Avoid setting the Glide singleton when it's half initialized

If we set the singleton before calling registerComponents, other threads
that call get() while registerComponents is being called may receive a
half initialized Glide object. Half initialized Glide objects may be
missing critical components and using them in other threads may break
lots of things.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161232290

library/src/main/java/com/bumptech/glide/Glide.java

@@ -92,6 +92,7 @@ public class Glide implements ComponentCallbacks2 {
   private static final String DEFAULT_DISK_CACHE_DIR = "image_manager_disk_cache";
   private static final String TAG = "Glide";
   private static volatile Glide glide;
+  private static volatile boolean isInitializing;
 
   private final Engine engine;
   private final BitmapPool bitmapPool;
@@ -151,7 +152,15 @@ public static Glide get(Context context) {
     if (glide == null) {
       synchronized (Glide.class) {
         if (glide == null) {
+          // In the thread running initGlide(), one or more classes may call Glide.get(context).
+          // Without this check, those calls could trigger infinite recursion.
+          if (isInitializing) {
+            throw new IllegalStateException("You cannot call Glide.get() in registerComponents(),"
+                + " use the provided Glide instance instead");
+          }
+          isInitializing = true;
           initGlide(context);
+          isInitializing = false;
         }
       }
     }
@@ -160,12 +169,12 @@ public static Glide get(Context context) {
   }
 
   @VisibleForTesting
-  public static void init(Glide glide) {
+  public static synchronized void init(Glide glide) {
     Glide.glide = glide;
   }
 
   @VisibleForTesting
-  public static void tearDown() {
+  public static synchronized void tearDown() {
     glide = null;
   }
 
@@ -212,13 +221,14 @@ private static void initGlide(Context context) {
     if (annotationGeneratedModule != null) {
       annotationGeneratedModule.applyOptions(applicationContext, builder);
     }
-    glide = builder.build(applicationContext);
+    Glide glide = builder.build(applicationContext);
     for (GlideModule module : manifestModules) {
       module.registerComponents(applicationContext, glide, glide.registry);
     }
     if (annotationGeneratedModule != null) {
       annotationGeneratedModule.registerComponents(applicationContext, glide, glide.registry);
     }
+    Glide.glide = glide;
   }
 
   @Nullable