Using the output path to avoid repeating previously failued builds results to changes in fixed output paths not being rebuilt (e.g., updating a src url). Use the derivation path instead to fix this.

This seems to have resolved the issue on our hydra server (i.e., it now performs rebuilds even if only the source url is changed). I'm not an expert on this code base at all, so please do double check.

Here is a top-level example (usually this occurs on the src line for a package and not at top-level) of a package that exhibits the problem. Initially you have

drvtest = fetchurl {
  url = "https://www.sharcnet.ca/~tyson-oops/git.pdf";
  sha256 = "0w319fv5aanllf92m083ldh11vz21cdjnpyb7mnwmlyi06i628cy";
};

then you realize the url has changed (or is wrong), so you update it to the correct one

drvtest = fetchurl {
  url = "https://www.sharcnet.ca/~tyson/git.pdf";
  sha256 = "0w319fv5aanllf92m083ldh11vz21cdjnpyb7mnwmlyi06i628cy";
};

but hydra doesn't rebuild it because both have the same fixed output path. With this patch it does rebuild because they don't both have the same drv path. Here is the test case I ran on our hydra server with this patch in place

• Bad URL build that fails
• Correct URL build that succeeds

Note, under the Details tab, how both builds have the same Output store paths.