--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -1825,8 +1825,12 @@ class localrepository(repo.repository):
         except ValueError:
             raise util.UnexpectedOutput(
                 _('Unexpected response from remote server:'), l)
-        if resp != 0:
+        if resp == 1:
             raise util.Abort(_('operation forbidden by server'))
+        elif resp == 2:
+            raise util.Abort(_('locking the remote repository failed'))
+        elif resp != 0:
+            raise util.Abort(_('the server sent an unknown error code'))
         self.ui.status(_('streaming all changes\n'))
         l = fp.readline()
         try:

--- a/mercurial/streamclone.py
+++ b/mercurial/streamclone.py
@@ -7,7 +7,7 @@
 
 from demandload import demandload
 from i18n import gettext as _
-demandload(globals(), "os stat util")
+demandload(globals(), "os stat util lock")
 
 # if server supports streaming clone, it advertises "stream"
 # capability with value that is version+flags of repo it is serving.
@@ -65,18 +65,23 @@ def stream_out(repo, fileobj):
         fileobj.write('1\n')
         return
 
-    fileobj.write('0\n')
-
     # get consistent snapshot of repo. lock during scan so lock not
     # needed while we stream, and commits can happen.
-    lock = repo.lock()
+    try:
+        repolock = repo.lock()
+    except (lock.LockHeld, lock.LockUnavailable), inst:
+        repo.ui.warn('locking the repository failed: %s\n' % (inst,))
+        fileobj.write('2\n')
+        return
+
+    fileobj.write('0\n')
     repo.ui.debug('scanning\n')
     entries = []
     total_bytes = 0
     for name, size in walkrepo(repo.path):
         entries.append((name, size))
         total_bytes += size
-    lock.release()
+    repolock.release()
 
     repo.ui.debug('%d files, %d bytes to transfer\n' %
                   (len(entries), total_bytes))