What's Wrong This Time? A Debugging Mystery in Three Parts

Part I: The Idea

"I have an idea"*, I said to myself.

"What if, instead of adding arbitrary dates to blog posts as I write them, I just got the dates from git?"

Git is a version control system used by software developers which tracks changes to files, as well as creation and deletion of files. Every change, addition, or deletion is timestamped and has a hash which uniquely identifies those changes, like a fingerprint.

"Each git commit has a date, so I should be able to programatically determine when a file in my repo was added, or edited."

That was my idea for simplifying the metadata ("front matter") that I add to each of my blog post files on my website. Before this adventure, that metadata looked something like this:

---
title: 'Hello, World!'
description: 'Rewriting My Personal Website in Next.js and TypeScript'
date: '2022-01-01'
---

What I wanted to do was get rid of that arbitrary date field I was adding to every blog post, and just let git tell me when I published or updated a file. I could pull this information from the history of file changes that git keeps track of.

Sounds relatively simple, right?

Well, there were a few roadblocks to getting this up and running. Before we get into them, let's take a look at my initial, quick-and-dirty solution.

First Attempt

Git background

When working locally (on your personal computer), you can interact with git using the command line. One command, git log, gives a list of all of the changes saved ("committed") to the project

$ git log --oneline -n 5
b6eea85 (HEAD -> master, origin/master, origin/HEAD) Merge pull request #15 from awwsmm/development
bc222b3 (origin/development, development) automatically date blog posts using git history
88c4208 order blog posts on index page reverse chronologically
9368f24 new blog post about nominal, structural, and duck typing
6bbe307 Merge pull request #1 from awwsmm/development

The options I passed to git log, --oneline and -n 5, tell git that I only want one line of minimal information for each commit (just the hash and the first line of the commit message), and that I only want the five most recent commits. By default, git shows something like

$ git log
commit b6eea85256fe54f34013f17aca4e892aba0778dd (HEAD -> master, origin/master, origin/HEAD)
Merge: 88c4208 bc222b3
Author: Andrew <aww@awwsmm.com>
Date:   Fri Jan 14 19:01:11 2022 +0000

    Merge pull request #15 from awwsmm/development

    automatically date blog posts using git history

...with the commit hash, the commit message, the date of the commit, the author, and so on. But lots of other options can be set for git log, for instance, specifying a hash will show only that commit and its ancestors (commits that came before it), and --name-only shows the names of all files modified in that commit

$ git log bc222b3 --name-only -n 1
commit bc222b37913ec2cdd7ff5f152dfaca06d33ec9f3 (origin/development, development)
Author: Andrew Watson <aww@awwsmm.com>
Date:   Fri Jan 14 18:54:39 2022 +0000

    automatically date blog posts using git history

.gitignore
README.md
blog/cache.json
blog/git-commit-hooks.md
blog/hello-world.md
blog/nominal-vs-structural-vs-duck-typing.md
lib/blog/FrontMatterFactory.ts
lib/blog/SlugFactory.ts
package-lock.json
package.json
pages/blog/[slug].tsx
pages/index.tsx

You can also pass a file path to git log and it will only show you commits in which that file was modified

$ git log blog/hello-world.md
commit bc222b37913ec2cdd7ff5f152dfaca06d33ec9f3 (origin/development, development)
Author: Andrew Watson <aww@awwsmm.com>
Date:   Fri Jan 14 18:54:39 2022 +0000

    automatically date blog posts using git history

commit ec96618d38c71134f4a9ed14d6ae6d7a2b5c9e59
Author: Andrew Watson <aww@awwsmm.com>
Date:   Sun Jan 2 12:11:54 2022 +0000

    attempting to deploy to Vercel

That's all well and good, but I wanted to get this information when building my website, which is written in TypeScript. So I would need a TypeScript or a JavaScript library to be able to do this easily. That's where simple-git comes in.

Git in TypeScript

simple-git is a JavaScript library which allows you to run git commands from within TypeScript / JavaScript code.

For instance, instead of the above git log blog/hello-world.md, we can write something like

import simpleGit from 'simple-git'

const git = simpleGit()
const logResult = await git.log({ file: 'blog/hello-world.md' })
const commits = logResult.all

Then, we can loop over all commits and easily access each commit's data

commits.forEach(commit => {        // example:
  console.log(commit.author_email) // aww@awwsmm.com
  console.log(commit.author_name)  // Andrew Watson
  console.log(commit.body)         //
  console.log(commit.date)         // 2022-01-14T18:54:39+00:00
  console.log(commit.diff)         // null
  console.log(commit.hash)         // bc222b37913ec2cdd7ff5f152dfaca06d33ec9f3
  console.log(commit.message)      // automatically date blog posts using git history
  console.log(commit.refs)         // origin/development, development
})

So that should be it: when compiling the project, we should be able to use this library to get all log lines which refer to a specific file. The most recent one should be the lastUpdated date of the file, while the least recent one should be the published date of the file.

I quickly coded this up, ran it locally, and everything looked fine. I thought that would be the end of the story.

But then I pushed it to production.

First Problem

The first problem I hit was that all of my blog posts now had the same date (which at the time was the current date):

Do you see any problems with the code I wrote here?

What's going on here?

Check out the answer in the second part of this series.

---

* This is the traditional phrase for conjuring bugs in your programs. See also: "I'll just do a little refactoring".