streamclone: ensure the server sends the right amount of data

Otherwise, the client would fail with some confusing error. I have
seen an error which I think is this, perhaps due to a concurrent
revlog split, which streamclones do not handle correctly and would
result in a short read of the index of the revlog being split.

Differential Revision: https://phab.mercurial-scm.org/D11236

mercurial/streamclone.py

@@ -583,7 +583,7 b' def _emit2(repo, entries, totalfilesize)'
         # copy is delayed until we are in the try
         entries = [_filterfull(e, copy, vfsmap) for e in entries]
         yield None  # this release the lock on the repository
-        seen = 0
+        totalbytecount = 0
 
         for src, name, ftype, data in entries:
             vfs = vfsmap[src]
@@ -595,6 +595,7 b' def _emit2(repo, entries, totalfilesize)'
             elif ftype == _filefull:
                 fp = open(data, b'rb')
                 size = util.fstat(fp).st_size
+            bytecount = 0
             try:
                 yield util.uvarintencode(size)
                 yield name
@@ -603,9 +604,20 b' def _emit2(repo, entries, totalfilesize)'
                 else:
                     chunks = util.filechunkiter(fp, limit=size)
                 for chunk in chunks:
-                    seen += len(chunk)
-                    progress.update(seen)
+                    bytecount += len(chunk)
+                    totalbytecount += len(chunk)
+                    progress.update(totalbytecount)
                     yield chunk
+                if bytecount != size:
+                    # Would most likely be caused by a race due to `hg strip` or
+                    # a revlog split
+                    raise error.Abort(
+                        _(
+                            b'clone could only read %d bytes from %s, but '
+                            b'expected %d bytes'
+                        )
+                        % (bytecount, name, size)
+                    )
             finally:
                 fp.close()