pgconn: improve comment explaining nil, nil path in Pipeline.Close

Expand the comment on the results == nil guard in Pipeline.Close() to
explain exactly when and why getResults() returns (nil, nil):

When the server sends FATAL errors that consume queued request slots
(e.g. Prepare, Sync) without ever sending ReadyForQuery, the request
event queue is exhausted while expectedReadyForQueryCount remains > 0.
ExtractFrontRequestType() returns pipelineNil, getResults() returns
(nil, nil), and without this guard the loop would spin indefinitely.

Author: veeceey

pgconn/pgconn.go

@@ -2836,8 +2836,11 @@ func (p *Pipeline) Close() error {
 				break
 			}
 		} else if results == nil {
-			// No more results available but we haven't received all expected ReadyForQuery messages.
-			// This means the connection is in a bad state (e.g. server closed the connection).
+			// getResults returns (nil, nil) when the request queue is exhausted (pipelineNil) but
+			// ExpectedReadyForQuery is still > 0. This happens when the server sends FATAL errors that
+			// consume queued request slots (e.g. Prepare, Sync) without ever sending ReadyForQuery --
+			// so expectedReadyForQueryCount is never decremented. Without this check the loop would spin
+			// indefinitely because there are no more requests to issue and no ReadyForQuery to receive.
 			p.conn.asyncClose()
 			if p.err == nil {
 				p.err = errors.New("pipeline: no more results but expected ReadyForQuery")