Fix bugs in libpq's GSSAPI encryption support.

The critical issue fixed here is that if a GSSAPI-encrypted connection
is successfully made, pqsecure_open_gss() cleared conn->allow_ssl_try,
as an admittedly-hacky way of preventing us from then trying to tunnel
SSL encryption over the already-encrypted connection.  The problem
with that is that if we abandon the GSSAPI connection because of a
failure during authentication, we would not attempt SSL encryption
in the next try with the same server.  This can lead to unexpected
connection failure, or silently getting a non-encrypted connection
where an encrypted one is expected.

Fortunately, we'd only manage to make a GSSAPI-encrypted connection
if both client and server hold valid tickets in the same Kerberos
infrastructure, which is a relatively uncommon environment.
Nonetheless this is a very nasty bug with potential security
consequences.  To fix, don't reset the flag, instead adding a
check for conn->gssenc being already true when deciding whether
to try to initiate SSL.

While here, fix some lesser issues in libpq's GSSAPI code:

* Use the need_new_connection stanza when dropping an attempted
GSSAPI connection, instead of partially duplicating that code.
The consequences of this are pretty minor: AFAICS it could only
lead to auth_req_received or password_needed remaining set when
they shouldn't, which is not too harmful.

* Fix pg_GSS_error() to not repeat the "mprefix" it's given multiple
times, and to notice any failure return from gss_display_status().

* Avoid gratuitous dependency on NI_MAXHOST in
pg_GSS_load_servicename().

Per report from Mikael Gustavsson.  Back-patch to v12 where
this code was introduced.

Discussion: https://postgr.es/m/e5b0b6ed05764324a2f3fe7acfc766d5@smhi.se

src/interfaces/libpq/fe-connect.c

@@ -2909,11 +2909,16 @@ keep_going:						/* We will come back to here until there is
 #ifdef USE_SSL
 
 				/*
-				 * If SSL is enabled and we haven't already got it running,
-				 * request it instead of sending the startup message.
+				 * If SSL is enabled and we haven't already got encryption of
+				 * some sort running, request SSL instead of sending the
+				 * startup message.
 				 */
 				if (conn->allow_ssl_try && !conn->wait_ssl_try &&
-					!conn->ssl_in_use)
+					!conn->ssl_in_use
+#ifdef ENABLE_GSS
+					&& !conn->gssenc
+#endif
+					)
 				{
 					ProtocolVersion pv;
 
@@ -3042,6 +3047,7 @@ keep_going:						/* We will come back to here until there is
 						}
 						/* Otherwise, proceed with normal startup */
 						conn->allow_ssl_try = false;
+						/* We can proceed using this connection */
 						conn->status = CONNECTION_MADE;
 						return PGRES_POLLING_WRITING;
 					}
@@ -3139,8 +3145,7 @@ keep_going:						/* We will come back to here until there is
 						 * don't hang up the socket, though.
 						 */
 						conn->try_gss = false;
-						pqDropConnection(conn, true);
-						conn->status = CONNECTION_NEEDED;
+						need_new_connection = true;
 						goto keep_going;
 					}
 
@@ -3158,6 +3163,7 @@ keep_going:						/* We will come back to here until there is
 						}
 
 						conn->try_gss = false;
+						/* We can proceed using this connection */
 						conn->status = CONNECTION_MADE;
 						return PGRES_POLLING_WRITING;
 					}
@@ -3186,8 +3192,7 @@ keep_going:						/* We will come back to here until there is
 					 * the current connection to do so, though.
 					 */
 					conn->try_gss = false;
-					pqDropConnection(conn, true);
-					conn->status = CONNECTION_NEEDED;
+					need_new_connection = true;
 					goto keep_going;
 				}
 				return pollres;
@@ -3354,10 +3359,9 @@ keep_going:						/* We will come back to here until there is
 					 */
 					if (conn->gssenc && conn->gssencmode[0] == 'p')
 					{
-						/* postmaster expects us to drop the connection */
+						/* only retry once */
 						conn->try_gss = false;
-						pqDropConnection(conn, true);
-						conn->status = CONNECTION_NEEDED;
+						need_new_connection = true;
 						goto keep_going;
 					}
 #endif

src/interfaces/libpq/fe-gssapi-common.c

@@ -20,10 +20,10 @@
 
 /*
  * Fetch all errors of a specific type and append to "str".
+ * Each error string is preceded by a space.
  */
 static void
-pg_GSS_error_int(PQExpBuffer str, const char *mprefix,
-				 OM_uint32 stat, int type)
+pg_GSS_error_int(PQExpBuffer str, OM_uint32 stat, int type)
 {
 	OM_uint32	lmin_s;
 	gss_buffer_desc lmsg;
@@ -31,9 +31,10 @@ pg_GSS_error_int(PQExpBuffer str, const char *mprefix,
 
 	do
 	{
-		gss_display_status(&lmin_s, stat, type,
-						   GSS_C_NO_OID, &msg_ctx, &lmsg);
-		appendPQExpBuffer(str, "%s: %s\n", mprefix, (char *) lmsg.value);
+		if (gss_display_status(&lmin_s, stat, type, GSS_C_NO_OID,
+							   &msg_ctx, &lmsg) != GSS_S_COMPLETE)
+			break;
+		appendPQExpBuffer(str, " %s", (char *) lmsg.value);
 		gss_release_buffer(&lmin_s, &lmsg);
 	} while (msg_ctx);
 }
@@ -46,12 +47,11 @@ pg_GSS_error(const char *mprefix, PGconn *conn,
 			 OM_uint32 maj_stat, OM_uint32 min_stat)
 {
 	resetPQExpBuffer(&conn->errorMessage);
-
-	/* Fetch major error codes */
-	pg_GSS_error_int(&conn->errorMessage, mprefix, maj_stat, GSS_C_GSS_CODE);
-
-	/* Add the minor codes as well */
-	pg_GSS_error_int(&conn->errorMessage, mprefix, min_stat, GSS_C_MECH_CODE);
+	appendPQExpBuffer(&conn->errorMessage, "%s:", mprefix);
+	pg_GSS_error_int(&conn->errorMessage, maj_stat, GSS_C_GSS_CODE);
+	appendPQExpBufferChar(&conn->errorMessage, ':');
+	pg_GSS_error_int(&conn->errorMessage, min_stat, GSS_C_MECH_CODE);
+	appendPQExpBufferChar(&conn->errorMessage, '\n');
 }
 
 /*
@@ -103,7 +103,7 @@ pg_GSS_load_servicename(PGconn *conn)
 	 * Import service principal name so the proper ticket can be acquired by
 	 * the GSSAPI system.
 	 */
-	maxlen = NI_MAXHOST + strlen(conn->krbsrvname) + 2;
+	maxlen = strlen(conn->krbsrvname) + strlen(host) + 2;
 	temp_gbuf.value = (char *) malloc(maxlen);
 	if (!temp_gbuf.value)
 	{

src/interfaces/libpq/fe-secure-gssapi.c

@@ -647,17 +647,14 @@ pqsecure_open_gss(PGconn *conn)
 	if (output.length == 0)
 	{
 		/*
-		 * We're done - hooray!  Kind of gross, but we need to disable SSL
-		 * here so that we don't accidentally tunnel one over the other.
+		 * We're done - hooray!  Set flag to tell the low-level I/O routines
+		 * to do GSS wrapping/unwrapping.
 		 */
-#ifdef USE_SSL
-		conn->allow_ssl_try = false;
-#endif
+		conn->gssenc = true;
 
 		/* Clean up */
 		gss_release_cred(&minor, &conn->gcred);
 		conn->gcred = GSS_C_NO_CREDENTIAL;
-		conn->gssenc = true;
 		gss_release_buffer(&minor, &output);
 
 		/*