[PATCH] help: document sharing of revlog header with revision 0

Gregory Szorc 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
on revlogs.

diff --git a/mercurial/help/internals/revlogs.txt b/mercurial/help/internals/revlogs.txt
--- 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
 File Format
 -----------
 
 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
+entry.
 
 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
+below.
 
 RevlogNG Format
 ---------------
 
@@ -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.
+
 Delta Chains
 ------------
 
 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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