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Differential Revision: https://phab.mercurial-scm.org/D6413

Revision logs - or *revlogs* - are an append only data structure for
storing discrete entries, or *revisions*. They are the primary storage
mechanism of repository data.

Revlogs effectively model a directed acyclic graph (DAG). Each node
has edges to 1 or 2 *parent* nodes. Each node contains metadata and
the raw value for that node.

Revlogs consist of entries which have metadata and revision data.
Metadata includes the hash of the revision's content, sizes, and
links to its *parent* entries. The collective metadata is referred
to as the *index* and the revision data is the *data*.

Revision data is stored as a series of compressed deltas against
ancestor revisions.

Revlogs are written in an append-only fashion. We never need to rewrite
a file to insert nor do we need to remove data. Rolling back in-progress
writes can be performed by truncating files. Read locks can be avoided
using simple techniques. This means that references to other data in
the same revlog *always* refer to a previous entry.

Revlogs can be modeled as 0-indexed arrays. The first revision is
revision #0 and the second is revision #1. The revision -1 is typically
used to mean *does not exist* or *not defined*.

File Format
===========

A revlog begins with a 32-bit big endian integer holding version info
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.

The following values for the format/version short are defined:

0
   The original revlog version.
1
   RevlogNG (*next generation*). It replaced version 0 when it was
   implemented in 2006.
2
   In-development version incorporating accumulated knowledge and
   missing features from 10+ years of revlog version 1.
57005 (0xdead)
   Reserved for internal testing of new versions. No defined format
   beyond 32-bit header.

The feature flags short consists of bit flags. Where 0 is the least
significant bit. The bit flags vary by revlog version.

Version 0 revlogs have no defined flags and the presence of a flag
is considered an error.

Version 1 revlogs have the following flags at the specified bit offsets:

0
   Store revision data inline.
1
   Generaldelta encoding.

Version 2 revlogs have the following flags at the specified bit offsets:

0
   Store revision data inline.

The following header values are common:

00 00 00 01
   v1
00 01 00 01
   v1 + inline
00 02 00 01
   v1 + generaldelta
00 03 00 01
   v1 + inline + generaldelta

Following the 32-bit header is the remainder 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.

Version 1 Format
================

Version 1 (RevlogNG) begins with an index describing the revisions in
the revlog. If the ``inline`` flag is set, revision data is stored inline,
or between index entries (as opposed to in a separate container).

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. The following bit offsets define:

   0: REVIDX_ISCENSORED revision has censor metadata, must be verified.

   1: REVIDX_ELLIPSIS revision hash does not match its data. Used by
   narrowhg

   2: REVIDX_EXTSTORED revision data is stored externally.

8-11 (4 bytes)
   Compressed length of revision data / chunk as stored in revlog.

12-15 (4 bytes)
   Uncompressed length of revision data. This is the size of the full
   revision data, not the size of the chunk post decompression.

16-19 (4 bytes)
   Base or previous revision this revision's delta was produced against.
   This revision holds full text (as opposed to a delta) if it points to
   itself. For generaldelta repos, this is the previous revision in the
   delta chain. For non-generaldelta repos, this is the base or first
   revision in the delta chain.

20-23 (4 bytes)
   A revision this revision is *linked* to. This allows a revision in
   one revlog to be forever associated with a revision in another
   revlog. For example, a file's revlog may point to the changelog
   revision that introduced it.

24-27 (4 bytes)
   Revision of 1st parent. -1 indicates no parent.

28-31 (4 bytes)
   Revision of 2nd parent. -1 indicates no 2nd parent.

32-63 (32 bytes)
   Hash of revision's full text. Currently, SHA-1 is used and only
   the first 20 bytes of this field are used. The rest of the bytes
   are ignored and should be stored as \0.

If inline revision data is being stored, the compressed revision data
(of length from bytes offset 8-11 from the index entry) immediately
follows the index entry. There is no header on the revision data. There
is no padding between it and the index entries before and after.

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.

Version 2 Format
================

(In development. Format not finalized or stable.)

Version 2 is identical to version 2 with the following differences.

There is no dedicated *generaldelta* revlog format flag. Instead,
the feature is implied enabled by default.

Delta Chains
============

Revision data is encoded as a chain of *chunks*. Each chain begins with
the compressed original full text for that revision. Each subsequent
*chunk* is a *delta* against the previous revision. We therefore call
these chains of chunks/deltas *delta chains*.

The full text for a revision is reconstructed by loading the original
full text for the base revision of a *delta chain* and then applying
*deltas* until the target revision is reconstructed.

*Delta chains* are limited in length so lookup time is bound. They are
limited to ~2x the length of the revision's data. The linear distance
between the base chunk and the final chunk is also limited so the
amount of read I/O to load all chunks in the delta chain is bound.

Deltas and delta chains are either computed against the previous
revision in the revlog or another revision (almost certainly one of
the parents of the revision). Historically, deltas were computed against
the previous revision. The *generaldelta* revlog feature flag (enabled
by default in Mercurial 3.7) activates the mode where deltas are
computed against an arbitrary revision (almost certainly a parent revision).

File Storage
============

Revlogs logically consist of an index (metadata of entries) and
revision data. This data may be stored together in a single file or in
separate files. The mechanism used is indicated by the ``inline`` feature
flag on the revlog.

Mercurial's behavior is to use inline storage until a revlog reaches a
certain size, at which point it will be converted to non-inline. The
reason there is a size limit on inline storage is to establish an upper
bound on how much data must be read to load the index. It would be a waste
to read tens or hundreds of extra megabytes of data just to access the
index data.

The actual layout of revlog files on disk is governed by the repository's
*store format*. Typically, a ``.i`` file represents the index revlog
(possibly containing inline data) and a ``.d`` file holds the revision data.

Revision Entries
================

Revision entries consist of an optional 1 byte header followed by an
encoding of the revision data. The headers are as follows:

\0 (0x00)
   Revision data is the entirety of the entry, including this header.
u (0x75)
   Raw revision data follows.
x (0x78)
   zlib (RFC 1950) data.

   The 0x78 value is actually the first byte of the zlib header (CMF byte).

Hash Computation
================

The hash of the revision is stored in the index and is used both as a primary
key and for data integrity verification.

Currently, SHA-1 is the only supported hashing algorithm. To obtain the SHA-1
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.