changes to avoid deadlock

ChangeLog

@@ -1,3 +1,7 @@
+2020-02-25 Richard Frith-Macdonald  <rfm@gnu.org>
+
+	* SQLClient.m: Changes t -release to avoid deadlock purging pool.
+
 2020-02-14 Richard Frith-Macdonald  <rfm@gnu.org>
 
 	* SQLClient.m: Re-implement SQLString as a true subclass rather than

SQLClient.m

@@ -2484,19 +2484,22 @@ static int	        poolConnections = 0;
 
 - (oneway void) release
 {
-  /* We lock the table while checking, to prevent
-   * another thread from grabbing this object while we are
-   * checking it.
-   * If we are going to deallocate the object, we first remove
-   * it from the table so that no other thread will find it
-   * and try to use it while it is being deallocated.
+  /* We lock the table while checking, to prevent another thread
+   * from grabbing this object while we are checking it.
    */
   [clientsLock lock];
   if (nil != _pool && [self retainCount] == 1)
     {
-      /* If this is the only reference to a client associated with
+      /* This is the only reference to a client associated with
        * a connection pool we put this client back to the pool.
        *
+       * That being the case, we know that this thread 'owns'
+       * the client and it's not going to be deallocated and not
+       * going to have the _pool iinstance variable changed, so it
+       * is safe to unlock clientsLock before returning the client
+       * to the pool.  This avoids a possible deadlock when a pool
+       * is being purged.
+       *
        * wl 2019-05-01: The original implementation was calling this code
        * when NSDecrementExtraRefCountWasZero returns YES, but
        * this does not work with newer versions of the GNUstep
@@ -2504,13 +2507,14 @@ static int	        poolConnections = 0;
        * Objective-C runtime, which both don't handle resurrection
        * gracefully for objects whose retain count has become zero.
        */
+      [clientsLock unlock];
       [_pool _swallowClient: self explicit: NO];
     }
   else
     {
       [super release];
+      [clientsLock unlock];
     }
-  [clientsLock unlock];
 }
 
 - (SQLClientPool*) pool