Remove unnecessary overhead in backend's large-object operations.

Do read/write permissions checks at most once per large object descriptor,
not once per lo_read or lo_write call as before.  The repeated tests were
quite useless in the read case since the snapshot-based tests were
guaranteed to produce the same answer every time.  In the write case,
the extra tests could in principle detect revocation of write privileges
after a series of writes has started --- but there's a race condition there
anyway, since we'd check privileges before performing and certainly before
committing the write.  So there's no real advantage to checking every
single time, and we might as well redefine it as "only check the first
time".

On the same reasoning, remove the LargeObjectExists checks in inv_write
and inv_truncate.  We already checked existence when the descriptor was
opened, and checking again doesn't provide any real increment of safety
that would justify the cost.

2 files changed, 78 insertions, 85 deletions

diff --git a/src/backend/libpq/be-fsstubs.c b/src/backend/libpq/be-fsstubs.c
index c4ac1a650f5..dbc00b450d1 100644
--- a/src/backend/libpq/be-fsstubs.c
+++ b/src/backend/libpq/be-fsstubs.c
@@ -157,24 +157,32 @@ int
 lo_read(int fd, char *buf, int len)
 {
 	int			status;
+	LargeObjectDesc *lobj;
 
 	if (fd < 0 || fd >= cookies_size || cookies[fd] == NULL)
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_OBJECT),
 				 errmsg("invalid large-object descriptor: %d", fd)));
+	lobj = cookies[fd];
 
-	/* Permission checks */
-	if (!lo_compat_privileges &&
-		pg_largeobject_aclcheck_snapshot(cookies[fd]->id,
-										 GetUserId(),
-										 ACL_SELECT,
-									   cookies[fd]->snapshot) != ACLCHECK_OK)
-		ereport(ERROR,
-				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-				 errmsg("permission denied for large object %u",
-						cookies[fd]->id)));
+	/* We don't bother to check IFS_RDLOCK, since it's always set */
+
+	/* Permission checks --- first time through only */
+	if ((lobj->flags & IFS_RD_PERM_OK) == 0)
+	{
+		if (!lo_compat_privileges &&
+			pg_largeobject_aclcheck_snapshot(lobj->id,
+											 GetUserId(),
+											 ACL_SELECT,
+											 lobj->snapshot) != ACLCHECK_OK)
+			ereport(ERROR,
+					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+					 errmsg("permission denied for large object %u",
+							lobj->id)));
+		lobj->flags |= IFS_RD_PERM_OK;
+	}
 
-	status = inv_read(cookies[fd], buf, len);
+	status = inv_read(lobj, buf, len);
 
 	return status;
 }
@@ -183,30 +191,36 @@ int
 lo_write(int fd, const char *buf, int len)
 {
 	int			status;
+	LargeObjectDesc *lobj;
 
 	if (fd < 0 || fd >= cookies_size || cookies[fd] == NULL)
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_OBJECT),
 				 errmsg("invalid large-object descriptor: %d", fd)));
+	lobj = cookies[fd];
 
-	if ((cookies[fd]->flags & IFS_WRLOCK) == 0)
+	if ((lobj->flags & IFS_WRLOCK) == 0)
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 			  errmsg("large object descriptor %d was not opened for writing",
 					 fd)));
 
-	/* Permission checks */
-	if (!lo_compat_privileges &&
-		pg_largeobject_aclcheck_snapshot(cookies[fd]->id,
-										 GetUserId(),
-										 ACL_UPDATE,
-									   cookies[fd]->snapshot) != ACLCHECK_OK)
-		ereport(ERROR,
-				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-				 errmsg("permission denied for large object %u",
-						cookies[fd]->id)));
+	/* Permission checks --- first time through only */
+	if ((lobj->flags & IFS_WR_PERM_OK) == 0)
+	{
+		if (!lo_compat_privileges &&
+			pg_largeobject_aclcheck_snapshot(lobj->id,
+											 GetUserId(),
+											 ACL_UPDATE,
+											 lobj->snapshot) != ACLCHECK_OK)
+			ereport(ERROR,
+					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+					 errmsg("permission denied for large object %u",
+							lobj->id)));
+		lobj->flags |= IFS_WR_PERM_OK;
+	}
 
-	status = inv_write(cookies[fd], buf, len);
+	status = inv_write(lobj, buf, len);
 
 	return status;
 }
@@ -230,8 +244,8 @@ lo_lseek(PG_FUNCTION_ARGS)
 	if (status != (int32) status)
 		ereport(ERROR,
 				(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
-				 errmsg("lo_lseek result out of range for large-object descriptor %d",
-						fd)));
+		errmsg("lo_lseek result out of range for large-object descriptor %d",
+			   fd)));
 
 	PG_RETURN_INT32((int32) status);
 }
@@ -303,8 +317,8 @@ lo_tell(PG_FUNCTION_ARGS)
 	if (offset != (int32) offset)
 		ereport(ERROR,
 				(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
-				 errmsg("lo_tell result out of range for large-object descriptor %d",
-						fd)));
+		 errmsg("lo_tell result out of range for large-object descriptor %d",
+				fd)));
 
 	PG_RETURN_INT32((int32) offset);
 }
@@ -558,30 +572,48 @@ lo_export(PG_FUNCTION_ARGS)
  * lo_truncate -
  *	  truncate a large object to a specified length
  */
-Datum
-lo_truncate(PG_FUNCTION_ARGS)
+static void
+lo_truncate_internal(int32 fd, int64 len)
 {
-	int32		fd = PG_GETARG_INT32(0);
-	int32		len = PG_GETARG_INT32(1);
+	LargeObjectDesc *lobj;
 
 	if (fd < 0 || fd >= cookies_size || cookies[fd] == NULL)
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_OBJECT),
 				 errmsg("invalid large-object descriptor: %d", fd)));
+	lobj = cookies[fd];
 
-	/* Permission checks */
-	if (!lo_compat_privileges &&
-		pg_largeobject_aclcheck_snapshot(cookies[fd]->id,
-										 GetUserId(),
-										 ACL_UPDATE,
-									   cookies[fd]->snapshot) != ACLCHECK_OK)
+	if ((lobj->flags & IFS_WRLOCK) == 0)
 		ereport(ERROR,
-				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-				 errmsg("permission denied for large object %u",
-						cookies[fd]->id)));
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+			  errmsg("large object descriptor %d was not opened for writing",
+					 fd)));
+
+	/* Permission checks --- first time through only */
+	if ((lobj->flags & IFS_WR_PERM_OK) == 0)
+	{
+		if (!lo_compat_privileges &&
+			pg_largeobject_aclcheck_snapshot(lobj->id,
+											 GetUserId(),
+											 ACL_UPDATE,
+											 lobj->snapshot) != ACLCHECK_OK)
+			ereport(ERROR,
+					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+					 errmsg("permission denied for large object %u",
+							lobj->id)));
+		lobj->flags |= IFS_WR_PERM_OK;
+	}
 
-	inv_truncate(cookies[fd], len);
+	inv_truncate(lobj, len);
+}
 
+Datum
+lo_truncate(PG_FUNCTION_ARGS)
+{
+	int32		fd = PG_GETARG_INT32(0);
+	int32		len = PG_GETARG_INT32(1);
+
+	lo_truncate_internal(fd, len);
 	PG_RETURN_INT32(0);
 }
 
@@ -591,24 +623,7 @@ lo_truncate64(PG_FUNCTION_ARGS)
 	int32		fd = PG_GETARG_INT32(0);
 	int64		len = PG_GETARG_INT64(1);
 
-	if (fd < 0 || fd >= cookies_size || cookies[fd] == NULL)
-		ereport(ERROR,
-				(errcode(ERRCODE_UNDEFINED_OBJECT),
-				 errmsg("invalid large-object descriptor: %d", fd)));
-
-	/* Permission checks */
-	if (!lo_compat_privileges &&
-		pg_largeobject_aclcheck_snapshot(cookies[fd]->id,
-										 GetUserId(),
-										 ACL_UPDATE,
-									   cookies[fd]->snapshot) != ACLCHECK_OK)
-		ereport(ERROR,
-				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-				 errmsg("permission denied for large object %u",
-						cookies[fd]->id)));
-
-	inv_truncate(cookies[fd], len);
-
+	lo_truncate_internal(fd, len);
 	PG_RETURN_INT32(0);
 }
 
diff --git a/src/backend/storage/large_object/inv_api.c b/src/backend/storage/large_object/inv_api.c
index 5dd17823c23..ad8424b9bfc 100644
--- a/src/backend/storage/large_object/inv_api.c
+++ b/src/backend/storage/large_object/inv_api.c
@@ -420,8 +420,8 @@ inv_seek(LargeObjectDesc *obj_desc, int64 offset, int whence)
 	if (newoffset < 0 || newoffset > MAX_LARGE_OBJECT_SIZE)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg_internal("invalid large object seek target: " INT64_FORMAT,
-								 newoffset)));
+		   errmsg_internal("invalid large object seek target: " INT64_FORMAT,
+						   newoffset)));
 
 	obj_desc->offset = newoffset;
 	return newoffset;
@@ -562,18 +562,7 @@ inv_write(LargeObjectDesc *obj_desc, const char *buf, int nbytes)
 	Assert(buf != NULL);
 
 	/* enforce writability because snapshot is probably wrong otherwise */
-	if ((obj_desc->flags & IFS_WRLOCK) == 0)
-		ereport(ERROR,
-				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-				 errmsg("large object %u was not opened for writing",
-						obj_desc->id)));
-
-	/* check existence of the target largeobject */
-	if (!LargeObjectExists(obj_desc->id))
-		ereport(ERROR,
-				(errcode(ERRCODE_UNDEFINED_OBJECT),
-				 errmsg("large object %u was already dropped",
-						obj_desc->id)));
+	Assert(obj_desc->flags & IFS_WRLOCK);
 
 	if (nbytes <= 0)
 		return 0;
@@ -767,18 +756,7 @@ inv_truncate(LargeObjectDesc *obj_desc, int64 len)
 	Assert(PointerIsValid(obj_desc));
 
 	/* enforce writability because snapshot is probably wrong otherwise */
-	if ((obj_desc->flags & IFS_WRLOCK) == 0)
-		ereport(ERROR,
-				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-				 errmsg("large object %u was not opened for writing",
-						obj_desc->id)));
-
-	/* check existence of the target largeobject */
-	if (!LargeObjectExists(obj_desc->id))
-		ereport(ERROR,
-				(errcode(ERRCODE_UNDEFINED_OBJECT),
-				 errmsg("large object %u was already dropped",
-						obj_desc->id)));
+	Assert(obj_desc->flags & IFS_WRLOCK);
 
 	/*
 	 * use errmsg_internal here because we don't want to expose INT64_FORMAT