Checks snapshot validity using a hash of source file instead of a version string.

tools/make_version.py

 # Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 #
 # This python script creates a version string in a C++ file.
 
+import hashlib
+import os
 import sys
 import time
 from optparse import OptionParser
 import utils
 
 def debugLog(message):
   print >> sys.stderr, message
   sys.stderr.flush()
 
+# When these files change, snapshots created by the VM are potentially no longer
+# backwards-compatible.
+VM_SNAPSHOT_FILES=[
+  # Header files.
+  'datastream.h',
+  'object.h',
+  'raw_object.h',
+  'snapshot.h',
+  'snapshot_ids.h',
+  'symbols.h',
+  # Source files.
+  'object.cc',
+  'raw_object.cc',
+  'raw_object_snapshot.cc',
+  'snapshot.cc',
+  'symbols.cc',

[siva, 2014/09/08 23:47:45]

I think we should also include dart_api_impl.cc it has function
to create snapshot/read snapshot etc. changes to that could potentially
make things incompatible.

dart.cc also has some snapshot related stuff when initializing an
isolate.

[zra, 2014/09/09 01:43:03]

Done.

+]
+
 def makeVersionString():
   version_string = utils.GetVersion()
   debugLog("Returning version string: %s " % version_string)
   return version_string
 
 
+def makeSnapshotHashString():
+  vmhash = hashlib.md5()

[Ivan Posva, 2014/09/09 00:08:03]

Anything better than MD5? Should we even care?

[zra, 2014/09/09 01:43:03]

Since the hash is not being used for any security purpose, I think we don't
care.

+  for vmfilename in VM_SNAPSHOT_FILES:
+    vmfilepath = os.path.join(utils.DART_DIR, 'runtime', 'vm', vmfilename)
+    with open(vmfilepath) as vmfile:
+      vmhash.update(vmfile.read())
+  return vmhash.hexdigest()
+
+
 def makeFile(output_file, input_file):
   version_cc_text = open(input_file).read()
   version_string = makeVersionString()
   version_cc_text = version_cc_text.replace("{{VERSION_STR}}",
                                             version_string)
   version_time = time.ctime(time.time())
   version_cc_text = version_cc_text.replace("{{BUILD_TIME}}",
                                             version_time)
+  snapshot_hash = makeSnapshotHashString()
+  version_cc_text = version_cc_text.replace("{{SNAPSHOT_HASH}}",
+                                            snapshot_hash)
   open(output_file, 'w').write(version_cc_text)
   return True
 
 
 def main(args):
   try:
     # Parse input.
     parser = OptionParser()
     parser.add_option("--output",
                       action="store", type="string",
@@ skipping 20 matching lines @@
     return 0
   except Exception, inst:
     sys.stderr.write('make_version.py exception\n')
     sys.stderr.write(str(inst))
     sys.stderr.write('\n')
     return -1
 
 if __name__ == '__main__':
   exit_code = main(sys.argv)
   sys.exit(exit_code)