hghave: feature absence can be checked by prefixing with 'no-'

tests/hghave

@@ -1,81 +1,88 b''
 #!/usr/bin/env python
 """Test the running system for features availability. Exit with zero
-if all features are there, non-zero otherwise.
+if all features are there, non-zero otherwise. If a feature name is
+prefixed with "no-", the absence of feature is tested.
 """
 import optparse
 import os
 import sys
 import tempfile
 
 def has_symlink():
     return hasattr(os, "symlink")
 
 def has_fifo():
     return hasattr(os, "mkfifo")
 
 def has_executablebit():
     fd, path = tempfile.mkstemp()
     os.close(fd)
     try:
         s = os.lstat(path).st_mode
         os.chmod(path, s | 0100)
         return (os.lstat(path).st_mode & 0100 != 0)
     finally:
         os.remove(path)
 
 def has_eol_in_paths():
     try:
         fd, path = tempfile.mkstemp(suffix='\n\r')
         os.close(fd)
         os.remove(path)
         return True
     except:
         return False
 
 checks = {
     "symlink": (has_symlink, "symbolic links"),
     "fifo": (has_fifo, "named pipes"),
     "execbit": (has_executablebit, "executable bit"),
     "eol-in-paths": (has_eol_in_paths, "end-of-lines in paths"),
 }
 
 def list_features():
     for name, feature in checks.iteritems():
         desc = feature[1]
         print name + ':', desc
 
 parser = optparse.OptionParser("%prog [options] [features]")
 parser.add_option("--list-features", action="store_true",
                   help="list available features")
 parser.add_option("-q", "--quiet", action="store_true",
                   help="check features silently")
 
 if __name__ == '__main__':
     options, args = parser.parse_args()
     if options.list_features:
         list_features()
         sys.exit(0)
 
     quiet = options.quiet
 
     failures = 0
 
     def error(msg):
         global failures
         if not quiet:
             sys.stderr.write(msg + '\n')
         failures += 1
 
     for feature in args:
+        negate = feature.startswith('no-')
+        if negate:
+            feature = feature[3:]
+        
         if feature not in checks:
             error('hghave: unknown feature: ' + feature)
             continue
 
         check, desc = checks[feature]
-        if not check():
+        if not negate and not check():
             error('hghave: missing feature: ' + desc)
+        elif negate and check():
+            error('hghave: unexpected feature: ' + desc)
 
     if failures != 0:
         sys.exit(1)