Improve MongoTimeoutException thrown by DefaultConnectionPool (#1767)

JAVA-5910

Author: stIncMale

driver-core/src/main/com/mongodb/MongoTimeoutException.java

@@ -18,6 +18,7 @@
 
 import com.mongodb.annotations.Alpha;
 import com.mongodb.annotations.Reason;
+import com.mongodb.lang.Nullable;
 
 /**
  * An exception indicating that the driver has timed out waiting for either a server or a connection to become available.
@@ -42,7 +43,7 @@ public MongoTimeoutException(final String message) {
  * @since 5.2
  */
  @Alpha(Reason.CLIENT)
- public MongoTimeoutException(final String message, final Throwable cause) {
+ public MongoTimeoutException(final String message, @Nullable final Throwable cause) {
  super(message, cause);
  }
 }

driver-core/src/main/com/mongodb/internal/connection/DefaultConnectionPool.java

@@ -345,7 +345,7 @@ private PooledConnection getPooledConnection(final Timeout waitQueueTimeout, fin
  }
  return new PooledConnection(internalConnection);
  } catch (MongoTimeoutException e) {
- throw createTimeoutException(startTime);
+ throw createTimeoutException(startTime, e);
  }
  }
 
@@ -359,13 +359,14 @@ private PooledConnection getPooledConnectionImmediate() {
  return internalConnection == null ? null : new PooledConnection(internalConnection);
  }
 
- private MongoTimeoutException createTimeoutException(final StartTime startTime) {
+ private MongoTimeoutException createTimeoutException(final StartTime startTime, @Nullable final MongoTimeoutException cause) {
  long elapsedMs = startTime.elapsed().toMillis();
  int numPinnedToCursor = pinnedStatsManager.getNumPinnedToCursor();
  int numPinnedToTransaction = pinnedStatsManager.getNumPinnedToTransaction();
  if (numPinnedToCursor == 0 && numPinnedToTransaction == 0) {
  return new MongoTimeoutException(format("Timed out after %d ms while waiting for a connection to server %s.",
- elapsedMs, serverId.getAddress()));
+ elapsedMs, serverId.getAddress()),
+ cause);
  } else {
  int maxSize = pool.getMaxSize();
  int numInUse = pool.getInUseCount();
@@ -375,7 +376,7 @@ private MongoTimeoutException createTimeoutException(final StartTime startTime)
  * - numInUse > 0
  * we consider at least one of `numPinnedToCursor`, `numPinnedToTransaction` to be positive,
  * so if we observe `numInUse` to be 0, we have to estimate it based on `numPinnedToCursor` and `numPinnedToTransaction`;
- * - numInUse < maxSize
+ * - numInUse <= maxSize
  * `numInUse` must not exceed the limit in situations when we estimate `numInUse`;
  * - numPinnedToCursor + numPinnedToTransaction <= numInUse
  * otherwise the numbers do not make sense.
@@ -399,7 +400,8 @@ private MongoTimeoutException createTimeoutException(final StartTime startTime)
  + "connections in use by other operations: %d",
  elapsedMs, serverId.getAddress(),
  sizeToString(maxSize), numPinnedToCursor, numPinnedToTransaction,
- numOtherInUse));
+ numOtherInUse),
+ cause);
  }
  }
 
@@ -1067,7 +1069,7 @@ private PooledConnection acquirePermitOrGetAvailableOpenedConnection(final boole
  & (availableConnection = tryGetAvailable ? tryGetAvailableConnection() : null) == null) {
 
  Timeout.onExistsAndExpired(waitQueueTimeout, () -> {
- throw createTimeoutException(startTime);
+ throw createTimeoutException(startTime, null);
  });
  waitQueueTimeout.awaitOn(permitAvailableOrHandedOverOrClosedOrPausedCondition,
  () -> "acquiring permit or getting available opened connection");
@@ -1406,7 +1408,7 @@ void failAsClosed() {
  }
 
  void failAsTimedOut() {
- doComplete(() -> createTimeoutException(startTime));
+ doComplete(() -> createTimeoutException(startTime, null));
  }
 
  private void doComplete(final Supplier<RuntimeException> failureSupplier) {