How to Patch an Ebuild

Background

ChromeOS uses a package management system from Gentoo Linux called Portage.

Sometimes, an upstream package does not quite serve our specific needs. When that is the case, you can patch the package in order to bend it to your will.

Variables used in this document

Testing	Blah
`$REPO_BASE`	The path to your chromiumos checkout.
`$BOARD`	The board for which you are building, such as grunt or octopus.
`$CATEGORY`	The category of your Gentoo package, such as chromeos-base or dev-python.
`$PKG`	The name of your Gentoo package, such as autotest or protobuf-python.
`$REV`	The revision number of your Gentoo package.
`$EBUILD_DIR`	`$REPO_BASE/src/third_party/chromiumos-overlay/$CATEGORY/$PKG/`
`$EBUILD_FILE`	`$EBUILD_DIR/$PKG(-$REV).ebuild`
`$PATCH_NAME`	The basename of your patch, such as `foobar.patch`

Process

1. Unpack a local copy of the ebuild

In order to modify the ebuild, you need to unpack a local copy. This is easy.

Create a local copy in /build/$BOARD/tmp/portage/$CATEGORY/$PKG(-$REV)/work/$PKG(-$REV):

$ ebuild-$BOARD $(equery which $CATEGORY/$PKG) unpack

Move that local copy somewhere that it won’t get clobbered:

$ cp /build/$BOARD/tmp/portage/$CATEGORY/$PKG(-$REV)/work/$PKG(-$REV) /tmp/my_cool_pkg/ -R

Initialize a git repo in your local copy, so that you can diff it later:

$ cd /tmp/my_cool_pkg/
$ git init
$ git add *
$ git commit -m “Initial commit”

2. Make some changes and create a patch

You can modify the files in /tmp/my_cool_pkg as you please.

When you’re ready to give your change a try, it’s time to create a patch file. Conveniently, patch files look exactly like git diffs. (That is why we ran git init above).

$ git diff | tee /tmp/$PATCH_NAME

3. Include your patch in the ebuild

Create the directory $EBUILD_FILE/files/. Your ebuild knows that directory as ${FILESDIR}. Your patch file should go in there.

$ mkdir $EBUILD_DIR/files
$ cp /tmp/$PATCH_NAME $EBUILD_DIR/files/$PATCH_NAME

If the ebuild is EAPI 6 or higher and calls a default src_prepare() function, then src_prepare() will automatically apply every patch file declared in an array named PATCHES. If that describes your situation, then declare an array called PATCHES before any functions are declared:

PATCHES = (
    “${FILESDIR}”/$PATCH_NAME
)

Otherwise—which is to say, if your ebuild is EAPI 5 or lower, or it does not call a default src_prepare() function—then you will need to manually apply your patch:

Inherit eutils. If the package already inherits some eclasses, add eutils to that list (example: inherit cros-workon linux-info eutils). If the package does not yet inherit any eclasses, then add the following line at the beginning of the ebuild, after initial comments:

inherit eutils

At the beginning of the src_prepare() step, add the following line:

epatch "${FILESDIR}"/$PATCH_NAME

4. Test your change

$ emerge-$BOARD $CATEGORY/$PKG

If your patch worked, goto step 5. Else, goto step 2.

5. Comment your patch, uprev it, and commit it

You can add comments at the start of your patch by prefixing them with octothorpes (#). Do that.

You will need to uprev the package. If the ebuild already has a basename like $PKG-r1.ebuild, just increment the number after the r. If there is no -r#, then create a symlink to the original ebuild:

ln -s $PKG.ebuild $PKG-r1.ebuild

Then rejoice, for your patch is ready for the CQ.