unbundle: extract checkheads in its own function

We are going to refactor the unbundle function to have it working on
a local repository too. Having this function extracted will ease the process.

In the case of non-matching heads, the function now directly raises an
exception. The top level of the function is catching it.

mercurial/exchange.py

@@ -611,3 +611,20 b' def getbundle(repo, source, heads=None, '
         temp.write(c)
     temp.seek(0)
     return bundle2.unbundle20(repo.ui, temp)
+
+class PushRaced(RuntimeError):
+    """An exception raised during unbunding that indicate a push race"""
+
+def check_heads(repo, their_heads, context):
+    """check if the heads of a repo have been modified
+
+    Used by peer for unbundling.
+    """
+    heads = repo.heads()
+    heads_hash = util.sha1(''.join(sorted(heads))).digest()
+    if not (their_heads == ['force'] or their_heads == heads or
+            their_heads == ['hashed', heads_hash]):
+        # someone else committed/pushed/unbundled while we
+        # were transferring data
+        raise PushRaced('repository changed while %s - '
+                        'please try again' % context)

mercurial/wireproto.py

@@ -9,7 +9,7 b' import urllib, tempfile, os, sys'
 from i18n import _
 from node import bin, hex
 import changegroup as changegroupmod
-import peer, error, encoding, util, store
+import peer, error, encoding, util, store, exchange
 
 
 class abstractserverproto(object):
@@ -754,18 +754,10 b' def stream(repo, proto):'
 def unbundle(repo, proto, heads):
     their_heads = decodelist(heads)
 
-    def check_heads():
-        heads = repo.heads()
-        heads_hash = util.sha1(''.join(sorted(heads))).digest()
-        return (their_heads == ['force'] or their_heads == heads or
-                their_heads == ['hashed', heads_hash])
-
+    try:
     proto.redirect()
 
-    # fail early if possible
-    if not check_heads():
-        return pusherr('repository changed while preparing changes - '
-                       'please try again')
+        exchange.check_heads(repo, their_heads, 'preparing changes')
 
     # write bundle data to temporary file because it can be big
     fd, tempname = tempfile.mkstemp(prefix='hg-unbundle-')
@@ -775,11 +767,7 b' def unbundle(repo, proto, heads):'
         proto.getfile(fp)
         lock = repo.lock()
         try:
-            if not check_heads():
-                # someone else committed/pushed/unbundled while we
-                # were transferring data
-                return pusherr('repository changed while uploading changes - '
-                               'please try again')
+                exchange.check_heads(repo, their_heads, 'uploading changes')
 
             # push can proceed
             fp.seek(0)
@@ -797,3 +785,5 b' def unbundle(repo, proto, heads):'
     finally:
         fp.close()
         os.unlink(tempname)
+    except exchange.PushRaced, exc:
+        return pusherr(str(exc))