view mercurial/help/internals/censor.txt @ 39558:b0e0db1565d1

internals: extract frame-based protocol docs to own document

wireprotocol.txt is quite long and difficult to digest. The
frame-based protocol is effectively a standalone concept (and could
even be used outside of Mercurial). So this commit extracts its
docs to a standalone file.

The first few paragraphs were rewritten as part of the extraction.
Sections headers were adjusted accordingly.

Existing referalls in wireprotocol.txt were updated to refer to the
new doc / concept, which I've started referring to as `hgrpc`.

I'm on the fence as to whether to move the HTTP and SSH transport
details to the new doc as well. For now, I'm leaving them in
wireprotocol.txt.

Differential Revision: https://phab.mercurial-scm.org/D4443

The censor system allows retroactively removing content from
files. Actually censoring a node requires using the censor extension,
but the functionality for handling censored nodes is partially in core.

Censored nodes in a filelog have the flag ``REVIDX_ISCENSORED`` set,
and the contents of the censored node are replaced with a censor
tombstone. For historical reasons, the tombstone is packed in the
filelog metadata field ``censored``. This allows censored nodes to be
(mostly) safely transmitted through old formats like changegroup
versions 1 and 2. When using changegroup formats older than 3, the
receiver is required to re-add the ``REVIDX_ISCENSORED`` flag when
storing the revision. This depends on the ``censored`` metadata key
never being used for anything other than censoring revisions, which is
true as of January 2017. Note that the revlog flag is the
authoritative marker of a censored node: the tombstone should only be
consulted when looking for a reason a node was censored or when revlog
flags are unavailable as mentioned above.

The tombstone data is a free-form string. It's expected that users of
censor will want to record the reason for censoring a node in the
tombstone. Censored nodes must be able to fit in the size of the
content being censored.