdb tool with the expected symbol paths',
+             category='devenv')
+    @CommandArgument('--release', action='store_true', help="Use release build symbols")
+    @CommandArgument('--target', action='store', default="armv7-linux-androideabi",
+                     help="Build target")
+    def ndk_gdb(self, release, target):
+        env = self.build_env(target)
+        self.handle_android_target(target)
+        ndk_gdb = path.join(env["ANDROID_NDK"], "ndk-gdb")
+        adb_path = path.join(env["ANDROID_SDK"], "platform-tools", "adb")
+        sym_paths = [
+            path.join(
+                getcwd(),
+                "target",
+                target,
+                "release" if release else "debug",
+                "apk",
+                "obj",
+                "local",
+                self.config["android"]["lib"]
+            ),
+            path.join(
+                getcwd(),
+                "target",
+                target,
+                "release" if release else "debug",
+                "apk",
+                "libs",
+                self.config["android"]["lib"]
+            ),
+        ]
+        env["NDK_PROJECT_PATH"] = path.join(getcwd(), "support", "android", "apk")
+        signal.signal(signal.SIGINT, signal.SIG_IGN)
+
+        with tempfile.NamedTemporaryFile(delete=False) as f:
+            f.write('\n'.join([
+                "python",
+                "param = gdb.parameter('solib-search-path')",
+                "param += ':{}'".format(':'.join(sym_paths)),
+                "gdb.execute('set solib-search-path ' + param)",
+                "end",
+            ]))
+
+        p = subprocess.Popen([
+            ndk_gdb,
+            "--adb", adb_path,
+            "--project", "support/android/apk/servoapp/src/main/",
+            "--launch", "com.mozilla.servo.MainActivity",
+            "-x", f.name,
+            "--verbose",
+        ], env=env)
+        return p.wait()

support/android/apk/jni/Application.mk

@@ -1,4 +1,5 @@
-NDK_TOOLCHAIN_VERSION := 4.9
+NDK_TOOLCHAIN_VERSION := clang
 APP_MODULES := c++_shared servojni gstreamer
 APP_PLATFORM := android-18
 APP_STL:= c++_shared
+APP_ABI:= armeabi-v7a x86