clone

Go library for programs that keep local checkouts of HTTPS Git repositories. Clone, fetch, remote queries, command retries, and blob reads shell out to the git binary, which must be on PATH. Programs that read many blobs can use the optional gogit module for in-process object access. Both modules support Go 1.25 or later.

Install

go get github.com/git-pkgs/clone

Clone or update a checkout

Ensure creates a shallow clone on its first call, then fetches and resets the existing checkout on later calls. The ref can be a branch, tag, commit ID, or an empty string for the remote's default branch.

ctx := context.Background()
url := "https://github.com/git-pkgs/clone"
dst := "/var/cache/my-tool/clone"

if err := clone.Ensure(ctx, clone.Retry{}, url, dst, "main", false); err != nil {
    log.Fatal(err)
}

fmt.Println(clone.Head(ctx, dst))

Pass true as the final argument for a full clone, including when an existing shallow checkout needs to be unshallowed. Clone and fetch errors are returned as *clone.UnreachableError, except when the context was canceled or reached its deadline. errors.As retrieves the URL and underlying Git error. ValidateURL accepts https:// URLs, while ValidateRef rejects leading hyphens, .., and characters outside letters, digits, ., _, /, and -.

Temporarily check out an exact tag

CheckoutTag resolves only a local tag with the given exact name, peels an annotated tag to its commit, and checks it out with detached HEAD. The returned function restores the previous commit and reattaches its branch if that branch has not moved. Both operations discard tracked changes.

restore, err := clone.CheckoutTag(ctx, dst, "v1.2.3")
if err != nil {
    log.Fatal(err)
}
defer func() {
    if err := restore(context.Background()); err != nil {
        log.Printf("restore checkout: %v", err)
    }
}()

Use errors.Is(err, clone.ErrTagNotFound) to distinguish an absent tag from an invalid tag or another local repository error. CheckoutTag does not fetch.

Persistent cache

Cache stores one checkout per URL under Root. Prepare holds a per-URL lock while updating the shallow checkout, then replaces dst with a copy and returns its commit. The destination must be outside Root.

cache := clone.Cache{
    Root: "/var/cache/my-tool/repositories",
}

commit, err := cache.Prepare(ctx,
    "https://github.com/git-pkgs/clone",
    "",
    "/tmp/job/src",
)
if err != nil {
    log.Fatal(err)
}

fmt.Println(commit, cache.DiskBytes("https://github.com/git-pkgs/clone"))

When a historical commit is missing from the shallow cache, EnsureCommit unshallows the checkout:

if err := cache.EnsureCommit(ctx, url, commit); err != nil {
    log.Fatal(err)
}

Read a file from a commit

clone.InspectBlob runs git show and returns at most maxBytes. The optional github.com/git-pkgs/clone/gogit module reads the object in process and falls back to git show for unsupported object formats and repository layouts. Install it separately when repeated process startup is costly:

go get github.com/git-pkgs/clone/gogit

Both implementations use the object's size to distinguish content exactly at the limit from truncated content. Complete reads use magic.Detect; truncated reads use magic.DetectPrefix so the result can report that later bytes may change the classification. They retain returned content for text, binary, and unknown results, and validate commits and paths before reading the repository. ValidCommit and SanitizePath are also available when callers need to validate input earlier:

path, ok := clone.SanitizePath("cmd/tool/main.go")
if !ok || !clone.ValidCommit(commit) {
    log.Fatal("invalid commit or path")
}

result, err := gogit.InspectBlob(
    ctx,
    filepath.Join(cache.Dir(url), "src"),
    commit,
    path,
    2<<20,
)
if err != nil {
    log.Fatal(err)
}
if result.Detection.Kind == magic.KindText &&
    result.Detection.Encoding == "utf-8" {
    fmt.Printf("%s", result.Content)
}
fmt.Println("truncated:", result.Truncated)

clone.Blob and gogit.Blob are available for callers that only need the original NUL-based binary flag. They return nil content when a NUL occurs within the returned range; a NUL beyond maxBytes is not observed.

Remote queries

RemoteBranches returns sorted branch names from git ls-remote --heads. It disables terminal prompts and the ambient credential helper so a supplied URL cannot trigger credential lookup. RemoteHead returns the SHA advertised for HEAD and keeps ambient non-interactive credentials available.

branches, err := clone.RemoteBranches(ctx, clone.Retry{}, url)
if err != nil {
    log.Fatal(err)
}

head, err := clone.RemoteHead(ctx, clone.Retry{}, url)
if err != nil {
    log.Fatal(err)
}

Retry policy

The zero value of Retry allows three attempts with exponential backoff and positive jitter. Do retries only recognized network and remote-service failures. Permanent markers win when output contains both kinds, and an unknown message stops after the first attempt.

retry := clone.Retry{
    Notify: func(notice clone.Notice) {
        log.Printf(
            "retrying %s after attempt %d of %d in %s",
            notice.Label,
            notice.Attempt,
            notice.Attempts,
            notice.Delay,
        )
    },
}

out, err := retry.Do(ctx, clone.Command{
    Label: "ls-remote",
    Env:   []string{"GIT_TERMINAL_PROMPT=0"},
    Args:  []string{"ls-remote", "--", url, "HEAD"},
})

TransientFailure exposes the same fail-closed classifier for callers with their own command loop. RunnerWithWaitDelay creates a Runner with a custom bound for transport child processes that retain Git's output pipe.

License

MIT