Add an observer to log blocked threads.

core-util/src/main/java/org/signal/core/util/concurrent/DeadlockDetector.kt

@@ -0,0 +1,60 @@
+package org.signal.core.util.concurrent
+
+import android.os.Handler
+import android.os.SystemClock
+import org.signal.core.util.logging.Log
+import java.util.concurrent.TimeUnit
+
+/**
+ * A class that polls active threads at a set interval and logs when multiple threads are BLOCKED.
+ */
+class DeadlockDetector(private val handler: Handler, private val pollingInterval: Long) {
+
+  private var running = false
+
+  fun start() {
+    Log.d(TAG, "Beginning deadlock monitoring.");
+    running = true
+    handler.postDelayed(this::poll, pollingInterval)
+  }
+
+  fun stop() {
+    Log.d(TAG, "Ending deadlock monitoring.");
+    running = false
+    handler.removeCallbacksAndMessages(null)
+  }
+
+  private fun poll() {
+    val threads = Thread.getAllStackTraces()
+    val blocked = threads.filterKeys { thread -> thread.state == Thread.State.BLOCKED }
+
+    if (blocked.size > 1) {
+      Log.w(TAG, buildLogString(blocked))
+    }
+
+    if (running) {
+      handler.postDelayed(this::poll, pollingInterval)
+    }
+  }
+
+  companion object {
+    private val TAG = Log.tag(DeadlockDetector::class.java)
+
+    private fun buildLogString(blocked: Map<Thread, Array<StackTraceElement>>): String {
+      val stringBuilder = StringBuilder()
+      stringBuilder.append("Found multiple blocked threads! Possible deadlock.\n")
+
+      for (entry in blocked) {
+        stringBuilder.append("-- [${entry.key.id}] ${entry.key.name}\n")
+
+        for (element in entry.value) {
+          stringBuilder.append("$element\n")
+        }
+
+        stringBuilder.append("\n")
+      }
+
+      return stringBuilder.toString()
+    }
+  }
+}