I just tried to build this but something is going wrong.

hash mismatch in fixed-output derivation '/nix/store/qg2803fwr0fvg6k2cdvb58vm4ggkh2f6-03f036cbccdd8699f5fe8540ef317595a35bceb8.patch.drv':
  specified: sha256-grrxQdmoUgpFpzYtSP7c2tLhQn+gmFwl06+VCqlHVyw=                                                 
     got:    sha256-wVe7tlUaTnIMP6ugBeG3LkaS0wTG/14OaFaRvUcZfco=

Hmm weird, just tried it again (nixos-rebuild), and it works for me...
I have also run

nix-prefetch-url https://github.com/openzfs/zfs/commit/03f036cbccdd8699f5fe8540ef317595a35bceb8.patch

which outputs this:

0b2p8ylhm5dgscjmr650gx1f3lnsvkz4hb9nlx2hllm8v50z3fl2
Could it be that the patch probably changes depending on where (physical location) this url is accessed?
I have already wondered, why I had to change the hash from the original issue, which seems to be different as well to this hash mismatch.
Anything I can try to reproduce this?

I simply ran

nix-shell -p nixpkgs-review --run "nixpkgs-review pr --no-shell --post-result 111375"

EDIT: I checked the store path and indeed, the patch header seems to be missing. Instead of a patch I only got a diff, so this might be a problem on GitHub's side. I'm deleting the store paths right now and then retry.

I checked out your branch and tried to build a single derivation but it still fails.

$ git log -1
commit 5794ba8b652b862f19b0662acb7dabe37c0535ef (HEAD -> fix-zfs-path-sanitizer-patch, refs/nixpkgs-review/1)
Author: Philipp Mildenberger <philipp.mildenberger@koeln.de>
Date:   Sun Jan 31 02:00:06 2021 +0100

    zfs: fix zfs legacy mount
    
    Closes #110376
$ nix-build -A zfs
these 2 derivations will be built:
  /nix/store/qg2803fwr0fvg6k2cdvb58vm4ggkh2f6-03f036cbccdd8699f5fe8540ef317595a35bceb8.patch.drv
  /nix/store/xnmc86mn26idv3gjhyc2gjn52vfn5hhh-zfs-user-2.0.1.drv
building '/nix/store/qg2803fwr0fvg6k2cdvb58vm4ggkh2f6-03f036cbccdd8699f5fe8540ef317595a35bceb8.patch.drv'...

trying https://github.com/openzfs/zfs/commit/03f036cbccdd8699f5fe8540ef317595a35bceb8.patch
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  3594  100  3594    0     0  18151      0 --:--:-- --:--:-- --:--:-- 18060
error: --- Error --- nix-daemon
hash mismatch in fixed-output derivation '/nix/store/qg2803fwr0fvg6k2cdvb58vm4ggkh2f6-03f036cbccdd8699f5fe8540ef317595a35bceb8.patch.drv':
  specified: sha256-grrxQdmoUgpFpzYtSP7c2tLhQn+gmFwl06+VCqlHVyw=
     got:    sha256-wVe7tlUaTnIMP6ugBeG3LkaS0wTG/14OaFaRvUcZfco=
error: --- Error -------------------------------------------------------------------------------------- nix-build
1 dependencies of derivation '/nix/store/xnmc86mn26idv3gjhyc2gjn52vfn5hhh-zfs-user-2.0.1.drv' failed to build

Maybe somebody else can reproduce. Here is the output of nix-shell -p nix-info --run "nix-info -m":

• system: "x86_64-linux"
• host os: Linux 5.9.16, NixOS, 20.09.2787.6e7f25001fe (Nightingale)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.4pre20201201_5a6ddb3
• channels(root): ""
• nixpkgs: /etc/channels/nixos

Maybe it helps to compare the patches itself, this is the content of the url when I access it:
https://pastebin.com/4xSsBQQh

Is there a more reproducable way to access patches in github?

nix-prefetch-url https://github.com/openzfs/zfs/commit/03f036cbccdd8699f5fe8540ef317595a35bceb8.patch

which outputs this:

0b2p8ylhm5dgscjmr650gx1f3lnsvkz4hb9nlx2hllm8v50z3fl2
Could it be that the patch probably changes depending on where (physical location) this url is accessed?

no, this is because fetchpatch will do some normalization before saving it to the store path, thus it changes the sha.

I have already wondered, why I had to change the hash from the original issue, which seems to be different as well to this hash mismatch.
Anything I can try to reproduce this?

change the sha to something invalid, I usually just change something to an "a". and run the build again, the expression will then be evaluated, instead of referencing an existing store path.

nix-prefetch-url https://github.com/openzfs/zfs/commit/03f036cbccdd8699f5fe8540ef317595a35bceb8.patch

which outputs this:

0b2p8ylhm5dgscjmr650gx1f3lnsvkz4hb9nlx2hllm8v50z3fl2

nix-prefetch-url will add the contents to the nix store. which means that you will have a fixed-output-derivation that is different from what fetchpatch uses. That's why it works for you locally, as your nix-prefetch-url store contents is being referenced, but that was done locally so it can't be reproduced by @hmenke

Thanks, something new learned again.
I could now reproduce the same sha256sum.

there was an attempt to make the fixed-output-derivations a little more ergonomic in NixOS/rfcs#84 . However, once you make a content addressed derivation more than just it's content.... then you have issues when you want to have alternate sources, and a few other issues.

For now, the rule of thumb is just to make the sha invalid, and take what nix says it wanted. As there's also other issues like if the content is packed or unpack (fetchFromGithub will unpack the source) which will also not align with nix-prefetch-url.

Result of nixpkgs-review pr 111375 1

do we need a backport?

@Mic92 Yes, #111529