[PATCH] branchmap: handle nullrev in setcachedata

[Durham Goode]

# HG changeset patch
# User Durham Goode <durham at fb.com>
# Date 1489618137 25200
#      Wed Mar 15 15:48:57 2017 -0700
# Node ID 6257ea07d629e83e85f68d0c83202b4111f6d4fd
# Parent  e83302d437485620c9bc25921f724af34cd5dd55
branchmap: handle nullrev in setcachedata

906be86990 recently changed to switch from:

  self._rbcrevs[rbcrevidx:rbcrevidx + _rbcrecsize] = rec

to

  pack_into(_rbcrecfmt, self._rbcrevs, rbcrevidx, node, branchidx)

This causes an exception if rbcrevidx is -1 (i.e. the nullrev). The old code
handled this because python handles out of bound sets to arrays gracefully. The
new code throws because the self._rbcrevs buffer isn't long enough to write 8
bytes to.  Normally it would've been resized by the immediately preceding line,
but because the 0 length buffer is greater than the idx (-1) times the size, no
resize happens.

Setting the branch for the nullrev doesn't make sense anyway, so let's skip it.
This was caught by external tests in the Facebook extensions repo, but I've
added a test here that catches the issue.

diff --git a/mercurial/branchmap.py b/mercurial/branchmap.py
--- a/mercurial/branchmap.py
+++ b/mercurial/branchmap.py
@@ -452,6 +452,8 @@ class revbranchcache(object):
 
     def _setcachedata(self, rev, node, branchidx):
         """Writes the node's branch data to the in-memory cache data."""
+        if rev == nullrev:
+            return
         rbcrevidx = rev * _rbcrecsize
         if len(self._rbcrevs) < rbcrevidx + _rbcrecsize:
             self._rbcrevs.extend('\0' *
diff --git a/tests/test-branches.t b/tests/test-branches.t
--- a/tests/test-branches.t
+++ b/tests/test-branches.t
@@ -1,5 +1,10 @@
   $ hg init a
   $ cd a
+
+Verify checking branch of nullrev before the cache is created doesnt crash
+  $ hg log -r 'branch(.)' -T '{branch}\n'
+
+Basic test
   $ echo 'root' >root
   $ hg add root
   $ hg commit -d '0 0' -m "Adding root node"