Add PullCommand to githttp package

Adding PullCommand through HTTP client.

I got one last MR to go. Implementing running gitlab.com/gitlab-org/gitlab-shell/v14/internal/command/pull PullCommand#execute in gitlab.com/gitlab-org/gitlab-shell/v14/internal/command/uploadpack Command#execute before closing this issue.

Related issue: #656 (closed)

Before

When a user performs a Git pull/clone over SSH request to secondary node, Gitlab Shell performs requests to secondary Workhorse & Rails which performs a Git over HTTP request to primary.

sequenceDiagram
    participant C as Git on client
    participant S as GitLab Shell
    participant I as Workhorse & Rails
    participant P as Workhorse & Rails

    Note left of C: git pull/clone
    Note over S,I: Secondary site
    Note over P: Primary site
    C->>+S: ssh git upload-pack request
    S->>I: SSH key validation (api/v4/internal/authorized_keys?key=..)
    I-->>S: HTTP/1.1 300 (custom action status) with {endpoint, msg, primary_repo}
    S->>I: POST /api/v4/geo/proxy_git_ssh/info_refs_upload_pack
    I->>P: POST $PRIMARY/foo/bar.git/info/refs/?service=git-upload-pack
    P-->>I: HTTP/1.1 200 OK
    I-->>S: <response>
    S-->>C: return Git response from primary
    C-->>S: stream Git data to pull
    S->>I: POST /api/v4/geo/proxy_git_ssh/upload_pack
    I->>P: POST $PRIMARY/foo/bar.git/git-upload-pack
    P-->>I: HTTP/1.1 200 OK
    I-->>S: <response>
    S-->>-C: gitlab-shell upload-pack response

After

When a user performs a Git pull/clone over SSH request to secondary node, Gitlab Shell performs Git over HTTP requests to primary Workhorse & Rails.

sequenceDiagram
    participant C as Git on client
    participant S as GitLab Shell
    participant I as Workhorse & Rails
    participant P as Workhorse & Rails

    Note left of C: git pull/clone
    Note over S,I: Secondary site
    Note over P: Primary site
    C->>+S: ssh git upload-pack request
    S->>I: SSH key validation (api/v4/internal/authorized_keys?key=..)
    I-->>S: HTTP/1.1 300 (custom action status) with {endpoint, msg, primary_repo, authorization headers}
    S->>P: POST $PRIMARY/foo/bar.git/info/refs/?service=git-upload-pack
    P-->>S: HTTP/1.1 200 OK
    P-->>S: <response>
    S-->>C: return Git response from primary
    C-->>S: stream Git data to pull
    S->>P: POST $PRIMARY/foo/bar.git/git-upload-pack
    P-->>S: HTTP/1.1 200 OK
    P-->>S: <response>
    S-->>-C: gitlab-shell upload-pack response

Solution

The solution is similar to the one provided in Perform Git over HTTP request to primary repo (!716 - merged).

• We need to modify internal/command/uploadpack/uploadpack.go and call a command similar to this one. It probably should be controlled by a different feature flag.
• A separate PullCommand should be implemented, similar to PushCommand. The info-refs and git-upload-pack responses should be converted according to this logic. It's similar to the following transformation (Rails logic -> Gitlab Shell logic)
• HTTP client for info-refs can be reused, an additional one for upload-pack should be implemented similar to receive-pack

This MR implements this solution.

Availability and Testing

• Install Geo https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/geo.md
• In the secondary GDK/gitlab-shell directory, checkout this MR's branch
• gdk start
• gdk stop sshd (because we will compile it and run it manually)
• make && bin/gitlab-sshd -config-dir=. from GitLab Shell
• Perform Git operations over SSH against the secondary's GitLab Shell
• Observe logs
• Enable geo_proxy_fetch_direct_to_primary feature flag on Rails on the primary GDK (it should propagate to the secondary)
• Perform Git operations again
• Observe logs-- the trip through the secondary's GitLab Rails should now be skipped