Fix hg clone race with writer

Most read operations in hg don't need locks because we order reads and
writes for consistency. Clone is an exception to this as we're copying
entire file histories and could end up with more file history copied
than we have commits.

For now, make clone take a lock on the source repo. Non-hardlinked
clone should eventually be changed to use lockless pull.

mercurial/commands.py

@@ -7,7 +7,7 b''
 
 from demandload import demandload
 demandload(globals(), "os re sys signal shutil")
-demandload(globals(), "fancyopts ui hg util")
+demandload(globals(), "fancyopts ui hg util lock")
 demandload(globals(), "fnmatch hgweb mdiff random signal time traceback")
 demandload(globals(), "errno socket version struct atexit")
 
@@ -494,6 +494,9 b' def clone(ui, source, dest=None, **opts)'
                     and getattr(os, 'link', None) or shutil.copy2)
         if copyfile is not shutil.copy2:
             ui.note("cloning by hardlink\n")
+        # we use a lock here because because we're not nicely ordered
+        l = lock.lock(os.path.join(source, ".hg", "lock"))
+
         util.copytree(os.path.join(source, ".hg"), os.path.join(dest, ".hg"),
                       copyfile)
         try:

mercurial/util.py

@@ -187,7 +187,7 b' def copytree(src, dst, copyfile):'
         elif os.path.isfile(srcname):
             copyfile(srcname, dstname)
         else:
-            raise IOError("Not a regular file: %r" % srcname)
+            pass
 
 def _makelock_file(info, pathname):
     ld = os.open(pathname, os.O_CREAT | os.O_WRONLY | os.O_EXCL)