[PATCH 1 of 2 V2] hgweb: add a hook for processing LFS Batch API requests

[Matt Harbison]

# HG changeset patch
# User Matt Harbison <matt_harbison at yahoo.com>
# Date 1519274700 18000
#      Wed Feb 21 23:45:00 2018 -0500
# Node ID 89382cb20bb19e089513b2ce69ef8acfa1f523fd
# Parent  6dab3bdb1f00932a80ffa07f80ba240c3a4b48df
hgweb: add a hook for processing LFS Batch API requests

There really isn't a clean way to give LFS a crack at intercepting the requests
without hardcoding some LFS knowledge in the core.  The rationale for this URI
is that the spec for the Batch API[1] defines the URL as the LFS server url +
'/objects/batch'.  The default git URLs are:

    Git remote: https://git-server.com/foo/bar
    LFS server: https://git-server.com/foo/bar.git/info/lfs
    Batch API: https://git-server.com/foo/bar.git/info/lfs/objects/batch

'.git/' seems like it's not something a user would normally track.  If we adhere
to how git defines the URLs, then the hg-git extension should be able to talk to
a git based server without any additional work.

I'm not sure if checking the User-Agent is a good idea, but this needs a
specialized client, and it seems like everyone else is doing it (3d48ae1aaa5e,
e7bb5fc4570c).  We can always back this off if it becomes a nuisance.  A web
browser will see "400: bad method", the same as it would before this change.

I'm not sure if None or 'pull' is the proper permission check, but the only
difference is whether or not `hgweb.allowpull` is checked.  Since nothing of
particular interest is transferred here, and the next phase handles the read or
write, treating this like web interface request seems fine.

[1] https://github.com/git-lfs/git-lfs/blob/master/docs/api/batch.md

diff --git a/mercurial/hgweb/hgweb_mod.py b/mercurial/hgweb/hgweb_mod.py
--- a/mercurial/hgweb/hgweb_mod.py
+++ b/mercurial/hgweb/hgweb_mod.py
@@ -90,6 +90,12 @@
         urlel = os.path.dirname(urlel)
     return reversed(breadcrumb)
 
+def _processlfsbatchreq(repo, req):
+    """A hook for the LFS extension to wrap that handles requests to the Batch
+    API, and returns the appropriate JSON response.
+    """
+    raise ErrorResponse(HTTP_NOT_FOUND)
+
 class requestcontext(object):
     """Holds state/context for an individual request.
 
@@ -376,6 +382,21 @@
             except ErrorResponse as inst:
                 return protohandler['handleerror'](inst)
 
+        # Route LFS Batch API requests to the appropriate handler
+
+        if req.env.get(r'HTTP_USER_AGENT', r'').startswith(r'git-lfs/'):
+            try:
+                path = req.env.get(r'PATH_INFO')
+                if path == '/.git/info/lfs/objects/batch':
+                    self.check_perm(rctx, req, None)
+                    return _processlfsbatchreq(rctx.repo, req)
+                else:
+                    raise ErrorResponse(HTTP_NOT_FOUND)
+            except ErrorResponse as inst:
+                req.respond(inst, 'text/plain; charset=utf-8')
+                # No body, since only lfs clients are allowed here
+                return ['']
+
         # translate user-visible url structure to internal structure
 
         args = query.split('/', 2)