pgsql: Disentangle concurrent stash flushes explicitly.

- Solves the last remaining one-deadlock-per-month problem.
- Speeds up the critical path with regard to stash-flushing.

lib/modules/Sql.pmod/pgsql_util.pmod

@@ -320,6 +320,7 @@ class bufcon {
      realbuffer->socket->query_fd(), mode, realbuffer->stashflushmode);
     if (mode > realbuffer->stashflushmode)
       realbuffer->stashflushmode = mode;
+    lock = 0;
     dirty = 0;
     this->clear();
     if (lock = realbuffer->nostash->trylock(1)) {
@@ -475,10 +476,7 @@ class conxion {
 #endif
       started = lock;
       lock = 0;				// Force release before acquiring next
-      lock = shortmux->lock();
-      mode = getstash(KEEP);
-      lock = 0;
-      if (mode > KEEP)
+      if (sizeof(stash) && getstash(KEEP) > KEEP)
         sendcmd(mode);			// Force out stash to the server
 #ifdef PG_DEBUGRACE
       return sess;
@@ -501,7 +499,7 @@ class conxion {
   }
 
   private int getstash(int mode) {
-    if (sizeof(stash)) {
+    Thread.MutexKey lock = shortmux->lock();
     add(stash); stash->clear();
     foreach (stashqueue->try_read_array(); ; int|sql_result portal)
       if (intp(portal))
@@ -513,7 +511,6 @@ class conxion {
     if (stashflushmode > mode)
       mode = stashflushmode;
     stashflushmode = KEEP;
-    }
     return mode;
   }
 
@@ -538,8 +535,9 @@ unfinalised:
       }
       qportals->write(synctransact++);
     } while (0);
-    Thread.MutexKey lock = shortmux->lock();
+    if (sizeof(stash))
       mode = getstash(mode);
+    for(;;) {
 #ifdef PG_DEBUG
       mixed err =
 #endif
@@ -563,9 +561,20 @@ outer:
           }
         } while (0);
         started = 0;
+        if (sizeof(stash) && (started = nostash->trylock(1))) {
+#ifdef PG_DEBUGRACE
+          conxsess sess = conxsess(this);
+          sess->sendcmd(SENDOUT);
+#else
+          mode = getstash(SENDOUT);
+          continue;
+#endif
+        }
         return;
       };
-    lock = 0;
+      break;
+    }
+    started = 0;
     PD("Sendcmd failed %s\n", describe_backtrace(err));
     destruct(this);
   }