Add large object functions catering to SQL callers.

With these, one need no longer manipulate large object descriptors and
extract numeric constants from header files in order to read and write
large object contents from SQL.

Pavel Stehule, reviewed by Rushabh Lathia.

src/backend/libpq/be-fsstubs.c

@@ -754,3 +754,154 @@ deleteLOfd(int fd)
 {
 	cookies[fd] = NULL;
 }
+
+/*****************************************************************************
+ *	Wrappers oriented toward SQL callers
+ *****************************************************************************/
+
+/*
+ * Read [offset, offset+nbytes) within LO; when nbytes is -1, read to end.
+ */
+static bytea *
+lo_get_fragment_internal(Oid loOid, int64 offset, int32 nbytes)
+{
+	LargeObjectDesc *loDesc;
+	int64		loSize;
+	int64		result_length;
+	int			total_read PG_USED_FOR_ASSERTS_ONLY;
+	bytea	   *result = NULL;
+
+	/*
+	 * We don't actually need to store into fscxt, but create it anyway to
+	 * ensure that AtEOXact_LargeObject knows there is state to clean up
+	 */
+	CreateFSContext();
+
+	loDesc = inv_open(loOid, INV_READ, fscxt);
+
+	/* Permission check */
+	if (!lo_compat_privileges &&
+		pg_largeobject_aclcheck_snapshot(loDesc->id,
+										 GetUserId(),
+										 ACL_SELECT,
+										 loDesc->snapshot) != ACLCHECK_OK)
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 errmsg("permission denied for large object %u",
+						loDesc->id)));
+
+	/*
+	 * Compute number of bytes we'll actually read, accommodating nbytes == -1
+	 * and reads beyond the end of the LO.
+	 */
+	loSize = inv_seek(loDesc, 0, SEEK_END);
+	if (loSize > offset)
+	{
+		if (nbytes >= 0 && nbytes <= loSize - offset)
+			result_length = nbytes;		/* request is wholly inside LO */
+		else
+			result_length = loSize - offset;	/* adjust to end of LO */
+	}
+	else
+		result_length = 0;		/* request is wholly outside LO */
+
+	/*
+	 * A result_length calculated from loSize may not fit in a size_t.  Check
+	 * that the size will satisfy this and subsequently-enforced size limits.
+	 */
+	if (result_length > MaxAllocSize - VARHDRSZ)
+		ereport(ERROR,
+				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
+				 errmsg("large object read request is too large")));
+
+	result = (bytea *) palloc(VARHDRSZ + result_length);
+
+	inv_seek(loDesc, offset, SEEK_SET);
+	total_read = inv_read(loDesc, VARDATA(result), result_length);
+	Assert(total_read == result_length);
+	SET_VARSIZE(result, result_length + VARHDRSZ);
+
+	inv_close(loDesc);
+
+	return result;
+}
+
+/*
+ * Read entire LO
+ */
+Datum
+lo_get(PG_FUNCTION_ARGS)
+{
+	Oid			loOid = PG_GETARG_OID(0);
+	bytea	   *result;
+
+	result = lo_get_fragment_internal(loOid, 0, -1);
+
+	PG_RETURN_BYTEA_P(result);
+}
+
+/*
+ * Read range within LO
+ */
+Datum
+lo_get_fragment(PG_FUNCTION_ARGS)
+{
+	Oid			loOid = PG_GETARG_OID(0);
+	int64		offset = PG_GETARG_INT64(1);
+	int32		nbytes = PG_GETARG_INT32(2);
+	bytea	   *result;
+
+	if (nbytes < 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("requested length cannot be negative")));
+
+	result = lo_get_fragment_internal(loOid, offset, nbytes);
+
+	PG_RETURN_BYTEA_P(result);
+}
+
+/*
+ * Create LO with initial contents
+ */
+Datum
+lo_create_bytea(PG_FUNCTION_ARGS)
+{
+	Oid			loOid = PG_GETARG_OID(0);
+	bytea	   *str = PG_GETARG_BYTEA_PP(1);
+	LargeObjectDesc *loDesc;
+	int			written PG_USED_FOR_ASSERTS_ONLY;
+
+	CreateFSContext();
+
+	loOid = inv_create(loOid);
+	loDesc = inv_open(loOid, INV_WRITE, fscxt);
+	written = inv_write(loDesc, VARDATA_ANY(str), VARSIZE_ANY_EXHDR(str));
+	Assert(written == VARSIZE_ANY_EXHDR(str));
+	inv_close(loDesc);
+
+	PG_RETURN_OID(loOid);
+}
+
+/*
+ * Update range within LO
+ */
+Datum
+lo_put(PG_FUNCTION_ARGS)
+{
+	Oid			loOid = PG_GETARG_OID(0);
+	int64		offset = PG_GETARG_INT64(1);
+	bytea	   *str = PG_GETARG_BYTEA_PP(2);
+	LargeObjectDesc *loDesc;
+	int			written PG_USED_FOR_ASSERTS_ONLY;
+
+	CreateFSContext();
+
+	loDesc = inv_open(loOid, INV_WRITE, fscxt);
+	inv_seek(loDesc, offset, SEEK_SET);
+	written = inv_write(loDesc, VARDATA_ANY(str), VARSIZE_ANY_EXHDR(str));
+	Assert(written == VARSIZE_ANY_EXHDR(str));
+	inv_close(loDesc);
+
+	PG_RETURN_VOID();
+}