Make SASL max message length configurable

The proposed OAUTHBEARER SASL mechanism will need to allow larger
messages in the exchange, since tokens are sent directly by the
client.  Move this limit into the pg_be_sasl_mech struct so that
it can be changed per-mechanism.

Author: Jacob Champion <jacob.champion@enterprisedb.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAOYmi+nqX_5=Se0W0Ynrr55Fha3CMzwv_R9P3rkpHb=1kG7ZTQ@mail.gmail.com

src/backend/libpq/auth-sasl.c

@@ -20,14 +20,6 @@
 #include "libpq/pqformat.h"
 #include "libpq/sasl.h"
 
-/*
- * Maximum accepted size of SASL messages.
- *
- * The messages that the server or libpq generate are much smaller than this,
- * but have some headroom.
- */
-#define PG_MAX_SASL_MESSAGE_LENGTH	1024
-
 /*
  * Perform a SASL exchange with a libpq client, using a specific mechanism
  * implementation.
@@ -103,7 +95,7 @@ CheckSASLAuth(const pg_be_sasl_mech *mech, Port *port, char *shadow_pass,
 
 		/* Get the actual SASL message */
 		initStringInfo(&buf);
-		if (pq_getmessage(&buf, PG_MAX_SASL_MESSAGE_LENGTH))
+		if (pq_getmessage(&buf, mech->max_message_length))
 		{
 			/* EOF - pq_getmessage already logged error */
 			pfree(buf.data);

src/backend/libpq/auth-scram.c

@@ -113,7 +113,9 @@ static int	scram_exchange(void *opaq, const char *input, int inputlen,
 const pg_be_sasl_mech pg_be_scram_mech = {
 	scram_get_mechanisms,
 	scram_init,
-	scram_exchange
+	scram_exchange,
+
+	PG_MAX_SASL_MESSAGE_LENGTH
 };
 
 /*