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Add troubleshooting to build_p4_fusion.md (#48763)

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Ran into a problem with Nix and archive hashes when building p4-fusion
locally; found a solution and updated the docs with a troubleshooting
section in case others hit the same problem.



## Test plan
Run docsite and make sure the page displays as expected.
<!-- All pull requests REQUIRE a test plan:
https://docs.sourcegraph.com/dev/background-information/testing_principles
-->

---------

Co-authored-by: Noah S-C <noah@sourcegraph.com>
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Peter Guy 2023-03-06 13:29:40 -08:00 committed by GitHub
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doc/dev/background-information/build_p4_fusion.md
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@ -11,7 +11,55 @@ In order to import Perforce depots into Sourcegraph we first convert them into g
Below are the instructions for building p4-fusion locally, assuming you have the [Sourcegraph repository](https://github.com/sourcegraph/sourcegraph) checked out.
1. Follow [these instruction](https://nixos.org/download.html) to install Nix. (Tested with version 2.11.1)
2. Navigate to the root of your Sourcegraph directory
3. Run `nix build ".#p4-fusion" --verbose --extra-experimental-features nix-command --extra-experimental-features flakes`
1. Navigate to the root of your Sourcegraph directory
1. Run `nix build ".#p4-fusion" --verbose --extra-experimental-features nix-command --extra-experimental-features flakes`
If the build completes successfully you should have a `p4-fusion` binary in `./result/bin/p4-fusion` which you can copy somewhere in your `$PATH`
## Troubleshooting
### `nix build` fails with "hash mismatch" error referencing `helix-core-api.drv`
The `p4-fusion` dependencies specified in [the `srcs` array of `dev/nix/p4-fusion.nix`](https://sourcegraph.com/github.com/sourcegraph/sourcegraph@8d513301f8a12f7c7e0b5a66ed20ba14b6679cca/-/blob/dev/nix/p4-fusion.nix?L45-77) are sometimes updated without being versioned, so their hashes change, which causes "hash mismatch" errors.
**Example**
```bash
building '/nix/store/cb2ssx8vykl1ghb4k87yp3q6wnvfvjj2-helix-core-api.drv'...
error: hash mismatch in fixed-output derivation '/nix/store/cb2ssx8vykl1ghb4k87yp3q6wnvfvjj2-helix-core-api.drv':
specified: sha256-8yX9sjE1f798ns0UmHXz5I8kdpUXHS01FG46SU8nsZw=
got: sha256-gaYvQOX8nvMIMHENHB0+uklyLcmeXT5gjGGcVC9TTtE=
error: 1 dependencies of derivation '/nix/store/m409z1rq40bwzvvndbnghrrxm000zd9v-p4-fusion.drv' failed to build
```
To update the hashes, we use `nix` tools to calculate the hashes of the unpacked archives, and convert the format to an SRI representation that includes the type of the hash.
Get the URL of the archive from the part of the `srcs` array that matches the OS and architecture of your local system.
**Example**
Replace `<url of archive>` with the value of the archive url from `srcs`
```bash
$ hash_type=sha256
$ hash_value=$(nix-prefetch-url --type "${hash_type}" --unpack <url of archive>)
$ nix --extra-experimental-features nix-command hash to-sri "${hash_type}:${hash_value}"
```
Copy the output from that sequence of commands and paste it into the value of the `hash` field in the `fetchzip` attribute set.
Since when one changes, they all probably change, here is an example of getting the updated hashes for all of the archives ([current archive URLs; double-check that they are still correct](https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/dev/nix/p4-fusion.nix))
```bash
hash_type=sha256
for url in \
"https://cdist2.perforce.com/perforce/r22.2/bin.macosx12arm64/p4api-openssl1.1.1.tgz" \
"https://cdist2.perforce.com/perforce/r22.2/bin.macosx12x86_64/p4api-openssl1.1.1.tgz" \
"https://cdist2.perforce.com/perforce/r22.2/bin.linux26x86_64/p4api-glibc2.3-openssl1.1.1.tgz"
do
echo "${url}"
hash_value=$(nix-prefetch-url --type "${hash_type}" --unpack "${url}")
nix --extra-experimental-features nix-command hash to-sri "${hash_type}:${hash_value}"
echo
done
```