Implementation Details about Changesets Evolution

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For a user perspective have a look at the ChangesetEvolution page.

1. Obsstore Format

Markers are stored in an append-only file stored in '.hg/store/obsstore'.

(see in line document for latest data)

quick summary:

<number-of-successors(=N)><metadata-lenght(=M)><bits-field><precursor>(<successor>*N)<metadata>
B, I, B, 20s, (20s*N), s*M

The file starts with a version header:

The header is followed by the markers. Each marker is made of:

1 unsigned byte: number of new changesets "N", can be zero.
1 unsigned 32-bits integer: metadata size "M" in bytes.
1 byte: a bit field. It is reserved for flags used in common
- obsolete marker operations, to avoid repeated decoding of metadata entries.
20 bytes: obsoleted changeset identifier.
N*20 bytes: new changesets identifiers.
M bytes: metadata as a sequence of nul-terminated strings. Each
- string contains a key and a value, separated by a colon ':', without additional encoding. Keys cannot contain '\0' or ':' and values cannot contain '\0'.

There is two extra information we would like to see in a second version of the format:

date: There is currently *always* a date in the meta data. So storing it explicitly would be more space efficient. It would also open the way to quickly access the date for sorting purpose (no use case yet but not crazy to think about it)
parents: When a changesets is pruned (obsoleted, no successors) we needs to records its parents. This is necessary to link the markers chain to the push/pull operation it is relevant to.