Patch applies fake diffs from commit messages

This has come up multiple times before [1], and more generally it's come up hundreds of times with Unix style tools in general. It's always been a stupid idea for every tool to have its own barely documented file format.

This wouldn't be an issue if patches were XML or JSON with a well defined schema, but everything must be a boutique undocumented format in the world of Unix tools.

Maybe the worst part about this is that it can entirely come from a patch being exported by git and then imported straight back in to git. If you can't even handle your own undocumented format then what hope do other tools have that want to work with it?

[1]: https://mas.to/@zekjur/116022397626943871

> This wouldn't be an issue if patches were XML or JSON with a well defined schema, but everything must be a boutique undocumented format in the world of Unix tools.

Patch files are readable by humans. Replacing them with XML or JSON would fix this problem, but at the expense of removing a core feature.

git-am(1) (apply patches) delimits the commit message from the patch/diff by looking for (1) a line `---` or (2) a line that starts with `diff -` or (3) a line that starts with `Index:SP` (SP is space). Only the first rule is necessary for patches generated git-format-patch(1). But git-am(1) is for applying patches, and you are free to bring patches from some other system. That’s why, I suppose, there are multiple options.

This means that it will try to apply any unindented diffs in the commit message. But you’re fine if you indent the diff. (Newschool code fencers will have a worse time here.)

I imagine that this worked fine for changes that were authored by one person and submitted by another person via email, or by their friend, or by someone trying to resurrect a previous attempt at getting something upstreamed. Someone is likely to notice that examples diffs are getting applied. But it won’t work well at all if you are some software distributor who is using patch files to apply modifications to packages.

Recall that git-am(1) will not apply indented diffs. Well have a look at my GNU patch 2.7.6:

    If the entire diff is indented by a consistent amount, if lines end in
    CRLF, or if a diff is encapsulated one or more times by prepending "- "
    to lines starting with "-" as specified by Internet RFC 934, this is
    taken into account.

Some may say that patch(1) should work like a more straightforward importer. But I’ve been itching to point out something else.

    Larry Wall wrote the original version of patch.

Is it surprising if patch(1) is a bit DWIM?

And a lot of these older tools are not meant to be fed untrusted, unvetted input. The patch shown there confused me for quite a bit.

Or, more snarky: tee is also a huge security problem if you pipe untrusted input into `tee -a /etc/passwd`, such as `curl | tee -a /etc/passwd`. Not many things are safe with a `curl |` in front of them. I think yes might be?

> Maybe the worst part about this is that it can entirely come from a patch being exported by git and then imported straight back in to git.

No one wants to apply diffs in commit messages. But some people use this technique via email:

    Finally fix it

    ---

    Changes in v2:

    - Proper formatting
    - Remove irrelevant typo fix

They’ve used the `---` commit message delimiter in the commit message itself so that everything after it won’t be applied by git-am(1). So that’s intentional loss of round tripping.

I would personally use Git notes instead though.

    Finally fix it

    ---

    Notes:
        Changes in v2: ...