Wednesday, March 29, 2017

Cancel a Stripe subscription in App Engine

Stripe's API for canceling subscriptions requires that you send an HTTP DELETE request.  If you want to cancel the subscription at the end of the current billing period (instead of immediately), you need to include a body parameter at_period_end=true in the request.  That's fine.  RFC 7231 section 4.3.5 says that a DELETE request is allowed to have a body but it "has no defined semantics".

Unfortunately, App Engine's urlfetch service silently removes the body from all outgoing DELETE requests.  Trying to cancel a Stripe subscription in App Engine always does it immediately, even if you asked to cancel at the end of the billing period.   Google's known about the problem since 2008, but never fixed it.  The glitch impacts many APIs other than Stripe.

Fortunately, you can use App Engine's sockets API to construct a workaround.  In Go, you build an HTTP client whose transport uses the socket API to make outbound network connections.  That uses Go's built in support for HTTP DELETE requests and avoids the bugs in App Engine's urlfetch service.

httpClient := &http.Client{
 Transport: &http.Transport{
  Dial: func(network, addr string) (net.Conn, error) {
   return socket.Dial(c, network, addr)

Tuesday, March 28, 2017

Programming Languages by Spec Size

I was curious which programming language has the smallest specification.  Which one has the largest?  For each language, I printed the spec to a PDF and counted the pages.  Go is the smallest.  C++ is the largest.

This is a very rough estimate of language complexity since each specification varies in style and purpose.  For example, Prolog includes Annex A which is only informative but doubles the size.  Haskell 2010 includes Part II - Libraries which defines the standard library, not the language.  Anyway, here's the list:

If you'd like me to add other languages, comment with a link to the spec and I'll update this post.

Monday, March 20, 2017

gsutil: No module named google_compute_engine

While trying to run gsutil ls on a Compute Engine VM, I received a stack trace like this:
Traceback (most recent call last):
ImportError: No module named google_compute_engine
It turns out that gsutil doesn't like Python installed from Linuxbrew.  It really wants to use the system Python.  The following works fine:
env CLOUDSDK_PYTHON=/usr/bin/python gsutil ls ...
Easy enough to fix.  Nobody else had documented trouble with this configuration so here you go Internet.

Wednesday, March 15, 2017

AGI Script completed, returning 4

I use Asterisk to run my phone system.  Most of my dialplan is an AGI script written in Perl.  In certain circumstances, I kept finding the following message in Asterisk's logs:
AGI Script ... completed, returning 4
My script should always return 0, so seeing an exit code of 4 was unexpected.  After this log message appeared, no further code in my AGI script was executed, also unexpected.

After a bunch of debugging, I discovered that Asterisk sends SIGHUP to your script if a caller hangs up.  The exit code 4 was Asterisk's interpretation of Perl's exit code when SIGHUP wasn't handled.  I installed a handler for that signal and now everything's grand.

Prior to tonight's debugging, I had never used the sigtrap package.  Its stack traces were very helpful.

Tuesday, March 07, 2017

Serving Git repositories from Go

I have a small web server written in Go.  I wanted to serve Git repositories from this server.  It turned out to be surprisingly easy since git-http-backend and the cgi package do all the hard work.

First, define a function for handling Git's HTTP requests:

func git(w http.ResponseWriter, r *http.Request) {
        username, password, ok := r.BasicAuth()
        if !ok || username != "john" || password != "secret" {
                w.Header().Set("Content-Type", "text/plain")
                w.Header().Set("WWW-Authenticate", "Basic realm=\" git\"")
                fmt.Fprintln(w, "Not authorized")

        const gitRoot = "/path/to/git/repositories/"
        h := &cgi.Handler{
                Path: "/usr/lib/git-core/git-http-backend",
                Root: "/git",
                Dir:  gitRoot,
                Env: []string{
                        "GIT_PROJECT_ROOT=" + gitRoot,
                        "GIT_HTTP_EXPORT_ALL=", // we require auth above
                        "REMOTE_USER=" + username,
        h.ServeHTTP(w, r)

Second, connect that function to the mux you're already using:

mux.HandleFunc("/git/", git)

Now I can clone from and push to my little Go server.

Saturday, March 04, 2017

Sharing is dangerous

Last week I tried pass as an open source replacement for 1Password.  It was almost exactly what I wanted.  Unfortunately, it uses GnuPG for encryption.  GnuPG is a pain and bloated (444k lines of code).  Since pass just runs the gpg binary, I wrote a quick script that implements the gpg shell interface but does encryption with a much smaller library.  The proof of concept worked.  I made a note to migrate to pass later and uninstalled it.  I forgot to remove my fake gpg script.

This morning I tried running "brew update".  It fetched Git repositories then stalled without hints about the cause.  After too much debugging time, I discovered that it was running my fake gpg which blocked waiting for input.  I deleted fake gpg then "brew update" proceeded fine.

Too much sharing

The wasted debug time was clearly my fault, but it reminded me how dangerous sharing is.  Too much of our software is a wobbly tower of dependencies.  Sure, you can change the bottom block, but it's risky.

The well-known costs of global mutable state are a symptom of problematic sharing.  Package management becomes NP-complete dependency hell when sharing is mandatory (assumption 4). The recent Cloudbleed vulnerability was mostly a problem because of sharing:

Because Cloudflare operates a large, shared infrastructure, an HTTP request to a Cloudflare web site that was vulnerable to this problem could reveal information about an unrelated other Cloudflare site. (emphasis added)

In other words, FitBit and Uber have a security vulnerability because some random WordPress blog generated bad HTML.

Don't share, copy

Some cultures don't like to share.  Docker containers isolate applications from each other.  Node prefers to load a module that's the least shared.  OpenBSD doesn't like to share file system partitions.  Chrome sandboxes each site to prevent sharing.  This is pretty nice.  If my Youtube tab has a problem, I don't have to consider debugging the Reddit tab.  If I upgrade one project's Node dependency, it doesn't break other Node projects.

Some cultures that used to favor sharing are moving away from it.  Perl (local::lib) and Go (vendor directories) come to mind.  Of course, everyone can still share (in the sense of using identical code), they just get their own local copy of it.  Go's default of building static binaries is a breath of fresh air.

All of this is just a reminder to me: if my code shares anything, find a way to make it either local (not shared) or immutable.

Thursday, March 02, 2017

Caller ID for SMS in XMPP

When you receive an SMS on a traditional mobile device, your phone looks up the number in your local address book and displays the corresponding name.  That works great for a small social network where you can maintain the list yourself.  As the number of potential contacts (coworkers, telemarketers, etc) increases, it becomes unwieldy.

For example, I routinely receive SMS from people in my ward or stake who I'm unlikely to contact on a regular basis.  That's a few thousand people and the population changes weekly.  I don't want all that info in my regular contact list.

Fortunately, I run my own little phone system that uses XMPP for sending and receiving SMS.  I wrote a gateway to handle the conversion back and forth between those networks.  That gateway can run a script to perform caller ID however I want.  So now I have something like this when an SMS arrives:

  1. check my personal address book
  2. check the stake directory
  3. check 800notes for records of telemarketing
I also have multiple external phone numbers.  One of them is a junk number that I give out when I fill out online forms.  If an SMS comes in to that number, the caller ID appears with a "(junk)" prefix.

It's a lot of fun to be able to write a couple shell scripts to customize how your phone works.

Wednesday, March 01, 2017

Signing up for Project Fi

In preparation for an extended visit to Europe, I signed up for Project Fi today.  Prior to this I had been using Ting.  I love Ting.  I do all my SMS and voice communications over IP, so I really just need a reliable, affordable data plan.  If Ting had better data rates in Europe, I would have stuck with them.  Anyway, here are some first impressions while signing up for Project Fi.

Charging $20 per month per phone line seems expensive.  That costs $6 on Ting.  Fi gives you unlimited calls and texts but since I always use zero of those, I'd rather have a cheaper base rate.

Fi charges $10 per GB for data.  As they say, "No matter your budget, you'll only pay for data you use."  So why are they asking me to set a budget?  It seems like an extra step without a purpose.  I chose the smallest budget allowed, 1 GB.  This combination costs $30 per month on Fi and $22 on Ting.  An extra $8 per month to get excellent data rates across Europe seems reasonable to me.

I already have a Google Pixel so I only needed a new SIM.  That was free.  As I recall, Ting charges $12 up front for that, so $0 is nice.

Since my public phone numbers are all managed through Twilio, it doesn't really matter to me what phone number Fi assigns me.  They gave me the choice of porting a number or choosing a new one.  I chose to receive a new one.  Unfortunately, they only offered numbers with area codes in my local area.  Some people might like to choose an arbitrary area code.  They didn't let me choose the specific number at this point.

Fi asked for my primary service address.  Since I'll be using this phone all over Europe, I wasn't entirely sure what to put.  I used my home address in Wyoming.  Fi didn't seem picky about that.  They even let me use a different shipping address for the SIM card.  That was convenient since I was traveling at the time I placed the order.

Overall the sign up process was smooth and the prices seem reasonable.  When the SIM arrives in 7 days, I'll put it through its paces.