Observatory build should try backup method if the SDK doesn't work.

tools/utils.py

 # Copyright (c) 2012, 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 file contains a set of utilities functions used by other Python-based
 # scripts.
 
 import commands
 import datetime
 import json
@@ skipping 547 matching lines @@
   print 'Executing: ' + ' '.join(cmd)
   pipe = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE,
       shell=IsWindows())
   output = pipe.communicate()
   if pipe.returncode != 0:
     raise Exception('Execution failed: ' + str(output))
   return pipe.returncode, output
 
 
 def DartBinary():
+  # TODO(24311): Replace all uses of this with CheckedInSdk[Fix]Executable().
   tools_dir = os.path.dirname(os.path.realpath(__file__))
   dart_binary_prefix = os.path.join(tools_dir, 'testing', 'bin')
   if IsWindows():
     return os.path.join(dart_binary_prefix, 'windows', 'dart.exe')
   else:
     arch = GuessArchitecture()
     system = GuessOS()
     if arch == 'armv5te':
       # TODO(zra): This binary does not exist, yet. Check one in once we have
       # sufficient stability.
       return os.path.join(dart_binary_prefix, system, 'dart-armv5te')
     elif arch == 'arm':
       return os.path.join(dart_binary_prefix, system, 'dart-arm')
     elif arch == 'arm64':
       return os.path.join(dart_binary_prefix, system, 'dart-arm64')
     elif arch == 'mips':
       return os.path.join(dart_binary_prefix, system, 'dart-mips')
     else:
       return os.path.join(dart_binary_prefix, system, 'dart')
 
 
 def DartSdkBinary():
   tools_dir = os.path.dirname(os.path.realpath(__file__))
   dart_binary_prefix = os.path.join(tools_dir, '..', 'sdk' , 'bin')
   return os.path.join(dart_binary_prefix, 'dart')
 
 
+# The checked-in SDKs are documented at
+#     https://github.com/dart-lang/sdk/wiki/The-checked-in-SDK-in--tools

[Ivan Posva, 2015/09/15 17:01:42]

ditto: --

[Bill Hesse, 2015/09/15 17:09:02]

This is the URL of the page.  I tracked it down, and found an extra space in the
page name.  Fixed.

 def CheckedInSdkPath():
   # We don't use the normal macos, linux, win32 directory names here, instead,
   # we use the names that the download_from_google_storage script uses.
   osdict = {'Darwin':'mac', 'Linux':'linux', 'Windows':'win'}
   system = platform.system()
   try:
     osname = osdict[system]
   except KeyError:
     print >>sys.stderr, ('WARNING: platform "%s" not supported') % (system)
     return None;
   tools_dir = os.path.dirname(os.path.realpath(__file__))
   return os.path.join(tools_dir,
                       'sdks',
                       osname,
                       'dart-sdk')
 
 
+def CheckedInSdkExecutable():
+  return os.path.join(CheckedInSdkPath(),
+                      'bin',
+                      'dart.exe' if IsWindows() else 'dart')
+
+
+def CheckedInSdkFixExecutable():

[Ivan Posva, 2015/09/15 17:01:42]

The documentation talks about a CheckedInSdkFixedExecutable, but here we are
missing the Fixed and instead have a Fix, which reads really strangely. Please
update to the documented name.

[Bill Hesse, 2015/09/15 17:09:02]

I like Fix, because it is an active verb, and this function has an effect.  But
I will change to Fixed if others agree.

[Bill Hesse, 2015/09/21 10:05:06]

Changed to CheckedInSdkCheckExecutable, since it now only checks whether
CheckedInSdkExecutable works or not.

+  executable = CheckedInSdkExecutable()
+  canary_script = os.path.join(os.path.dirname(os.path.realpath(__file__)),
+                               'canary.dart')
+  try:
+    if 42 == subprocess.call([executable, canary_script]):
+      return True
+  except OSError as e:
+    pass
+  arch = GuessArchitecture()
+  system = GuessOS()
+  if system == 'linux' and (arch == 'mips' or arch == 'arm'):
+    try:
+      shutil.copy(os.path.join(CheckedInSdkPath(), 'bin', 'dart-%s' % arch),
+                  executable)

[Ivan Posva, 2015/09/15 17:01:42]

Here you are overwriting the ia32 executable which is really a bad thing to do
as you now have completely disabled running on a ia32 or x64 machine. Also it
breaks the "do-not-change" your source repository rule.

[Bill Hesse, 2015/09/15 17:09:02]

We are already changing the source repository by checking out this SDK into it
and unzipping it, at gclient runhooks.  If you want, I can put this change into
the gclient runhooks, and do it at that point.  There is no way to have a single
SDK checkout work for multiple architectures at once.  If this checkout is on an
ARM, MIPS, or other machine, the SDK on it should be the one with the proper
dart executable.  Even if I had separate SDKs up in the cloud, they would
download to the same place, so "dart" would be the one for the host.

[Bill Hesse, 2015/09/15 22:37:05]

We can avoid doing this, if you absolutely don't want to change the "dart" file,
by running pub always with an explicitly given dart executable, and the snapshot
from the SDK, rather than from the runnable pub command.  We would still want to
test that this command works, and fall back to the bootstrap executable if pub
doesn't run this way.

[Bill Hesse, 2015/09/15 22:37:05]

For testing purposes, I added a dart-ia32 binary to the repository, and
supported copying that to "dart" when building on ia32 or x64.  This could be
checked in, and then building would function on whatever platform the build was
currently running on.

[Ivan Posva, 2015/09/15 23:32:59]

Summarizing offline discussion here:
Specifying a dart executable is preferred over the copying the currently valid
executable over the dart binary. Thinking about it a bit further, we already
have a function which gives us the CheckedInSdkExecutable so the smarts of
figuring out the currect architecture can be added right there. No?

[Bill Hesse, 2015/09/21 10:05:06]

Done.

+      if 42 == subprocess.call([executable, canary_script]):
+        return True
+    except (OSError, IOError) as e:
+      pass
+  return False
+
+
 def CheckedInPubPath():
   executable_name = 'pub'
   if platform.system() == 'Windows':
     executable_name = 'pub.bat'
   return os.path.join(CheckedInSdkPath(), 'bin', executable_name)
 
 
 class TempDir(object):
   def __init__(self, prefix=''):
     self._temp_dir = None
@@ skipping 16 matching lines @@
     os.chdir(self._working_directory)
 
   def __exit__(self, *_):
     print "Enter directory = ", self._old_cwd
     os.chdir(self._old_cwd)
 
 
 if __name__ == "__main__":
   import sys
   Main()