fix: review comments.

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

@@ -1665,8 +1665,6 @@ public PooledSession get() {
         try (Scope waitScope = tracer.withSpan(span)) {
           PooledSession s =
               pollUninterruptiblyWithTimeout(currentTimeout, options.getAcquireSessionTimeout());
-          // TODO clean up this code since acquireSessionTimeout will never be null and hence
-          // we won't have a use-case when s == null
           if (s == null) {
             // Set the status to DEADLINE_EXCEEDED and retry.
             numWaiterTimeouts.incrementAndGet();
@@ -1695,8 +1693,6 @@ private PooledSession pollUninterruptiblyWithTimeout(
       try {
         while (true) {
           try {
-            // TODO refactor this code since eventually acquireSessionTimeout will always have a
-            // default value and won't be null
             return acquireSessionTimeout == null
                 ? waiter.get(timeoutMillis, TimeUnit.MILLISECONDS)
                 : waiter.get(acquireSessionTimeout.toMillis(), TimeUnit.MILLISECONDS);

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

@@ -718,9 +718,18 @@ public Builder setWaitForMinSessions(Duration waitForMinSessions) {
 
     /**
      * If greater than zero, we wait for said duration when no sessions are available in the {@link
-     * SessionPool}. When no configuration is passed, the default is a 60s timeout.
+     * SessionPool}. When no configuration is passed, the default is a 60s timeout. To avoid setting
+     * a default value, set the value as null.
      */
     public Builder setAcquireSessionTimeout(Duration acquireSessionTimeout) {
+      try {
+        Preconditions.checkArgument(
+            acquireSessionTimeout.toMillis() > 0,
+            "acquireSessionTimeout should be greater than 0 ns");
+      } catch (ArithmeticException ex) {
+        throw new IllegalArgumentException(
+            "acquireSessionTimeout in millis should be lesser than Long.MAX_VALUE");
+      }
       this.acquireSessionTimeout = acquireSessionTimeout;
       return this;
     }

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

@@ -189,10 +189,27 @@ public void setNegativeIdleTimeThreshold() {
 
   @Test
   public void setAcquireSessionTimeout() {
-    SessionPoolOptions sessionPoolOptions =
+    SessionPoolOptions sessionPoolOptions1 =
         SessionPoolOptions.newBuilder().setAcquireSessionTimeout(Duration.ofSeconds(20)).build();
+    SessionPoolOptions sessionPoolOptions2 =
+        SessionPoolOptions.newBuilder()
+            .setAcquireSessionTimeout(Duration.ofMillis(Long.MAX_VALUE))
+            .build();
+
+    assertEquals(Duration.ofSeconds(20), sessionPoolOptions1.getAcquireSessionTimeout());
+    assertEquals(Duration.ofMillis(Long.MAX_VALUE), sessionPoolOptions2.getAcquireSessionTimeout());
+  }
+
+  @Test(expected = IllegalArgumentException.class)
+  public void setAcquireSessionTimeout_valueLessThanLowerBound() {
+    SessionPoolOptions.newBuilder().setAcquireSessionTimeout(Duration.ofMillis(0)).build();
+  }
 
-    assertEquals(Duration.ofSeconds(20), sessionPoolOptions.getAcquireSessionTimeout());
+  @Test(expected = IllegalArgumentException.class)
+  public void setAcquireSessionTimeout_valueMoreThanUpperBound() {
+    SessionPoolOptions.newBuilder()
+        .setAcquireSessionTimeout(Duration.ofSeconds(Long.MAX_VALUE))
+        .build();
   }
 
   @Test