cdatapack: fix empty revision reading

Summary:
The newly changed `LZ4_decompress_safe` is unhappy about empty buffer. So let's
check empty revision explicitly.

Test Plan: Added a test

Reviewers: #mercurial, ikostia

Reviewed By: ikostia

Subscribers: mjpieters

Differential Revision: https://phabricator.intern.facebook.com/D4986277

Signature: t1:4986277:1493743735:be2ad6047bb0b983425c6e49b7c5ddf2c94d1c1a

cdatapack/cdatapack.c

@@ -488,8 +488,8 @@ const get_delta_chain_link_result_t getdeltachainlink(
 }
 
 bool uncompressdeltachainlink(delta_chain_link_t *link) {
-  if (link->delta != NULL) {
-    // previously decompressed
+  if (link->delta != NULL || link->delta_sz == 0) {
+    // previously decompressed or no content to decompress
     return true;
   }
 

tests/remotefilelog-datapack.py

@@ -71,11 +71,10 @@ class datapacktestsbase(object):
         path = packer.close()
         return self.datapackreader(path)
 
-    def testAddSingle(self):
+    def _testAddSingle(self, content):
         """Test putting a simple blob into a pack and reading it out.
         """
         filename = "foo"
-        content = "abcdef"
         node = self.getHash(content)
 
         revisions = [(filename, node, nullid, content)]
@@ -86,6 +85,12 @@ class datapacktestsbase(object):
         chain = pack.getdeltachain(filename, node)
         self.assertEquals(content, chain[0][4])
 
+    def testAddSingle(self):
+        self._testAddSingle('')
+
+    def testAddSingleEmpty(self):
+        self._testAddSingle('abcdef')
+
     def testAddMultiple(self):
         """Test putting multiple unrelated blobs into a pack and reading them
         out.

tests/test-remotefilelog-t17796162.t

@@ -1,7 +0,0 @@
-Remotefilelog appears to have issues with specific (historical?) datapack
-files, see https://phabricator.intern.facebook.com/P57361795.
-
-This test is here only to ensure we don't push out a build with this bug.
-
-  $ echo FAIL
-  Please fix remotefilelog file parsing, see t17796162