Patch guard support?

[xmo-odoo]

Mercurial Queues have an extremely useful feature when trying to maintain a patch queue for e.g. multiple base branches: patch guards.

Basically, each patch can be "tagged" positively or negatively, then qselect can be used to enable (or disable) guards.

Tags which have a positive guard which is not enabled, or a negative guard which is, will not be applied on push. This is rather useful when maintaning a number of patches against multiple branches but some of those patches only apply to a subset e.g. they patch features which have since been removed, or patch features recently added.

Is there anything similar in StGit currently, or being planned? Or a way to handle the general use case of "multiple patch queues which are largely identical but have some minor divergences"?

[xmo-odoo]

Complement I wasn't aware of when I created this issue: the quilt distribution provides a guards(1) utility which processes "guarded" files, though I don't quite understand its format it seems different than mq's, and it's unclear whether that's intended as a "static" preprocessor or can be integrated to quilt the way mq does.

[jpgrayson]

With the latest change to format StGit's stack metadata in JSON instead of the prior custom format (7641796), it becomes a bit more straightforward to capture patch guards in the stack metadata.

For example:

{
  "version": 5,
  "prev": "113d9bb567523ae0fd4355534d4131da2b262c9f",
  "head": "5369f4cdf8cd0e516becab39ec559e36fbd8e73a",
  "applied": [
    "p0"
  ],
  "unapplied": [
    "p1"
  ],
  "hidden": [],
  "patches": {
    "p0": {
      "oid": "a2f7228037db176a0465d963699415b761857407"
    },
    "p1": {
      "oid": "fa0ccc54091a6405a20bcc803209b689e82a3959"
    }
  }
}

In this new format, each patch has a dict which can be used to associate arbitrary metadata with each patch. Currently we only associate the object id of the patch's commit, but we could easily extend to having a "guards" list for each patch.

At some point in the future I intend to look at implementing patch guards.