[SPARK-4715][Core] Make sure tryToAcquire won't return a negative value

core/src/main/scala/org/apache/spark/shuffle/ShuffleMemoryManager.scala

@@ -66,8 +66,9 @@ private[spark] class ShuffleMemoryManager(maxMemory: Long) extends Logging {
       val curMem = threadMemory(threadId)
       val freeMemory = maxMemory - threadMemory.values.sum
 
-      // How much we can grant this thread; don't let it grow to more than 1 / numActiveThreads
-      val maxToGrant = math.min(numBytes, (maxMemory / numActiveThreads) - curMem)
+      // How much we can grant this thread; don't let it grow to more than 1 / numActiveThreads;
+      // don't let it be negative
+      val maxToGrant = math.min(numBytes, math.max(0, (maxMemory / numActiveThreads) - curMem))
 
       if (curMem < maxMemory / (2 * numActiveThreads)) {
         // We want to let each thread get at least 1 / (2 * numActiveThreads) before blocking;

core/src/test/scala/org/apache/spark/shuffle/ShuffleMemoryManagerSuite.scala

@@ -159,7 +159,7 @@ class ShuffleMemoryManagerSuite extends FunSuite with Timeouts {
 
   test("threads can block to get at least 1 / 2N memory") {
     // t1 grabs 1000 bytes and then waits until t2 is ready to make a request. It sleeps
-    // for a bit and releases 250 bytes, which should then be greanted to t2. Further requests
+    // for a bit and releases 250 bytes, which should then be granted to t2. Further requests
     // by t2 will return false right away because it now has 1 / 2N of the memory.
 
     val manager = new ShuffleMemoryManager(1000L)
@@ -291,4 +291,19 @@ class ShuffleMemoryManagerSuite extends FunSuite with Timeouts {
       assert(state.t2WaitTime > 200, s"t2 waited less than 200 ms (${state.t2WaitTime})")
     }
   }
+
+  test("threads should not be granted a negative size") {
+    val manager = new ShuffleMemoryManager(1000L)
+    manager.tryToAcquire(700L)
+
+    val latch = new CountDownLatch(1)
+    startThread("t1") {
+      manager.tryToAcquire(300L)
+      latch.countDown()
+    }
+    latch.await() // Wait until `t1` calls `tryToAcquire`
+
+    val granted = manager.tryToAcquire(300L)
+    assert(0 === granted, "granted is negative")
+  }
 }