--- a/setup.py	Sun Oct 08 22:07:47 2017 +0200
+++ b/setup.py	Mon Oct 09 02:30:23 2017 -0700
@@ -516,6 +516,26 @@
 
 class buildhgexe(build_ext):
     description = 'compile hg.exe from mercurial/exewrapper.c'
+    user_options = build_ext.user_options + [
+        ('long-paths-support', None, 'enable support for long paths on '
+                                     'Windows (off by default and '
+                                     'experimental)'),
+    ]
+
+    LONG_PATHS_MANIFEST = """
+    <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+    <assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
+        <application>
+            <windowsSettings
+            xmlns:ws2="http://schemas.microsoft.com/SMI/2016/WindowsSettings">
+                <ws2:longPathAware>true</ws2:longPathAware>
+            </windowsSettings>
+        </application>
+    </assembly>"""
+
+    def initialize_options(self):
+        build_ext.initialize_options(self)
+        self.long_paths_support = False
 
     def build_extensions(self):
         if os.name != 'nt':
@@ -557,10 +577,45 @@
         objects = self.compiler.compile(['mercurial/exewrapper.c'],
                                          output_dir=self.build_temp)
         dir = os.path.dirname(self.get_ext_fullpath('dummy'))
-        target = os.path.join(dir, 'hg')
-        self.compiler.link_executable(objects, target,
+        self.hgtarget = os.path.join(dir, 'hg')
+        self.compiler.link_executable(objects, self.hgtarget,
                                       libraries=[],
                                       output_dir=self.build_temp)
+        if self.long_paths_support:
+            self.addlongpathsmanifest()
+
+    def addlongpathsmanifest(self):
+        """Add manifest pieces so that hg.exe understands long paths
+
+        This is an EXPERIMENTAL feature, use with care.
+        To enable long paths support, one needs to do two things:
+        - build Mercurial with --long-paths-support option
+        - change HKLM\SYSTEM\CurrentControlSet\Control\FileSystem\
+                 LongPathsEnabled to have value 1.
+
+        Please ignore 'warning 81010002: Unrecognized Element "longPathAware"';
+        it happens because Mercurial uses mt.exe circa 2008, which is not
+        yet aware of long paths support in the manifest (I think so at least).
+        This does not stop mt.exe from embedding/merging the XML properly.
+
+        Why resource #1 should be used for .exe manifests? I don't know and
+        wasn't able to find an explanation for mortals. But it seems to work.
+        """
+        exefname = self.compiler.executable_filename(self.hgtarget)
+        fdauto, manfname = tempfile.mkstemp(suffix='.hg.exe.manifest')
+        os.close(fdauto)
+        with open(manfname, 'w') as f:
+            f.write(self.LONG_PATHS_MANIFEST)
+        log.info("long paths manifest is written to '%s'" % manfname)
+        inputresource = '-inputresource:%s;#1' % exefname
+        outputresource = '-outputresource:%s;#1' % exefname
+        log.info("running mt.exe to update hg.exe's manifest in-place")
+        # supplying both -manifest and -inputresource to mt.exe makes
+        # it merge the embedded and supplied manifests in the -outputresource
+        self.spawn(['mt.exe', '-nologo', '-manifest', manfname,
+                    inputresource, outputresource])
+        log.info("done updating hg.exe's manifest")
+        os.remove(manfname)
 
     @property
     def hgexepath(self):