Too much magic in Gentoo

Christoph Berg

February 17, 2009

Five years after the release of patch 2.5.9, patch 2.6 was released two weeks ago. The most interesting feature is that it will now produce reject files in unified format when the input is in that format. Lucky for those of us whose brain hurts when trying to read context diffs.

Of course there are more changes, and one of them is affecting Gentoo in particular: Gentoo service announcement: keep clear of GNU patch-2.6. The linked Gentoo bug #293570 has attachments to reproduce the bug:

benz:~/tmp/tripwire-2.3.1-2 $ patch -p0 -F3 < ../tripwire-friend-classes.patch
patching file tripwire-2.3.1-2-p1/src/fco/fconame.h
Hunk #1 succeeded at 48 with fuzz 3 (offset -1 lines).
patching file tripwire-2.3.1-2-p1/src/fco/fcosetimpl.h
Hunk #1 succeeded at 45 with fuzz 3 (offset -1 lines).
patching file tripwire-2.3.1-2-p1/src/tw/fcoreport.h
Hunk #1 succeeded at 84 with fuzz 3 (offset -1 lines).

benz:~/tmp/tripwire-2.3.1-2 $ ls -l
drwxr-xr-x 4 cbe credativ 4096 18. Nov 05:03 src/
drwxr-xr-x 3 cbe credativ 4096  2. Dez 12:23 tripwire-2.3.1-2-p1/

Note that it created a new tripwire-2.3.1-2-p1 directory inside the existing one, which is consistent with -p0. The newly created file contain just the “+” lines from the patch. (There are no “-” lines.) patch 2.5.9 would have refused the patch because it couldn’t find the file to patch.

This is arguably a surprising behavior, but what really happens here is that -F3 (fuzz 3) will explicitly tell patch to ignore 3 context lines if it cannot apply the patch otherwise. diff’s default behavior is to create patches with 3 lines of context. patch’s default fuzz factor is one less than that, 2. Now, 3

3 is zero, which doesn’t leave any context left.

Apparently lots of Gentoo ebuilds use -F3, and try to guess the correct number of directories to strip, starting with -p0, -p1, … No wonder that things break badly for them now, as patches will suddenly succeed with -p0.

I’m not sure I like the new patch behavior, but in Gentoo’s case I’d say they are using way too much magic in their build system, and now get bitten by options they shouldn’t have put there in the first place.

Debian