[PATCH] help: document sharing of revlog header with revision 0
gregory.szorc at gmail.com
Sat Mar 19 18:17:50 EDT 2016
# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1458425853 25200
# Sat Mar 19 15:17:33 2016 -0700
# Node ID eab3e3960745c92f5d29c80ea1d456a28c440009
# Parent 1f3d9fe592151d4eab21282e87628ef655c67daf
help: document sharing of revlog header with revision 0
The previous docs were incorrect about there being a discrete header
diff --git a/mercurial/help/internals/revlogs.txt b/mercurial/help/internals/revlogs.txt
@@ -30,9 +30,10 @@ used to mean *does not exist* or *not de
A revlog begins with a 32-bit big endian integer holding version info
-and feature flags.
+and feature flags. This integer is shared with the first revision
This integer is logically divided into 2 16-bit shorts. The least
significant half of the integer is the format/version short. The other
short holds feature flags that dictate behavior of the revlog.
@@ -69,10 +70,12 @@ 00 02 00 01
RevlogNG + generaldelta
00 03 00 01
RevlogNG + inline + generaldelta
-Following the 32-bit header is *index* data. Inlined revision data is possibly
-located between index entries. More on this layout is described below.
+Following the 32-bit header is the remaining of the first index entry.
+Following that are remaining *index* data. Inlined revision data is
+possibly located between index entries. More on this layout is described
@@ -82,8 +85,10 @@ or between index entries (as opposed to
Each index entry is 64 bytes. The byte layout of each entry is as
follows, with byte 0 being the first byte (all data stored as big endian):
+0-3 (4 bytes) (rev 0 only)
+ Revlog header
0-5 (6 bytes)
Absolute offset of revision data from beginning of revlog.
6-7 (2 bytes)
Bit flags impacting revision behavior.
@@ -119,8 +124,11 @@ is no padding between it and the index e
If revision data is not inline, then raw revision data is stored in a
separate byte container. The offsets from bytes 0-5 and the compressed
length from bytes 8-11 define how to access this data.
+The first 4 bytes of the revlog are shared between the revlog header
+and the 6 byte absolute offset field from the first revlog entry.
Revision data is encoded as a chain of *chunks*. Each chain begins with
@@ -189,5 +197,5 @@ hash of a revision:
1. Hash the parent nodes
2. Hash the fulltext of the revision
-The 20 byte node ids of the parents are fed into the hasher in ascending order.
\ No newline at end of file
+The 20 byte node ids of the parents are fed into the hasher in ascending order.
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