Censor

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.