Move privilege check for SET SESSION AUTHORIZATION.

Presently, the privilege check for SET SESSION AUTHORIZATION is
performed in session_authorization's assign_hook.  A relevant
comment states, "It's OK because the check does not require catalog
access and can't fail during an end-of-transaction GUC
reversion..."  However, we plan to add a catalog lookup to this
privilege check in a follow-up commit.

This commit moves this privilege check to the check_hook for
session_authorization.  Like check_role(), we do not throw a hard
error for insufficient privileges when the source is PGC_S_TEST.

Author: Joseph Koshakow
Discussion: https://postgr.es/m/CAAvxfHc-HHzONQ2oXdvhFF9ayRnidPwK%2BfVBhRzaBWYYLVQL-g%40mail.gmail.com

src/backend/commands/variable.c

@@ -821,14 +821,16 @@ check_session_authorization(char **newval, void **extra, GucSource source)
 		return false;
 	}
 
+	/*
+	 * When source == PGC_S_TEST, we don't throw a hard error for a
+	 * nonexistent user name or insufficient privileges, only a NOTICE. See
+	 * comments in guc.h.
+	 */
+
 	/* Look up the username */
 	roleTup = SearchSysCache1(AUTHNAME, PointerGetDatum(*newval));
 	if (!HeapTupleIsValid(roleTup))
 	{
-		/*
-		 * When source == PGC_S_TEST, we don't throw a hard error for a
-		 * nonexistent user name, only a NOTICE.  See comments in guc.h.
-		 */
 		if (source == PGC_S_TEST)
 		{
 			ereport(NOTICE,
@@ -846,6 +848,28 @@ check_session_authorization(char **newval, void **extra, GucSource source)
 
 	ReleaseSysCache(roleTup);
 
+	/*
+	 * Only superusers may SET SESSION AUTHORIZATION a role other than itself.
+	 * Note that in case of multiple SETs in a single session, the original
+	 * authenticated user's superuserness is what matters.
+	 */
+	if (roleid != GetAuthenticatedUserId() &&
+		!GetAuthenticatedUserIsSuperuser())
+	{
+		if (source == PGC_S_TEST)
+		{
+			ereport(NOTICE,
+					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+					 errmsg("permission will be denied to set session authorization \"%s\"",
+							*newval)));
+			return true;
+		}
+		GUC_check_errcode(ERRCODE_INSUFFICIENT_PRIVILEGE);
+		GUC_check_errmsg("permission denied to set session authorization \"%s\"",
+						 *newval);
+		return false;
+	}
+
 	/* Set up "extra" struct for assign_session_authorization to use */
 	myextra = (role_auth_extra *) guc_malloc(LOG, sizeof(role_auth_extra));
 	if (!myextra)

src/backend/utils/init/miscinit.c

@@ -582,6 +582,16 @@ GetAuthenticatedUserId(void)
 	return AuthenticatedUserId;
 }
 
+/*
+ * Return whether the authenticated user was superuser at connection start.
+ */
+bool
+GetAuthenticatedUserIsSuperuser(void)
+{
+	Assert(OidIsValid(AuthenticatedUserId));
+	return AuthenticatedUserIsSuperuser;
+}
+
 
 /*
  * GetUserIdAndSecContext/SetUserIdAndSecContext - get/set the current user ID
@@ -889,28 +899,12 @@ system_user(PG_FUNCTION_ARGS)
 /*
  * Change session auth ID while running
  *
- * Only a superuser may set auth ID to something other than himself.  Note
- * that in case of multiple SETs in a single session, the original userid's
- * superuserness is what matters.  But we set the GUC variable is_superuser
- * to indicate whether the *current* session userid is a superuser.
- *
- * Note: this is not an especially clean place to do the permission check.
- * It's OK because the check does not require catalog access and can't
- * fail during an end-of-transaction GUC reversion, but we may someday
- * have to push it up into assign_session_authorization.
+ * Note that we set the GUC variable is_superuser to indicate whether the
+ * current role is a superuser.
  */
 void
 SetSessionAuthorization(Oid userid, bool is_superuser)
 {
-	/* Must have authenticated already, else can't make permission check */
-	Assert(OidIsValid(AuthenticatedUserId));
-
-	if (userid != AuthenticatedUserId &&
-		!AuthenticatedUserIsSuperuser)
-		ereport(ERROR,
-				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-				 errmsg("permission denied to set session authorization")));
-
 	SetSessionUserId(userid, is_superuser);
 
 	SetConfigOption("is_superuser",

src/include/miscadmin.h

@@ -357,6 +357,7 @@ extern Oid	GetUserId(void);
 extern Oid	GetOuterUserId(void);
 extern Oid	GetSessionUserId(void);
 extern Oid	GetAuthenticatedUserId(void);
+extern bool GetAuthenticatedUserIsSuperuser(void);
 extern void GetUserIdAndSecContext(Oid *userid, int *sec_context);
 extern void SetUserIdAndSecContext(Oid userid, int sec_context);
 extern bool InLocalUserIdChange(void);