help: document sharing of revlog header with revision 0

The previous docs were incorrect about there being a discrete header
on revlogs.

mercurial/help/internals/revlogs.txt

@@ -1,193 +1,201 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.
+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.
 
 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.
+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.
 
 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):
 
+0-3 (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.
 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.
 
+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
 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
+The 20 byte node ids of the parents are fed into the hasher in ascending order.