fix: update max_in_use_session at 10 mins interval (#3570)

This PR fixes the max_in_use_session value. Currently this value is only reset at client initialization.

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java

@@ -2048,6 +2048,7 @@ boolean isClosed() {
 
     // Does various pool maintenance activities.
     void maintainPool() {
+      Instant currTime;
       synchronized (lock) {
         if (SessionPool.this.isClosed()) {
           return;
@@ -2059,8 +2060,15 @@ void maintainPool() {
                   / (loopFrequency / 1000L);
         }
         this.prevNumSessionsAcquired = SessionPool.this.numSessionsAcquired;
+
+        currTime = clock.instant();
+        // Reset the start time for recording the maximum number of sessions in the pool
+        if (currTime.isAfter(SessionPool.this.lastResetTime.plus(Duration.ofMinutes(10)))) {
+          SessionPool.this.maxSessionsInUse = SessionPool.this.numSessionsInUse;
+          SessionPool.this.lastResetTime = currTime;
+        }
       }
-      Instant currTime = clock.instant();
+
       removeIdleSessions(currTime);
       // Now go over all the remaining sessions and see if they need to be kept alive explicitly.
       keepAliveSessions(currTime);
@@ -2309,6 +2317,9 @@ enum Position {
   @GuardedBy("lock")
   private int maxSessionsInUse = 0;
 
+  @GuardedBy("lock")
+  private Instant lastResetTime = Clock.INSTANCE.instant();
+
   @GuardedBy("lock")
   private long numSessionsAcquired = 0;
 

google-cloud-spanner/src/test/java/com/google/cloud/spanner/SessionPoolTest.java

@@ -2247,6 +2247,54 @@ public void testWaitOnMinSessionsThrowsExceptionWhenTimeoutIsReached() {
     pool.maybeWaitOnMinSessions();
   }
 
+  @Test
+  public void reset_maxSessionsInUse() {
+    Clock clock = mock(Clock.class);
+    when(clock.instant()).thenReturn(Instant.now());
+    options =
+        SessionPoolOptions.newBuilder()
+            .setMinSessions(1)
+            .setMaxSessions(3)
+            .setIncStep(1)
+            .setMaxIdleSessions(0)
+            .setPoolMaintainerClock(clock)
+            .build();
+    setupForLongRunningTransactionsCleanup(options);
+
+    pool = createPool(clock);
+    // Make sure pool has been initialized
+    pool.getSession().close();
+
+    // All 3 sessions used. 100% of pool utilised.
+    PooledSessionFuture readSession1 = pool.getSession();
+    PooledSessionFuture readSession2 = pool.getSession();
+    PooledSessionFuture readSession3 = pool.getSession();
+
+    // complete the async tasks
+    readSession1.get().setEligibleForLongRunning(false);
+    readSession2.get().setEligibleForLongRunning(false);
+    readSession3.get().setEligibleForLongRunning(true);
+
+    assertEquals(3, pool.getMaxSessionsInUse());
+    assertEquals(3, pool.getNumberOfSessionsInUse());
+
+    // Release 1 session
+    readSession1.get().close();
+
+    // Verify that numSessionsInUse reduces to 2 while maxSessionsInUse remain 3
+    assertEquals(3, pool.getMaxSessionsInUse());
+    assertEquals(2, pool.getNumberOfSessionsInUse());
+
+    // ensure that the lastResetTime for maxSessionsInUse > 10 minutes
+    when(clock.instant()).thenReturn(Instant.now().plus(11, ChronoUnit.MINUTES));
+
+    pool.poolMaintainer.maintainPool();
+
+    // Verify that maxSessionsInUse is reset to numSessionsInUse
+    assertEquals(2, pool.getMaxSessionsInUse());
+    assertEquals(2, pool.getNumberOfSessionsInUse());
+  }
+
   private void mockKeepAlive(ReadContext context) {
     ResultSet resultSet = mock(ResultSet.class);
     when(resultSet.next()).thenReturn(true, false);