feat(server): implement tags/list endpoint

[eonpatapon]

When a git source is used branch names are exposed in the tags list.

For other source types, only "latest" is exposed.

Fixes: #85

[eonpatapon]

Using a local clone of nixpkgs:

 skopeo --insecure-policy --debug inspect --tls-verify=false docker://localhost:3000/shell
DEBU[0000] reference rewritten from 'localhost:3000/shell:latest' to 'localhost:3000/shell:latest'
DEBU[0000] Trying to access "localhost:3000/shell:latest"
DEBU[0000] Credentials not found
DEBU[0000] Using registries.d directory /etc/containers/registries.d for sigstore configuration
DEBU[0000]  No signature storage configuration found for localhost:3000/shell:latest
DEBU[0000] Looking for TLS certificates and private keys in /etc/docker/certs.d/localhost:3000
DEBU[0000] GET https://localhost:3000/v2/
DEBU[0000] Ping https://localhost:3000/v2/ err Get https://localhost:3000/v2/: http: server gave HTTP response to HTTPS client (&url.Error{Op:"Get", URL:"https://localhost:3000/v2/", Err:(*errors.errorString)(0xc000331630)})
DEBU[0000] GET http://localhost:3000/v2/
DEBU[0000] Ping http://localhost:3000/v2/ status 200
DEBU[0000] GET http://localhost:3000/v2/shell/manifests/latest
DEBU[0000] Downloading /v2/shell/blobs/sha256:4cd79f85ab6250c3a111db2fa14c30d3444e00e4941aac99822f2e813113233e
DEBU[0000] GET http://localhost:3000/v2/shell/blobs/sha256:4cd79f85ab6250c3a111db2fa14c30d3444e00e4941aac99822f2e813113233e
DEBU[0000] Credentials not found
DEBU[0000] Using registries.d directory /etc/containers/registries.d for sigstore configuration
DEBU[0000]  No signature storage configuration found for localhost:3000/shell:latest
DEBU[0000] Looking for TLS certificates and private keys in /etc/docker/certs.d/localhost:3000
DEBU[0000] GET https://localhost:3000/v2/
DEBU[0000] Ping https://localhost:3000/v2/ err Get https://localhost:3000/v2/: http: server gave HTTP response to HTTPS client (&url.Error{Op:"Get", URL:"https://localhost:3000/v2/", Err:(*errors.errorString)(0xc000331af0)})
DEBU[0000] GET http://localhost:3000/v2/
DEBU[0000] Ping http://localhost:3000/v2/ status 200
DEBU[0000] GET http://localhost:3000/v2/shell/tags/list
{
    "Name": "localhost:3000/shell",
    "Digest": "sha256:80dd7c0162c5a7d3b21f39d30b89b7aa0e90d792d8a0c4d18449e9567a1277b9",
    "RepoTags": [
        "latest",
        "fix-waybar",
        "kapow",
        "khronos",
        "nextcloud-paths",
        "nixos-unstable",
        "pipewire",
        "postfix-exporter",
        "release-19.09",
        "safe",
        "terraform-provider-keycloak",
        "terraform-providers",
        "terragrunt",
        "update-gitAndTools.delta",
        "vim-cue",
        "vim-plugins",
        "waybar-0.9.0",
        "wireshark"
    ],
    "Created": "0001-01-01T00:00:00Z",
    "DockerVersion": "",
    "Labels": null,
    "Architecture": "amd64",
    "Os": "linux",
    "Layers": [
        "sha256:4154da7086872108190d44dee0f850201a7c0b934a574af5c18014a018204d92",
        "sha256:60f1476de294ba0a45d8015579da439b5106afb38eec5fc0ee90152457682ab0",
        "sha256:a3a9b38adbc40c72a55bff6b151c475e2af9a2d1fff2ea58963be895cf0896dc",
        "sha256:adfb766e6074c49187e5f855f01cab9f143ecc50d5a1a9a41e7f4b168a5c7a6a",
        "sha256:b56088c8a44a150629bbd7c98d91f235080b59ce9f66dd86727a318d97fe5074",
        "sha256:ae8407af708662d112d57e41ea9088e2e30b13ca86870f6adddd6ca9327ac771",
        "sha256:fa80bfd2dcf2085098b3a967df1b28212cddcfa056a426291d4134e162d65e7b",
        "sha256:19f82ae38f6d463d2bbfe1c4c17c3652964494f4569b5a5f6f920032431dfe3e",
        "sha256:cefc7fc57a1a09633bca23a917f76b6db90bfcaddead61014950a12dd332ef6a",
        "sha256:a38d5cea5f469a68cb3877e81e82a91f541795ef01c87a554992ca66a19dd50c"
    ],
    "Env": null
}

[tazjin]

Hey, thanks for this PR! Seems like a useful feature :-)

Before doing an in-depth review, I have a couple of remarks:

1. Should both branches and tags be listed?
2. The plainOpen function from go-git seems to open repositories from local file paths only, however the Nixery git package set can come from remote repositories (e.g. via SSH, HTTP or the git protocol), too. Since the fetching currently happens in Nix (via builtins.fetchGit) it might be necessary to turn the fetching into a two-phase operation (caching the fetches for repo settings where cacheKey != "").
3. Is the new dependency on go-git (and its transitive deps) needed? A git repository's local layout on the filesystem lists branches via the .git/refs/heads folder, and tags via .git/refs/tags. If the repository is present on the filesystem, looking into those to construct the list might be a better way.

[eonpatapon]

Hi, thanks for the feedback

1. Actually I included only branches names since I wasn't able to build an image using a git tag. I didn't really look into it. I've just supposed it is not supported atm:

When trying to build with some git tag (19.09):

{"cmd":"nixery-prepare-image","eventTime":"2020-02-26T10:02:42.248190084+01:00","image":"shell","message":"[nix] error: cannot update ref 'refs/heads/19.09': trying to write non-commit object 0d99a63fd353198d13b1f6c17eb747a9b28f47c5 to branch 'refs/heads/19.09'","serviceContext":{"service":"nixery","version":"devel"},"severity":"INFO"}

2. Are you suggesting to fetch the repos (HTTP, git://) before running the nix build ?

3. I agree, we don't need to pull this extra dependency. I was just a bit too lazy :)

[eonpatapon]

Ok I get why building git tags doesn't work.

In the Render function of GitSource:

    if commitRegex.MatchString(tag) {
        args["rev"] = tag
    } else {
        args["ref"] = tag
    }

And I found in the manual for builtins.fetchGit: By default, the ref value is prefixed with refs/heads/. As of Nix 2.3.0 Nix will not prefix refs/heads/ if ref starts with refs/.

So that means that nixery would need to know if the requested docker tag is a git tag or git branch of the current repo before calling fetchGit. Which is kind of tricky since you can have a tag and a branch that have the same name.

[tazjin]

Hm, interesting, thanks! Lets spin the tag issue out into a separate problem for now (#89).

Are you suggesting to fetch the repos (HTTP, git://) before running the nix build?

Yeah, currently this happens as part of the build. In order for the functionality implemented here to work for remote repos the fetching needs to be separate. I believe the call to PlainOpen would currently just fail if Nixery is configured with a remote URL.

As for the dependency on go-git, I would prefer if we could avoid it for now (both branches and tags can be listed just via the filesystem regardless).

[eonpatapon]

I've removed the dependency to go-git. I will have a look how we can fetch remote git repos before running the build when I have some time.