On OSX, use the TOR_BROWSER_TOR_DATA_DIR environment variable value
(if available) to determine the location of the meek browser profile.
This fixes a problem where meek-client-torbrowser attempted to use a
path under /Applications/TorBrowser-Data, to which regular users may
not have write access.

Suggested by Yawning Angel as a safeguard in addition to 125b0ca1:
https://bugs.torproject.org/20030#comment:8

Fixes "go vet" warnings:
helper_test.go:50: arg input for printf verb %q of wrong type: net/url.URL
helper_test.go:57: arg test.input for printf verb %q of wrong type: net/url.URL
helper_test.go:60: arg test.input for printf verb %q of wrong type: net/url.URL

See https://bugs.torproject.org/20030 for discussion. Since Go 1.6,
writes to fd 1 or 2 when they are closed causes the program to terminate
with a SIGPIPE. Because the default log location is stderr (fd 2), we
would get a SIGPIPE when logging once the parent process had died and
closed its stderr. This prevented meek-client-torbrowser from cleaning
up its subprocesses.

A better way to do this is with the -application flag to goapp.

To ensure that the meek-http-helper profile is up-to-date with respect
to the template (e.g., after Tor Browser has been updated), the
contents of the file meek-template-sha256sum.txt within the profile
are compared with the corresponding template file; if they differ, the
profile is deleted and recreated.

Fix a problem where copying the profile from the template failed
if TorBrowser-Data/Tor/PluggableTransports/ did not already exist
(before calling ioutil.TempDir(), the parent directory must exist).

Remove trailing semicolons and unneeded parens.

Use the regular Tor Browser instead of a symlinked copy and pass a
--invisible option to firefox. Tor Browser will be patched to
recognize that flag and hide the Mac OS dock icon as soon as possible.

Also, fix meek-client-torbrowser's embedded paths to match Tor Browser's
new Mac OS directory structure and create the meek-http-helper browser
profile on-the-fly by copying files from a template.

I don't know of anyone using it and I haven't tried it in a long time.
We can bring it back if there is a need.

With higher precedence.

This is a replacement for X-Forwarded-For because App Engine doesn't
allow to set X-Forwarded-For, and doesn't set any equivalent header by
default.

https://trac.torproject.org/projects/tor/ticket/13171#comment:7

Includes failing tests for X-Forwarded-For.

This is working towards #13171. Currently it's just getting
req.RemoteAddr as before, except that it's now setting the port number
to ":0" in all cases.

These used to be set using the web interface. Now you have to set them
in app.yaml. For example,
https://cloud.google.com/appengine/docs/adminconsole/performancesettings#Setting_the_Number_of_Idle_Instances
now redirects to
https://cloud.google.com/appengine/docs/developers-console/#module-settings
which says:
	Note: Many performance settings for modules are included in the
	module's configuration file (Java | Python | Go | PHP). These
	settings are made at deployment time and cannot be changed from
	the Developers Console.
So we've been running without the settings I set manually before for who
knows how long. Previously I had set:
  max_idle_instances: 2
  min_pending_latency: 1000ms
Now I set it to:
  max_idle_instances: 2
  min_pending_latency: 1000ms

The new URL for the documentation of these parameters is here:
https://cloud.google.com/appengine/docs/go/modules/#Go_Configuration

https://lists.torproject.org/pipermail/tor-talk/2015-July/038496.html
https://trac.torproject.org/projects/tor/ticket/16634

https://gitweb.torproject.org/pluggable-transports/goptlib.git/commit/?id=65fc8d72eccd82237562744ddc85760d426561d8

https://trac.torproject.org/projects/tor/ticket/14135
https://gitweb.torproject.org/pluggable-transports/goptlib.git/commit/?id=50b39b746c6ff34bf31977b658848d876ee84fbf

These were the cause of the profile error that you saw when restarting
Tor Browser after an update.
https://trac.torproject.org/projects/tor/ticket/13247#comment:17

Ensure that update-related browser preferences are written to
disk by flushing them during startup.