Revert "Don't truncate database and user names in startup packets."

This reverts commit 562bee0fc1.

We received a report from the field about this change in behavior,
so it seems best to revert this commit and to add proper
multibyte-aware truncation as a follow-up exercise.

Fixes bug #18711.

Reported-by: Adam Rauch
Reviewed-by: Tom Lane, Bertrand Drouvot, Bruce Momjian, Thomas Munro
Discussion: https://postgr.es/m/18711-7503ee3e449d2c47%40postgresql.org
Backpatch-through: 17

src/backend/tcop/backend_startup.c

@@ -830,6 +830,15 @@ ProcessStartupPacket(Port *port, bool ssl_done, bool gss_done)
 	if (port->database_name == NULL || port->database_name[0] == '\0')
 		port->database_name = pstrdup(port->user_name);
 
+	/*
+	 * Truncate given database and user names to length of a Postgres name.
+	 * This avoids lookup failures when overlength names are given.
+	 */
+	if (strlen(port->database_name) >= NAMEDATALEN)
+		port->database_name[NAMEDATALEN - 1] = '\0';
+	if (strlen(port->user_name) >= NAMEDATALEN)
+		port->user_name[NAMEDATALEN - 1] = '\0';
+
 	if (am_walsender)
 		MyBackendType = B_WAL_SENDER;
 	else