Prettier's encoding bug

About a month ago, a curious bug was reported on the prettier plugin for Ruby in this issue: prettier/plugin-ruby#596. It stated that the parser was failing with an error saying that there was an invalid byte sequence for US-ASCII encoding. What made things even more confusing was that it only occured when it was being run through the JavaScript side, not just through the Ruby side.

Encodings have always felt a big like black magic to me, and this issue was no exception. What follows is every rabbit hole I went down before finally figuring out the answer. My hope is that if someone ends up in a similar situation where they know nothing about encoding, this might serve as a bit of a guidepost.

  1. First, I just tried to replicate the issue. I installed the exact same version of Ruby, node, and VSCode. Then I ran every conceivable combination of printing and parsing files that I could think of. No dice, on my machine it continued to work normally.
  2. Next, I tried going down the plain Ruby route. I started checking out methods like String#encode and String#force_encoding trying to shoehorn the source string into the correct encoding. No matter what I tried I couldn’t get it to replicate.
  3. At this point I gave in and decided I had to have more information if I was going to continue. I started asking the folks reporting the issue to start printing out massive amounts of debugging information. This included ruby -e 'pp RbConfig::CONFIG' (to try to see if it was a Ruby configuration issue) and ruby -rripper -e "pp Ripper.sexp('ä')" (to see if it could parse non-ASCII strings at all). Eventually one of the reporters of the issue noticed that ruby -e "pp 'ä'.encoding" was always returning US-ASCII by default, which led to the next steps.

Once we determined it had to do with the default encoding for strings within Ruby, we got more narrowed down. I started looking into the Ruby source, trying to figure out how Ruby determines its default encoding. A quick search of the ruby/ruby repository for UTF-8 led me to this file: langinfo.c. This file has a couple of lines that included a certain header file if a special constant was defined. While this didn’t feel like the answer yet, it seemed like enough of a flag to catch my attention because those constant could be defined on a per-system level. To test whether or not I had the constant defined on my machine, I created a test.c file and added this code:

#include <stdio.h>


int main() {
  return 0;

Don’t mind the fact that the code doesn’t actually even work. The important part is that the file won’t even compile correctly if HAVE_LANGINFO_H is defined. After running gcc test.c and seeing that it did in fact work, I knew I didn’t have that constant defined. (For good measure I changed it to #ifndef and verified). A quick search of this constant actually led me to a python bug of all things. The description of which pointed me to the LC_ALL, LC_CTYPE, and LANG environment variables.

It is at this point that I should mention that there are plenty of programmers in the world that understanding encoding far better than I do, and would probably have hit this point much sooner. Regardless, we’re here now, so continuing on.

  1. Searching through ruby/ruby for those environment variables led me to a nice comment in encoding.c that clearly shows how you can set the LANG environment variable to control external encoding, which will in turn control the manner in which Ruby attempts to parse code. At this point it seemed clear that all I would have to do would be to pass the LANG variable down into the Ruby parsing process when it was being spawned and everything would work. Simple enough, here’s the code that I ended up writing:
  const child = spawnSync(
    ["--disable-gems", path.join(__dirname, "./ripper.rb")],
+     env: { LANG: "UTF-8" },
      input: text,
      maxBuffer: 10 * 1024 * 1024 // 10MB

This actually ended up working perfectly (on my machine). To make sure I didn’t end up shipping something that wasn’t going to up fixing it on others however, I wrote the following test:

const { spawnSync } = require("child_process");
const path = require("path");

// This is just a way to get the stderr to print out in the event that the
// process that we're testing failed with an unexpected error code.
  toHaveExitedCleanly(child) {
    return {
      pass: child.status === 0,
      message: () => child.stderr.toString()

test("different lang settings don't break", () => {
  const script = path.join(__dirname, "../../node_modules/.bin/prettier");
  const child = spawnSync(
    [script, "--plugin", ".", "--parser", "ruby"],
      env: {
        LANG: "US-ASCII"
      input: "'# あ'"


This test spawns the prettier process with the LANG environment variable for the overall system set to US-ASCII (which replicates the system of the users that initially reported the error). Fortunately it passes and failed depending on adding and removing that line in parser.js, so I assumed I was done.

Unfortunately, it failed in CI. As it turns out, UTF-8 is not actually a valid locale outside of Mac (the machine I used to develop). It turns out that Mac has special handling for this locale to switch it to en_US.UTF-8 automatically depending on your configuration. What linux expects is a different value: C.UTF-8. And after further searching it appears that Windows actually wants .UTF-8.

So, back to parser.js, we can extend it even further with:

+const LANG = {
+  aix: "C.UTF-8",
+  darwin: "UTF-8",
+  freebsd: "C.UTF-8",
+  linux: "C.UTF-8",
+  openbsd: "C.UTF-8",
+  sunos: "C.UTF-8",
+  win32: ".UTF-8"
 const child = spawnSync(
   ["--disable-gems", path.join(__dirname, "./ripper.rb")],
-    env: { LANG: "UTF-8" },
+    env: { LANG },
     input: text,
     maxBuffer: 10 * 1024 * 1024 // 10MB

Finally, it passed both locally and on CI.


Ruby infers the encoding in use by your system using the environment variables LC_ALL, LC_CTYPE, and LANG. If your system doesn’t have nl_langinfo, it will replicate it for you. If you’re going to spawn a Ruby process, make sure you have your encoding set correctly. The pull request that ended up fixing this is here: prettier/plugin-ruby#617.

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