Add a variable indicating the target as Android M or later.

build/config/android/rules.gni

Index: build/config/android/rules.gni
diff --git a/build/config/android/rules.gni b/build/config/android/rules.gni
index d72242c1225605a2b64775a2f8ccdd7e22281348..4a4471d55d194d5bf107e4dfa21bb9895edb600d 100644
--- a/build/config/android/rules.gni
+++ b/build/config/android/rules.gni
@@ -1232,6 +1232,9 @@ template("android_java_prebuilt") {
 #     all other asset files.
 #   alternative_locale_resource_dep: The locale resource target which overrides
 #     any exsting locale resources in dep graph.
+#   requires_sdk_api_level_23: If defined and true, the apk is intended for
+#     installation only on Android M or later. In these releases the system
+#     linker does relocation unpacking, so we can enable it unconditionally.
 #
 #   DEPRECATED_java_in_dir: Directory containing java files. All .java files in
 #     this directory will be included in the library. This is only supported to
@@ -1336,9 +1339,14 @@ template("android_apk") {
   assert(_use_chromium_linker || true)  # Mark as used.
   assert(_enable_relocation_packing || true)  # Mark as used.
 
-  assert(
-      !_load_library_from_apk || _use_chromium_linker,
-      "Loading library from the apk requires use" + " of the Chromium linker.")
+  assert(!_load_library_from_apk || _use_chromium_linker,
+         "Loading library from the apk requires use of the Chromium linker.")

[Torne, 2016/02/16 15:46:56]

It's okay to load library from APK if it's sdk 23+ also, right?

[simonb (inactive), 2016/02/16 16:54:08]

Yes, but at present only if the chromium linker is being used (requires path
shuffling to construct a full path_to_apk!/path_within_apk.so string, passed to
android_dlopen_ext). This differs from relocation unpacking, which the Android M
and later system linker handles transparently and no matter how/where the
library is loaded.

I'm planning other changes to separate load from apk out from the chromium
linker. For now this change targets only relocation packing.

[Torne, 2016/02/16 17:58:49]

OK.

+
+  # If the apk is for Android M or later, always enable relocation packing.
+  if (defined(invoker.requires_sdk_api_level_23) &&

[Torne, 2016/02/16 15:46:56]

Can we just make this setting work like the other one? i.e. just allow the
defining APK to specify whether it wants relocation packing, and then assert
that the linker will support it?

If we forcibly enable it here there won't be an easy way to build without it to
compare.

[simonb (inactive), 2016/02/16 16:54:08]

The assert condition that the linker supports unpacking is "uses chromium linker
OR apk's target android version is M or later". There isn't any easy way in gn
to check the second of these -- the apk's target version could be a string like
"M", or a string like "23", or an integer like 23, or something else entirely.

If there were a way to check for apk target version later than or equal to
android M that would be better used rather than a special flag. I don't believe
there is one, though.

[Torne, 2016/02/16 17:58:49]

No, I mean, can we just make the *logic* work the same way: let the person
defining the target set enable_relocation_packing = true, and then assert that
if they did that that either they're using the chromium linker or they also set
requires_sdk_api_level_23.

Currently this behaves completely differently to the other one: it's forced on
for api 23, and there's also no error if you try to enable it in a situation
where it doesn't work.

+      invoker.requires_sdk_api_level_23) {
+    _enable_relocation_packing = true
+  }
 
   # The dependency that makes the chromium linker, if any is needed.
   _native_libs_deps = []