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The git pickaxe
git
pickaxe
2014/02/09

I care a lot about commit messages. I try to write them following Tim Pope's example, using a short summary line, followed by one or more paragraphs of explanation. It's not unusual for my commit message to be longer than the diff. Why do I do this? Is it just some form of OCD? After all, who really reads commit messages?

The reason I care about commit messages is because I'm an avid user of the git pickaxe. If I'm ever confused about a line of code, and I want to know what was going through the mind of the developer when they were writing it, the pickaxe is the first tool I'll reach for. For example, let's say I was looking at this line from our puppet-graphite module:

exec <%= @root_dir %>/bin/carbon-cache.py --debug start

That --debug option looks suspect. I might think to myself: "Why are we running carbon-cache in --debug mode? Isn't that wasteful? Do we capture the output? Why was it added in the first place?" In order to answer these questions, I'd like to find the commit that added the switch. I could run git blame on the file, to find the last commit that touched the line. However that leads to a totally unrelated commit that had nothing to do with my --debug flag issue.

So I still want to find the commit that added that --debug switch, but git blame has got me nowhere. What next? It turns out there's an option to git log which will find any commit which introduces or removes a string from anywhere in its commit:

git log -p -S --debug

This will show me every commit that either introduced or removed the string --debug. (It's a slightly confusing example, because --debug is not being used as a command-line switch to git, but as a string argument to the -S switch instead. Nevertheless, git does the right thing.) The -p switch shows the commit diff as well. There are in fact a few matches for this search, but the third commit that comes up is the winner:

commit 5288d5804a3fc20dae4f3b2deeaa7f687595aff1
Author: Philip Potter <philip.g.potter@gmail.com>
Date:   Tue Dec 17 09:33:59 2013 +0000

    Re-add --debug option (reverts #11)

    The --debug option is somewhat badly named -- it *both* adds debug
    output, *and* causes carbon-cache to run in the foreground. Removing the
    option in #11 caused the upstart script to lose track of the process as
    carbon-cache started unexpectedly daemonizing.

    Ideally we want to have a way of running through upstart without the
    debug output, but this will fix the immediate problem.

diff --git a/templates/upstart/carbon-cache.conf b/templates/upstart/carbon-cache.conf
old mode 100644
new mode 100755
index 43a16ee..2322b2d
--- a/templates/upstart/carbon-cache.conf
+++ b/templates/upstart/carbon-cache.conf
@@ -12,4 +12,4 @@ pre-start exec rm -f '<%= @root_dir %>'/storage/carbon-cache.pid
 chdir '<%= @root_dir %>'
 env GRAPHITE_STORAGE_DIR='<%= @root_dir %>/storage'
 env GRAPHITE_CONF_DIR='<%= @root_dir %>/conf'
-exec python '<%= @root_dir %>/bin/carbon-cache.py' start
+exec python '<%= @root_dir %>/bin/carbon-cache.py' --debug start

Now I know exactly why --debug is there, and I know that I certainly don't want to remove it. But what if my commit message had just been "Re-add --debug option"? I'd be none the wiser. This is why I care so much about commit messages: because I have the tools to quickly get from a piece of code to the commit that introduced it, I spend much more time reading commit messages.

This example is also interesting because it raises another question: should this explanation have been in a code comment instead? The --debug flag is inherently confusing, and a comment could have answered my questions even quicker by being right there in the file.

However, a 6-line comment in the file would be quite a bit of noise whenever you weren't interested in the --debug switch, whereas a commit message can be as big as it needs to be to make the explanation clear. Comments and commit messages can be complementary: there could be a one-line comment saying that --debug causes carbon-cache to stay in the foreground, and a more detailed explanation in the commit message. In some ways I see commit messages as a type of expanded commenting system which is available at your fingertips whenever you need it but automatically hides when you just want to read the code.


A couple of small postscripts: I could have even narrowed down my search further by adding a path filter to my log command:

git log -p -S --debug templates/upstart/carbon-cache.conf

This search finds the commit in question instantly: it's the first result. But unlike the original git log command, it is not resilient against the file being renamed in an intervening commit. I tend not to use path filters for pickaxe searches, because I can normally find what I want easily enough anyway.

The -S switch takes a string match only. If you want to match a regex instead, you can add the --pickaxe-regex switch.

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