The original idea was to implement a git-fetcher in Nix's core that
supports content hashes[1]. In #3549[2] it has been suggested to
actually use `fetchTree` for this since it's a fairly generic wrapper
over the new fetcher-API[3] and already supports content-hashes.
This patch implements a new git-fetcher based on `fetchTree` by
incorporating the following changes:
* Removed the original `fetchGit`-implementation and replaced it with an
alias on the `fetchTree` implementation.
* Ensured that the `git`-fetcher from `libfetchers` always computes a
content-hash and returns an "empty" revision on dirty trees (the
latter one is needed to retain backwards-compatibility).
* The hash-mismatch error in the fetcher-API exits with code 102 as it
usually happens whenever a hash-mismatch is detected by Nix.
* Removed the `flakes`-feature-flag: I didn't see a reason why this API
is so tightly coupled to the flakes-API and at least `fetchGit` should
remain usable without any feature-flags.
* It's only possible to specify a `narHash` for a `git`-tree if either a
`ref` or a `rev` is given[4].
* It's now possible to specify an URL without a protocol. If it's missing,
`file://` is automatically added as it was the case in the original
`fetchGit`-implementation.
[1] https://github.com/NixOS/nix/pull/3216
[2] https://github.com/NixOS/nix/pull/3549#issuecomment-625194383
[3] https://github.com/NixOS/nix/pull/3459
[4] https://github.com/NixOS/nix/pull/3216#issuecomment-553956703
This provides a pluggable mechanism for defining new fetchers. It adds
a builtin function 'fetchTree' that generalizes existing fetchers like
'fetchGit', 'fetchMercurial' and 'fetchTarball'. 'fetchTree' takes a
set of attributes, e.g.
fetchTree {
type = "git";
url = "https://example.org/repo.git";
ref = "some-branch";
rev = "abcdef...";
}
The existing fetchers are just wrappers around this. Note that the
input attributes to fetchTree are the same as flake input
specifications and flake lock file entries.
All fetchers share a common cache stored in
~/.cache/nix/fetcher-cache-v1.sqlite. This replaces the ad hoc caching
mechanisms in fetchGit and download.cc (e.g. ~/.cache/nix/{tarballs,git-revs*}).
This also adds support for Git worktrees (c169ea5904).
Thus, -I / $NIX_PATH entries are now downloaded only when they are
needed for evaluation. An error to download an entry is a non-fatal
warning (just like non-existant paths).
This does change the semantics of builtins.nixPath, which now returns
the original, rather than resulting path. E.g., before we had
[ { path = "/nix/store/hgm3yxf1lrrwa3z14zpqaj5p9vs0qklk-nixexprs.tar.xz"; prefix = "nixpkgs"; } ... ]
but now
[ { path = "https://nixos.org/channels/nixos-16.03/nixexprs.tar.xz"; prefix = "nixpkgs"; } ... ]
Fixes#792.
