Always close io.Pipe when io.Copy is done (PostgresSQL engine) (#37220)

* fix(postgres): close pipe after io.Copy is done

* chore(postgres): add comment

lib/srv/db/postgres/engine.go

@@ -419,8 +419,6 @@ func (e *Engine) receiveFromServer(serverConn *pgconn.PgConn, serverErrCh chan<-
 	// parse and count the messages from the server in a separate goroutine,
 	// operating on a copy of the server message stream. the copy is arranged below.
 	copyReader, copyWriter := io.Pipe()
-	defer copyWriter.Close()
-
 	closeChan := make(chan struct{})
 
 	go func() {
@@ -461,6 +459,12 @@ func (e *Engine) receiveFromServer(serverConn *pgconn.PgConn, serverErrCh chan<-
 		log.WithError(err).Warn("Server -> Client copy finished with unexpected error.")
 	}
 
+	// We need to close the writer half of the pipe to notify the analysis
+	// goroutine that the connection is done. This will result in the goroutine
+	// receiving an io.ErrClosedPipe error, which will cause it to finish its
+	// execution. After that, wait until the closeChan is closed to ensure the
+	// goroutine is completed, avoiding data races.
+	copyWriter.Close()
 	<-closeChan
 
 	serverErrCh <- trace.Wrap(err)