view contrib/plan9/README @ 27142:060f83d219b9

extensions: refuse to load extensions if minimum hg version not met

As the author of several 3rd party extensions, I frequently see bug
reports from users attempting to run my extension with an old version
of Mercurial that I no longer support in my extension. Oftentimes, the
extension will import just fine. But as soon as we run extsetup(),
reposetup(), or get into the guts of a wrapped function, we encounter
an exception and abort. Today, Mercurial will print a message about
extensions that don't have a "testedwith" declaring explicit
compatibility with the current version.

The existing mechanism is a good start. But it isn't as robust as I
would like. Specifically, Mercurial assumes compatibility by default.
This means extension authors must perform compatibility checking in
their extsetup() or we wait and see if we encounter an abort at
runtime. And, compatibility checking can involve a lot of code and
lots of error checking. It's a lot of effort for extension authors.

Oftentimes, extension authors know which versions of Mercurial there
extension works on and more importantly where it is broken.

This patch introduces a magic "minimumhgversion" attribute in
extensions. When found, the extension loading mechanism will compare
the declared version against the current Mercurial version. If the
extension explicitly states we require a newer Mercurial version, a
warning is printed and the extension isn't loaded beyond importing
the Python module. This causes a graceful failure while alerting
the user of the compatibility issue.

I would be receptive to the idea of making the failure more fatal.
However, care would need to be taken to not criple every hg command.
e.g. the user may use `hg config` to fix the hgrc and if we aborted
trying to run that, the user would effectively be locked out of `hg`!

A potential future improvement to this functionality would be to catch
ImportError for the extension/module and parse the source code for
"minimumhgversion = 'XXX'" and do similar checking. This way we could
give more information about why the extension failed to load.

Mercurial for Plan 9 from Bell Labs
===================================

This directory contains support for Mercurial on Plan 9 from Bell Labs
platforms. It is assumed that the version of Python running on these
systems supports the ANSI/POSIX Environment (APE). At the time of this
writing, the bichued/python port is the most commonly installed version
of Python on these platforms. If a native port of Python is ever made,
some minor modification will need to be made to support some of the more
esoteric requirements of the platform rather than those currently made
(cf. posix.py).

By default, installations will have the factotum extension enabled; this
extension permits factotum(4) to act as an authentication agent for
HTTP repositories. Additionally, an extdiff command named 9diff is
enabled which generates diff(1) compatible output suitable for use with
the plumber(4).

Commit messages are plumbed using E if no editor is defined; users must
update the plumbed file to continue, otherwise the hg process must be
interrupted.

Some work remains with regard to documentation. Section 5 manual page
references for hgignore and hgrc need to be re-numbered to section 6 (file
formats) and a new man page writer should be written to support the
Plan 9 man macro set. Until these issues can be resolved, manual pages
are elided from the installation.

Basic install:

  % mk install      # do a system-wide install
  % hg debuginstall # sanity-check setup
  % hg              # see help

A proto(2) file is included in this directory as an example of how a
binary distribution could be packaged, ostensibly with contrib(1).

See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.