libpq: Trace frontend authentication challenges

If tracing was enabled during connection startup, these messages would
previously be listed in the trace output as something like this:

F	54	Unknown message: 70
mismatched message length: consumed 4, expected 54

With this commit their type and contents are now correctly listed:

F	36	StartupMessage	 3 0 "user" "foo" "database" "alvherre"
F	54	SASLInitialResponse	 "SCRAM-SHA-256" 32 'n,,n=,r=nq5zEPR/VREHEpOAZzH8Rujm'
F	108	SASLResponse	 'c=biws,r=nq5zEPR/VREHEpOAZzH8RujmVtWZDQ8glcrvy9OMNw7ZqFUn,p=BBwAKe0WjSvigB6RsmmArAC+hwucLeuwJrR5C/HQD5M='

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CAGECzQSoPHtZ4xe0raJ6FYSEiPPS+YWXBhOGo+Y1YecLgknF3g@mail.gmail.com

src/interfaces/libpq/libpq-int.h

@@ -331,6 +331,18 @@ typedef enum
 	PGQUERY_CLOSE				/* Close Statement or Portal */
 } PGQueryClass;
 
+
+/*
+ * valid values for pg_conn->current_auth_response.  These are just for
+ * libpq internal use: since authentication response types all use the
+ * protocol byte 'p', fe-trace.c needs a way to distinguish them in order
+ * to print them correctly.
+ */
+#define AUTH_RESPONSE_GSS			'G'
+#define AUTH_RESPONSE_PASSWORD		'P'
+#define AUTH_RESPONSE_SASL_INITIAL	'I'
+#define AUTH_RESPONSE_SASL			'S'
+
 /*
  * An entry in the pending command queue.
  */
@@ -490,7 +502,9 @@ struct pg_conn
 										 * codes */
 	bool		client_finished_auth;	/* have we finished our half of the
 										 * authentication exchange? */
-
+	char		current_auth_response;	/* used by pqTraceOutputMessage to
+										 * know which auth response we're
+										 * sending */
 
 	/* Transient state needed while establishing connection */
 	PGTargetServerType target_server_type;	/* desired session properties */