diff -r e340b556a13e -r b876f0bf7366 mercurial/revlogutils/constants.py
--- a/mercurial/revlogutils/constants.py	Mon May 03 20:22:34 2021 +0200
+++ b/mercurial/revlogutils/constants.py	Mon May 03 19:46:25 2021 +0200
@@ -119,6 +119,10 @@
 # These constants are used in revlog version >=2 to denote the compression used
 # for a chunk.
 
+# Chunk use no compression, the data stored on disk can be directly use as
+# chunk value. Without any header information prefixed.
+COMP_MODE_PLAIN = 0
+
 # Chunk use a compression mode stored "inline" at the start of the chunk
 # itself.  This is the mode always used for revlog version "0" and "1"
 COMP_MODE_INLINE = 2