I am trying to create a rpm using mock. https://fedoraproject.org/wiki/Projects/Mock
I am able to build an rpm through source rpm. Now I want to add a patch to this package and I have no idea how to proceed. Can you please let me know how can I go ahead with this? What is the way to modify/patch a package using mock?
The normal approach here is not to use mock to modify your package in any way. Mock is just a way to ensure that your package is built in a clean environment every time (a fresh chroot), and it's not really meant to do more than that.
The normal thing to do, then, would be to put the patch in the spec file for your RPM itself.
This requires two parts — first, the inclusion of the patch file as part of the package, and second, its application.
For the first, list the patch near the top of the spec file, usually right after your Source line (or lines). Each patch gets a number, and the normal convention is to start counting with 0, so if you have just one, that will look like this:
Patch0: packagename-version-terse_patch_description.patch
As with source files, anything up to the last / in that filename is stripped off, so you can use a URL if you want. The patch will need to be in your RPM sources directory (usually, next to the tarball.)
At this point, if you build a source RPM from your modified spec, the resulting src.rpm file will contain this patch file. (Try it — rpm -qlp packagename-ver-rel.src.rpm). But, it won't be applied. To do that, you need to use the %patch macro.
This goes in the %prep section of the specfile, usually right after the %setup macro line. Each %patch macro has a number corresponding to the Patch line in the header, so for your Patch0, add a line like this:
%patch0 -p1 -b .bugfix
Again by convention, patches used in RPM are made built one level up, so -p1 is appropriate. (Conveniently, this will be correct for diffs made with git, too.) And the -b .bugfix bit isn't necessary, but it's customary for debugging, and I guess serves as a sort of inline comment for what this specific patch macro does. (Replace the string "bugfix" with something appropriate to your actual patch.)
