~bzr-pqm/bzr/bzr.dev

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Extension to store annotations

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We might extend the revfile format in a future version to also store

annotations.  *This is not implemented yet.*

In previous versions, the  index file identified texts by their

SHA-1 digest.  This was unsatisfying for two reasons.  Firstly it

assumes that SHA-1 will not collide, which is not an assumption we

wish to make in long-lived files.  Secondly for annotations we need

to be able to map from file versions back to a revision.

Texts are identified by the name of the revfile and a UUID

corresponding to the first revision in which they were first

introduced.  This means that given a text we can identify which

revision it belongs to, and annotations can use the index within the

revfile to identify where a region was first introduced.

  We cannot identify texts by the integer revision number, because

  that would limit us to only referring to a file in a particular

  branch.

  I'd like to just use the revision-id, but those are variable-length

  strings, and I'd like the revfile index to be fixed-length and

  relatively short.  UUIDs can be encoded in binary as only 16 bytes.

  Perhaps we should just use UUIDs for revisions and be done?

Annotations

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Annotations indicate which revision of a file first inserted a line

(or region of bytes).

Given a string, we can write annotations on it like so: a sequence of

*(index, length)* pairs, giving the *index* of the revision which

introduced the next run of *length* bytes.  The sum of the lengths

must equal the length of the string.  For text files the regions will

typically fall on line breaks.  This can be transformed in memory to

other structures, such as a list of *(index, content)* pairs.

When a line was inserted from a merge revision then the annotation for

that line should still be the source in the merged branch, rather than

just being the revision in which the merge took place.

They can cheaply be calculated when inserting a new text, but are

expensive to calculate after the fact because that requires searching

back through all previous text and all texts which were merged in.  It

therefore seems sensible to calculate them once and store them.

To do this we need two operators which update an existing annotated

file:

A. Given an annotated file and a working text, update the annotation to

   mark regions inserted in the working file as new in this revision.

B. Given two annotated files, merge them to produce an annotated

   result.    When there are conflicts, both texts should be included

   and annotated.

These may be repeated: after a merge there may be another merge, or

there may be manual fixups or conflict resolutions.

So what we require is given a diff or a diff3 between two files, map

the regions of bytes changed into corresponding updates to the origin

annotations.

Annotations can also be delta-compressed; we only need to add new

annotation data when there is a text insertion.

    (It is possible in a merge to have a change of annotation when

    there is no text change, though this seems unlikely.  This can

    still be represented as a "pointless" delta, plus an update to the

    annotations.)

Index file

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In a proposed (not implemented) storage with annotations, the index

file is a series of fixed-length records::

  byte[16]     UUID of revision

  byte[20]     SHA-1 of expanded text (as binary, not hex)

  uint32       flags: 1=zlib compressed

  uint32       sequence number this is based on, or -1 for full text

  uint32       offset in text file of start

  uint32       length of compressed delta in text file

  uint32[3]    reserved

Total 64 bytes.

The header is also 64 bytes, for tidyness and easy calculation.  For

this format the header must be ``bzr revfile v2\n`` padded with

``\xff`` to 64 bytes.

The first record after the header is index 0.  A record's base index

must be less than its own index.

The SHA-1 is redundant with the inventory but stored just as a check

on the compression methods and so that the file can be validated

without reference to any other information.

Each byte in the text file should be included by at most one delta.

Deltas

------

In a proposed (not implemented) storage with annotations, deltas to

the text are stored as a series of variable-length records::

  uint32        idx

  uint32        m

  uint32        n

  uint32        l

  byte[l]       new

This describes a change originally introduced in the revision

described by *idx* in the index.

This indicates that the region [m:n] of the input file should be

replaced by the text *new*.  If m==n this is a pure insertion of l

bytes.  If l==0 this is a pure deletion of (n-m) bytes.

Open issues

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* Storing the annotations with the text is reasonably simple and

  compact, but means that we always need to process the annotation

  structure even when we only want the text.  In particular it means

  that full-texts cannot just simply be copied out but rather composed

  from chunks.  That seems inefficient since it is probably common to

  only want the text.

* Should annotations also indicate where text was deleted?

* This design calls for only one annotation per line, which seems

  standard.  However, this is lacking in at least two cases:

  - Lines which originate in the same way in more than one revision,

    through being independently introduced.  In this case we would

    apparently have to make an arbitrary choice; I suppose branches

    could prefer to assume lines originated in their own history.

  - It might be useful to directly indicate which mergers included

    which lines.  We do have that information in the revision history

    though, so there seems no need to store it for every line.