Make sure pending metadata is reset and is thread safe

Summary: We have gotton concurrent modification crashes from this

Reviewed By: lblasa

Differential Revision: D42343224

fbshipit-source-id: 9cf4046da63d40cbe6632c3ae24d95abd21081ba

android/src/main/java/com/facebook/flipper/plugins/uidebugger/UIDebuggerFlipperPlugin.kt

@@ -59,7 +59,7 @@ class UIDebuggerFlipperPlugin(
         MetadataUpdateEvent.name,
         Json.encodeToString(
             MetadataUpdateEvent.serializer(),
-            MetadataUpdateEvent(MetadataRegister.getPendingMetadata())))
+            MetadataUpdateEvent(MetadataRegister.extractPendingMetadata())))
 
     context.treeObserverManager.start()
   }

android/src/main/java/com/facebook/flipper/plugins/uidebugger/descriptors/MetadataRegister.kt

@@ -23,9 +23,11 @@ object MetadataRegister {
   const val TYPE_LAYOUT = "layout"
   const val TYPE_DOCUMENTATION = "documentation"
 
+  private val lock = "lock"
+
   private var generator: MetadataId = 0
   private val register: MutableMap<String, Metadata> = mutableMapOf()
-  private val pending: MutableSet<String> = mutableSetOf()
+  private val pendingKeys: MutableSet<String> = mutableSetOf()
 
   private fun key(namespace: String, name: String): String = "${namespace}_$name"
 
@@ -41,38 +43,40 @@ object MetadataRegister {
       return m.id
     }
 
+    synchronized(lock) {
       val identifier = ++generator
       val metadata = Metadata(identifier, type, namespace, name, mutable, possibleValues)
 
       register[key] = metadata
-    pending.add(key)
+      pendingKeys.add(key)
 
       return identifier
     }
+  }
 
   fun get(namespace: String, name: String): Metadata? {
     val key = key(namespace, name)
     return register[key]
   }
 
-  fun getMetadata(): Map<MetadataId, Metadata> {
-    val metadata: MutableMap<MetadataId, Metadata> = mutableMapOf()
-    register.forEach { entry -> metadata[entry.value.id] = entry.value }
-
-    return metadata
-  }
-
-  fun getPendingMetadata(): Map<MetadataId, Metadata> {
+  /** gets all pending metadata to be sent and resets the pending list */
+  fun extractPendingMetadata(): Map<MetadataId, Metadata> {
+    synchronized(lock) {
       val pendingMetadata: MutableMap<MetadataId, Metadata> = mutableMapOf()
-    pending.forEach { key ->
+
+      pendingKeys.forEach { key ->
         register[key]?.let { metadata -> pendingMetadata[metadata.id] = metadata }
       }
+      pendingKeys.clear()
 
       return pendingMetadata
     }
+  }
 
   fun reset() {
-    pending.clear()
-    register.forEach { entry -> pending.add(entry.key) }
+    synchronized(lock) {
+      pendingKeys.clear()
+      register.forEach { entry -> pendingKeys.add(entry.key) }
+    }
   }
 }

android/src/main/java/com/facebook/flipper/plugins/uidebugger/observers/TreeObserverManager.kt

@@ -90,7 +90,7 @@ class TreeObserverManager(val context: Context) {
   }
 
   private fun sendMetadata() {
-    val metadata = MetadataRegister.getPendingMetadata()
+    val metadata = MetadataRegister.extractPendingMetadata()
     if (metadata.size > 0) {
       context.connectionRef.connection?.send(
           MetadataUpdateEvent.name,