This is needed to make Mercurial read the HGEXTENSIONPATH to detect
third-party extensions.  It is called HGEXTENSIONPATH and not
HG_EXTENSION_PATH to keep it consistent with other environment variables for
Mercurial, e.g. HGENCODINGAMBIGUOUS, HGEDITOR ...  Hopefully I or someone else
will get this into Mercurial proper.

diff --git a/mercurial/extensions.py b/mercurial/extensions.py
--- a/mercurial/extensions.py
+++ b/mercurial/extensions.py
@@ -13,6 +13,7 @@
 import imp
 import inspect
 import os
+import sys
 
 from .i18n import (
     _,
@@ -108,6 +109,11 @@
 
 def _importh(name):
     """import and return the <name> module"""
+    # Read HGEXTENSIONSPATH environment variable when import extensions.
+    extension_path = os.getenv("HGEXTENSIONSPATH")
+    if extension_path is not None:
+        for path in extension_path:
+            sys.path.append(path)
     mod = __import__(pycompat.sysstr(name))
     components = name.split(b'.')
     for comp in components[1:]:
able>
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<div class='path'>path: <a href='/guix/log/?id=9b6d058ce134b8f0693e7d87c5612757c572db5e'>root</a>/<a href='/guix/log/gnu?id=9b6d058ce134b8f0693e7d87c5612757c572db5e'>gnu</a>/<a href='/guix/log/gnu/packages?id=9b6d058ce134b8f0693e7d87c5612757c572db5e'>packages</a>/<a href='/guix/log/gnu/packages/axoloti.scm?id=9b6d058ce134b8f0693e7d87c5612757c572db5e'>axoloti.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/packages/axoloti.scm?id=9b6d058ce134b8f0693e7d87c5612757c572db5e&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2023-01-08 23:20:59 +0100'>2023-01-08</span></td><td><a href='/guix/commit/gnu/packages/axoloti.scm?id=a4d9ce54880dc0d299e4489a168398440c9e7c33'>gnu: Move Java XML packages to new module.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/xml.scm (java-simple-xml, java-jaxp,
java-apache-xml-commons-resolver, java-jaxen, java-xom, java-xsdlib,
java-xpp3, java-xmlpull2, java-xmlpull-api-v1, java-dom4j, java-kxml2,
java-stax, java-jettison, java-jdom2, java-xstream, java-mxparser): Move these
variables from here...
* gnu/packages/java-xml.scm: ...to this new file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Register new file.
* gnu/packages/axoloti.scm,
gnu/packages/batik.scm,
gnu/packages/bioinformatics.scm,
gnu/packages/groovy.scm,
gnu/packages/java.scm,
gnu/packages/maven.scm: Adjust module imports.
</span></span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2022-11-11 18:09:58 +0100'>2022-11-11</span></td><td><a href='/guix/commit/gnu/packages/axoloti.scm?id=09562e0bead3313e3e46a5fc0763f027a6bced3a'>gnu: axoloti: Remove unused module import.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/axoloti.scm (axoloti-runtime, axoloti-patcher,
axoloti-patcher-next)[arguments]: Don't import (guix build syscalls).
</span></span></td><td>Marius Bakke</td></tr>