depot_tools: bail out if $PATH contains tildes

Bash and its descendants allow having $PATH entries containing literal tildes,
like:
  ...:~/depot_tools:...
If the user has their shell configured this way, their shell will be able to
invoke `fetch` and other depot_tools commands, but depot_tools will not in turn
be able to invoke subcommands via popen because popen does not have this
bash-specific behavior when not in shell mode.

This change has gclient detect this configuration problem and error out with a
descriptive error message. Not doing so leads to a puzzling condition where
gclient complains about being unable to find a program that (from the user's
perspective) is in their $PATH and invokable interactively.

It also has 'fetch' suppress its own failure stack trace if it runs gclient and
gclient fails; the 'fetch' stack trace pushes the messages from gclient (which
are actually diagnostic) off the screen.

Bug: chromium:952865
Change-Id: Ibba4d2fccee405aa68392ce141493f1de21ec018
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1573204
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>

fetch.py

@@ -70,7 +70,16 @@ class Checkout(object):
     if return_stdout:
       return subprocess.check_output(cmd, **kwargs)
     else:
-      subprocess.check_call(cmd, **kwargs)
+      try:
+        subprocess.check_call(cmd, **kwargs)
+      except subprocess.CalledProcessError as e:
+        # If the subprocess failed, it likely emitted its own distress message
+        # already - don't scroll that message off the screen with a stack trace
+        # from this program as well. Emit a terse message and bail out here;
+        # otherwise a later step will try doing more work and may hide the
+        # subprocess message.
+        print('Subprocess failed with return code %d.' % e.returncode)
+        sys.exit(e.returncode)
       return ''
 
 

gclient.py

@@ -3098,19 +3098,40 @@ def disable_buffering():
   sys.stdout = gclient_utils.MakeFileAnnotated(sys.stdout)
 
 
-def main(argv):
-  """Doesn't parse the arguments here, just find the right subcommand to
-  execute."""
+def path_contains_tilde():
+  for element in os.environ['PATH'].split(os.pathsep):
+    if element.startswith('~/'):
+      return True
+  return False
+
+
+def can_run_gclient_and_helpers():
   if sys.hexversion < 0x02060000:
     print(
         '\nYour python version %s is unsupported, please upgrade.\n' %
         sys.version.split(' ', 1)[0],
         file=sys.stderr)
-    return 2
+    return False
   if not sys.executable:
     print(
         '\nPython cannot find the location of it\'s own executable.\n',
         file=sys.stderr)
+    return False
+  if path_contains_tilde():
+    print(
+        '\nYour PATH contains a literal "~", which works in some shells ' +
+        'but will break when python tries to run subprocesses. ' +
+        'Replace the "~" with $HOME.\n' +
+        'See https://crbug.com/952865.\n',
+        file=sys.stderr)
+    return False
+  return True
+
+
+def main(argv):
+  """Doesn't parse the arguments here, just find the right subcommand to
+  execute."""
+  if not can_run_gclient_and_helpers():
     return 2
   fix_encoding.fix_encoding()
   disable_buffering()