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/extensions.py

@@ -101,6 +101,17 b' def load(ui, name, path):'
             if ui.debugflag:
                 ui.traceback()
             mod = importh(name)
+
+    # Before we do anything with the extension, check against minimum stated
+    # compatibility. This gives extension authors a mechanism to have their
+    # extensions short circuit when loaded with a known incompatible version
+    # of Mercurial.
+    minver = getattr(mod, 'minimumhgversion', None)
+    if minver and util.versiontuple(minver, 2) > util.versiontuple(n=2):
+        ui.warn(_('(third party extension %s requires version %s or newer '
+                  'of Mercurial; disabling)\n') % (shortname, minver))
+        return
+
     _extensions[shortname] = mod
     _order.append(shortname)
     for fn in _aftercallbacks.get(shortname, []):

tests/test-extension.t

@@ -1009,6 +1009,50 b" Test version number support in 'hg versi"
   
     throw  1.twentythree
 
+Refuse to load extensions with minimum version requirements
+
+  $ cat > minversion1.py << EOF
+  > from mercurial import util
+  > util.version = lambda: '3.5.2'
+  > minimumhgversion = '3.6'
+  > EOF
+  $ hg --config extensions.minversion=minversion1.py version
+  (third party extension minversion requires version 3.6 or newer of Mercurial; disabling)
+  Mercurial Distributed SCM (version 3.5.2)
+  (see https://mercurial-scm.org for more information)
+  
+  Copyright (C) 2005-* Matt Mackall and others (glob)
+  This is free software; see the source for copying conditions. There is NO
+  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+
+  $ cat > minversion2.py << EOF
+  > from mercurial import util
+  > util.version = lambda: '3.6'
+  > minimumhgversion = '3.7'
+  > EOF
+  $ hg --config extensions.minversion=minversion2.py version 2>&1 | egrep '\(third'
+  (third party extension minversion requires version 3.7 or newer of Mercurial; disabling)
+
+Can load version that is only off by point release
+
+  $ cat > minversion2.py << EOF
+  > from mercurial import util
+  > util.version = lambda: '3.6.1'
+  > minimumhgversion = '3.6'
+  > EOF
+  $ hg --config extensions.minversion=minversion3.py version 2>&1 | egrep '\(third'
+  [1]
+
+Can load minimum version identical to current
+
+  $ cat > minversion3.py << EOF
+  > from mercurial import util
+  > util.version = lambda: '3.5'
+  > minimumhgversion = '3.5'
+  > EOF
+  $ hg --config extensions.minversion=minversion3.py version 2>&1 | egrep '\(third'
+  [1]
+
 Restore HGRCPATH
 
   $ HGRCPATH=$ORGHGRCPATH