Improve error message for failing `nix-prefetch-git`

[Ma27]

So I was trying to add a git dependency in a shell where lon was available, but nix-prefetch-git wasn't.

The lon add git cilib ... command resulted in the following error:

Adding cilib...
Locked revision: 1580c9cef4ae54cf32ea951d82e122fe74894c83
Failed to compute hash for git@git.example.com:dependency@1580c9cef4ae54cf32ea951d82e122fe74894c83

The only way for me to find out that nix-prefetch-git was missing was to strace lon:

[pid 159593] execve("/nix/var/nix/profiles/default/bin/nix-prefetch-git", ["nix-prefetch-git", "--name", "source", "git@git.example.com:depe"..., "1580c9cef4ae54cf32ea951d82e122fe"...], 0x7ffdbbb4c3c0 /* 190 vars */) = -1 ENOENT (No such file or directory)
[pid 159593] execve("/run/current-system/sw/bin/nix-prefetch-git", ["nix-prefetch-git", "--name", "source", "git@git.example.com:depe"..., "1580c9cef4ae54cf32ea951d82e122fe"...], 0x7ffdbbb4c3c0 /* 190 vars */) = -1 ENOENT (No such file or directory)
[pid 159593] exit_group(127) = ?
[pid 159588] <... clone3 resumed>) = 159593
[pid 159593] +++ exited with 127 +++

I'd very much prefer if lon would tell me this directly.

Observed with lon 0.8.0.

[Tom-Hubrecht]

The issue is not exactly that nix-prefetch-git was not available in the shell as the lon binary is wrapped with nix-prefetch-git's path, the root cause is that the package did not provide the correct binary and lon assumes that packages added in its wrapper actually exist

[nikstur]

This should be fixed now with: NixOS/nixpkgs#488336

[0xf09f95b4]

And, for the record as I was just hitting the same issue, -v actually tells you about the problem's cause without needing strace:

$ lon -v update
Failed to execute nix-prefetch-git. Most likely it's not on PATH: No such file or directory (os error 2)