lfs: add a 'null' store that doesn't require any setup

Summary:
We have to use the dummy store to ensure we don't try to open an HTTP
connection during push, which can fail and must not happen during p4fastimport.
However `dummy` requires some configuration. So just add an internal `null`
store that allows us to operate LFS in a mode that will ignore all large files
completely.

Test Plan: used it in p4fastimport

Reviewers: #sourcecontrol, quark

Reviewed By: quark

Subscribers: quark, mjpieters

Differential Revision: https://phabricator.intern.facebook.com/D4963638

Signature: t1:4963638:1493316573:c012bd97794f9a57c3cf8c15d868a67ae3c03c31

hgext3rd/lfs/__init__.py

@@ -10,7 +10,7 @@ Configs::
     remoteuser = user
     # password for HTTP auth
     remotepassword = password
-    # blobstore type. "git-lfs", or "dummy" (test-only)
+    # blobstore type. "git-lfs", "dummy" (test-only), or "null" (interal use)
     remotestore = git-lfs
     # local filesystem path (only used by the dummy blobstore, test-only)
     remotepath = /tmp/test

hgext3rd/lfs/blobstore.py

@@ -209,9 +209,28 @@ class _dummyremote(object):
         filename = os.path.join(self._storepath, storeid.oid)
         return filename
 
+class _nullremote(object):
+    """Null store storing blobs to /dev/null."""
+
+    def __init__(self, repo):
+        pass
+
+    def write(self, storeid, data):
+        pass
+
+    def read(self, storeid):
+        raise NotImplementedError()
+
+    def writebatch(self, storeids, fromstore, ui=None, total=None):
+        pass
+
+    def readbatch(self, storeids, tostore, ui=None, total=None):
+        pass
+
 _storemap = {
     'git-lfs': _gitlfsremote,
     'dummy': _dummyremote,
+    'null': _nullremote,
 }
 
 def remote(repo):