internals: document revlog format

It seems like a good idea to document the revlog format.

There is a lot more that could be added to this documentation.
But you have to start somewhere.

mercurial/help/internals/revlogs.txt

@@ -0,0 +1,193 b''
+Revisions Logs
+==============
+
+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 previous
+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 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.
+
+Only 1 bit of the format/version short is currently used. Remaining
+bits are reserved for future use.
+
+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.
+
+The feature flags short consists of bit flags. Where 0 is the least
+significant bit, the following bit offsets define flags:
+
+0
+   Store revision data inline.
+1
+   Generaldelta encoding.
+
+2-15
+   Reserved for future use.
+
+The following header values are common:
+
+00 00 00 01
+   RevlogNG
+00 01 00 01
+   RevlogNG + inline
+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.
+
+RevlogNG Format
+---------------
+
+RevlogNG (version 1) 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):
+
+0-5 (6 bytes)
+   Absolute offset of revision data from beginning of revlog.
+6-7 (2 bytes)
+   Bit flags impacting revision behavior.
+8-11 (4 bytes)
+   Compressed length of revision data / chunk as stored in revlog.
+12-15 (4 bytes)
+   Uncompressed length of revision data / chunk.
+16-19 (4 bytes)
+   Base or previous revision this revision's delta was produced against.
+   -1 means this revision holds full text (as opposed to a delta).
+   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.
+
+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.
  No newline at end of file

mercurial/help.py

@@ -187,6 +187,8 b' internalstable = sorted(['
      loaddoc('bundles', subdir='internals')),
     (['changegroups'], _('representation of revlog data'),
      loaddoc('changegroups', subdir='internals')),
+    (['revlogs'], _('revision storage mechanism'),
+     loaddoc('revlogs', subdir='internals')),
 ])
 
 def internalshelp(ui):

tests/test-help.t

@@ -875,6 +875,7 b' internals topic renders index of availab'
   
        bundles       container for exchange of repository data
        changegroups  representation of revlog data
+       revlogs       revision storage mechanism
 
 sub-topics can be accessed
 
@@ -2716,6 +2717,13 b' Sub-topic indexes rendered properly'
   </td><td>
   representation of revlog data
   </td></tr>
+  <tr><td>
+  <a href="/help/internals.revlogs">
+  revlogs
+  </a>
+  </td><td>
+  revision storage mechanism
+  </td></tr>
   
   
   

tests/test-install.t

@@ -104,6 +104,7 b' path variables are expanded (~ is the sa'
     help/hgrc.5.txt
     help/internals/bundles.txt
     help/internals/changegroups.txt
+    help/internals/revlogs.txt
   Not tracked:
 
   $ python wixxml.py templates