Add Yoga JNI bindings to libcoldstart

Summary:
Yoga's JNI bindings are usually loaded during class loading, and can stall the UI thread.

Here, we try to mitigate the problem by adding the bindings to libcoldstart.

Reviewed By: michalgr

Differential Revision: D12956818

fbshipit-source-id: 9dda5cb6d26c2bae64606bc2d7c98ab8f7c05a30

java/BUCK

@@ -10,6 +10,7 @@ yoga_cxx_library(
     srcs = glob(["jni/*.cpp"]),
     headers = glob(["jni/*.h"]),
     header_namespace = "",
+    allow_jni_merging = True,
     compiler_flags = [
         "-fno-omit-frame-pointer",
         "-fexceptions",

java/com/facebook/yoga/YogaJNI.java

@@ -10,6 +10,7 @@ package com.facebook.yoga;
 import com.facebook.soloader.SoLoader;
 
 public class YogaJNI {
+  private static boolean isInitialized = false;
 
   // Known constants. 1-3 used in previous experiments. Do not reuse.
   public static int JNI_FAST_CALLS = 4;
@@ -19,12 +20,13 @@ public class YogaJNI {
 
   private static native void jni_bindNativeMethods(boolean useFastCall);
 
-  static boolean init() {
-    if (SoLoader.loadLibrary("yoga")) {
+  static synchronized boolean init() {
+    if (!isInitialized) {
+      isInitialized = true;
+      SoLoader.loadLibrary("yoga");
       jni_bindNativeMethods(useFastCall);
       return true;
     }
-
     return false;
   }
 }

tools/build_defs/oss/yoga_defs.bzl

@@ -174,6 +174,8 @@ def yoga_cxx_binary(*args, **kwargs):
 def yoga_cxx_library(*args, **kwargs):
     # Currently unused
     kwargs.pop("platforms", None)
+    kwargs.pop("allow_jni_merging", None)
+
     native.cxx_library(*args, **kwargs)
 
 def yoga_cxx_test(*args, **kwargs):