Fix a bug in the USAP Pool refill logic.

This CL addresses a bug in the USAP Pool refill logic that could cause
refill events to be delayed indefinitely.  This would not prevent future
refill events from triggering, but could cause temporary degradation in
USAP performance.

Test: launch applications from USAP pool and inspect log
Bug: 177795408
Merged-In: I2e46eae431a1d9671d31c29b3552cd0287c66239
Change-Id: I2e46eae431a1d9671d31c29b3552cd0287c66239

Author: chriswailes

core/java/com/android/internal/os/ZygoteServer.java

@@ -492,10 +492,12 @@ Runnable runSelectLoop(String abiList) {
                 long elapsedTimeMs = System.currentTimeMillis() - mUsapPoolRefillTriggerTimestamp;
 
                 if (elapsedTimeMs >= mUsapPoolRefillDelayMs) {
-                    // Normalize the poll timeout value when the time between one poll event and the
-                    // next pushes us over the delay value.  This prevents poll receiving a 0
-                    // timeout value, which would result in it returning immediately.
-                    pollTimeoutMs = -1;
+                    // The refill delay has elapsed during the period between poll invocations.
+                    // We will now check for any currently ready file descriptors before refilling
+                    // the USAP pool.
+                    pollTimeoutMs = 0;
+                    mUsapPoolRefillTriggerTimestamp = INVALID_TIMESTAMP;
+                    mUsapPoolRefillAction = UsapPoolRefillAction.DELAYED;
 
                 } else if (elapsedTimeMs <= 0) {
                     // This can occur if the clock used by currentTimeMillis is reset, which is
@@ -517,9 +519,11 @@ Runnable runSelectLoop(String abiList) {
             }
 
             if (pollReturnValue == 0) {
-                // The poll timeout has been exceeded.  This only occurs when we have finished the
-                // USAP pool refill delay period.
-
+                // The poll returned zero results either when the timeout value has been exceeded
+                // or when a non-blocking poll is issued and no FDs are ready.  In either case it
+                // is time to refill the pool.  This will result in a duplicate assignment when
+                // the non-blocking poll returns zero results, but it avoids an additional
+                // conditional in the else branch.
                 mUsapPoolRefillTriggerTimestamp = INVALID_TIMESTAMP;
                 mUsapPoolRefillAction = UsapPoolRefillAction.DELAYED;