diff -r 013c645dd28c -r 130c9f7ed914 mercurial/cext/revlog.c
--- a/mercurial/cext/revlog.c	Tue May 04 01:15:03 2021 +0200
+++ b/mercurial/cext/revlog.c	Mon May 03 18:19:16 2021 +0200
@@ -118,9 +118,9 @@
 static int index_find_node(indexObject *self, const char *node);
 
 #if LONG_MAX == 0x7fffffffL
-static const char *const tuple_format = PY23("Kiiiiiis#Ki", "Kiiiiiiy#Ki");
+static const char *const tuple_format = PY23("Kiiiiiis#KiB", "Kiiiiiiy#KiB");
 #else
-static const char *const tuple_format = PY23("kiiiiiis#ki", "kiiiiiiy#ki");
+static const char *const tuple_format = PY23("kiiiiiis#kiB", "kiiiiiiy#kiB");
 #endif
 
 /* A RevlogNG v1 index entry is 64 bytes long. */
@@ -132,6 +132,8 @@
 static const long format_v1 = 1; /* Internal only, could be any number */
 static const long format_v2 = 2; /* Internal only, could be any number */
 
+static const char comp_mode_inline = 2;
+
 static void raise_revlog_error(void)
 {
 	PyObject *mod = NULL, *dict = NULL, *errclass = NULL;
@@ -294,6 +296,7 @@
 	uint64_t offset_flags, sidedata_offset;
 	int comp_len, uncomp_len, base_rev, link_rev, parent_1, parent_2,
 	    sidedata_comp_len;
+	char data_comp_mode;
 	const char *c_node_id;
 	const char *data;
 	Py_ssize_t length = index_length(self);
@@ -340,9 +343,11 @@
 		sidedata_comp_len = getbe32(data + 72);
 	}
 
+	data_comp_mode = comp_mode_inline;
 	return Py_BuildValue(tuple_format, offset_flags, comp_len, uncomp_len,
 	                     base_rev, link_rev, parent_1, parent_2, c_node_id,
-	                     self->nodelen, sidedata_offset, sidedata_comp_len);
+	                     self->nodelen, sidedata_offset, sidedata_comp_len,
+	                     data_comp_mode);
 }
 /*
  * Pack header information in binary
@@ -443,6 +448,7 @@
 {
 	uint64_t offset_flags, sidedata_offset;
 	int rev, comp_len, uncomp_len, base_rev, link_rev, parent_1, parent_2;
+	char data_comp_mode;
 	Py_ssize_t c_node_id_len, sidedata_comp_len;
 	const char *c_node_id;
 	char *data;
@@ -450,8 +456,9 @@
 	if (!PyArg_ParseTuple(obj, tuple_format, &offset_flags, &comp_len,
 	                      &uncomp_len, &base_rev, &link_rev, &parent_1,
 	                      &parent_2, &c_node_id, &c_node_id_len,
-	                      &sidedata_offset, &sidedata_comp_len)) {
-		PyErr_SetString(PyExc_TypeError, "10-tuple required");
+	                      &sidedata_offset, &sidedata_comp_len,
+	                      &data_comp_mode)) {
+		PyErr_SetString(PyExc_TypeError, "11-tuple required");
 		return NULL;
 	}
 
@@ -459,6 +466,12 @@
 		PyErr_SetString(PyExc_TypeError, "invalid node");
 		return NULL;
 	}
+	if (data_comp_mode != comp_mode_inline) {
+		PyErr_Format(PyExc_ValueError,
+		             "invalid data compression mode: %i",
+		             data_comp_mode);
+		return NULL;
+	}
 
 	if (self->new_length == self->added_length) {
 		size_t new_added_length =
@@ -2761,9 +2774,9 @@
 		self->entry_size = v1_entry_size;
 	}
 
-	self->nullentry =
-	    Py_BuildValue(PY23("iiiiiiis#ii", "iiiiiiiy#ii"), 0, 0, 0, -1, -1,
-	                  -1, -1, nullid, self->nodelen, 0, 0);
+	self->nullentry = Py_BuildValue(PY23("iiiiiiis#iiB", "iiiiiiiy#iiB"), 0,
+	                                0, 0, -1, -1, -1, -1, nullid,
+	                                self->nodelen, 0, 0, comp_mode_inline);
 
 	if (!self->nullentry)
 		return -1;