dynamic-config-git

Read dynamic-config configuration from a git repository — GitHub, GitLab, Azure DevOps, Gitea, Bitbucket, or a bare git@host:repo.git. One file, or a set of them read out of one commit.

[dependencies]
dynamic-config = "0.6.1"
dynamic-config-git = "0.6.1"

use dynamic_config_git::{Credential, GitSource};

AppConfig::set_remote(
    GitSource::builder("https://github.com/acme/config.git")
        .branch("main")
        .path("services/api/config.yaml")
        .credential(Credential::token(std::env::var("GITHUB_TOKEN")?))
        .build()?,
);

// Fetching is explicit; the load that follows touches no network.
AppConfig::refresh_remote()?;
AppConfig::builder("app").init()?;

Configuration in git is how a great many teams already work: review, history, blame and rollback come free, and nobody runs etcd for a file that changes twice a month.

Why git rather than five REST APIs

GitHub, GitLab, Azure DevOps, Gitea and Bitbucket all speak git. Their file APIs are five clients, five auth models and five ways of spelling this ref. "Compatible with all of them" is only reachable through the protocol they share, and the extra round trip it costs is irrelevant at configuration cadence.

The implementation is gix — pure Rust, no libgit2, no C toolchain and no OpenSSL question. The exception is SSH, which gix carries by spawning the system ssh exactly as git does, so ssh must be on the host for an ssh:// remote.

This is the blocking RemoteSource trait. A git fetch is blocking work — negotiation, decompression, index writing — and an async program loses nothing: refresh_remote_async() puts a blocking source on off_thread.

What a fetch does

A shallow, single-ref fetch into a bare object database — never a clone, and never a checkout.

1. Connect and read the ref advertisement. This is what git ls-remote costs: a few hundred bytes, no objects.
2. If the commit is already in the object database, stop — an unchanged ref transfers nothing.
3. Otherwise ask for that one commit at depth 1.
4. Read one blob out of the tree, in memory.

What it costs. The first fetch transfers the repository's whole tree at that commit, because that is what the protocol delivers; a monorepo with a gigabyte of files will transfer a gigabyte once. Later fetches transfer one commit's worth. This crate is comfortable with a configuration repository and slow to start against a monorepo.

Filtering by path would cut that first transfer to the files actually read, and it is not implemented because nothing below this crate can express it: gix 0.86 exposes no filter on a fetch — the protocol argument lives one layer down, on a type only gix's own fetch ever holds — and the filter the large hosts serve is blob:none, which answers with a tree whose blobs are absent, so reading one needs a lazy fetch from a promisor remote that nothing in this dependency graph implements. Two upstream features away, not one call.

How long it may take. .with_timeout(..) is thirty seconds by default and bounds one attempt. What it reaches depends on the transport: with .tls(..) this crate builds the HTTP client, so the number is the connect deadline and the stall deadline on every read; without it, gix's own transport hardcodes a twenty-second connect timeout, exposes no other, and a host that accepts the connection and then says nothing is bounded by nothing. gix's interrupt flag — which is what bounds the pack — is checked between packets, and a silent host sends none.

Which ref

A branch is the default because a configuration store's reason to exist is that the configuration changes; pinning a SHA and then starting a watcher asks a loop to wait for something that cannot happen.

A SHA is fetched by asking the host for that object directly, which needs uploadpack.allowReachableSHA1InWant — GitHub, GitLab and Azure DevOps allow it; a self-hosted server may not, and the error says so.

Authenticating

Every credential can come from a callable, because the ones that matter expire.

Credential::expiring is the one this crate exists for. A store that takes token: String at construction works in a demo and fails at three in the morning on the first refresh: an installation token lives one hour and a watcher lives for the life of the process. The refresh machinery is dynamic-config-store-core's, shared with the Vault, Consul and Firestore crates — obtained once, reused until a minute before expiry, refreshed under one lock, and thrown away the moment the host refuses it.

let credential = Credential::expiring(|_previous| {
    let (token, lives_for) = installation_token()?;   // your GitHub client

    Ok(Issued {
        value: Auth::Https { username: "x-access-token".into(), password: token },
        ttl: Some(lives_for),
    })
});

The JWT-to-installation-token exchange is not in this crate. Signing an RS256 JWT needs an RSA implementation, and the pure-Rust one carries an unpatched timing-sidechannel advisory this workspace's licence and advisory gate rejects. A program that talks to GitHub almost certainly has a client that does the exchange already; what this crate owes that flow is the refresh.

An SSH key passphrase is not accepted either. ssh has no way to take one that does not put it on a command line, where ps can read it. Use an agent — ssh-add the key once — which is what an agent is for.

Several files as one document

.path(..) takes a path — or a Keys, for a set of them. A bare string is still one file, so nothing that already worked changed.

A directory, not a string prefix: a git tree has directories, so Keys::prefix("conf") reads conf/db.yaml and does not read conf-old.yaml. The walk is recursive, bounded at 512 files, and every file it finds has to parse — point it at a directory that holds configuration and nothing else. A directory has no extension, so it needs .format(..); a list whose members name two different formats is refused at build() rather than parsed as whichever came first.

This is the one store here whose multi-file sources can also be watched. Everywhere else a watch on a set is refused, because waking on one key and then re-reading the rest collects a document that never existed at any instant. What moves here is a ref, what a ref names is a commit, and every file is read out of that one commit's tree — so a deployment that writes four files in one commit is delivered as one document, and there is no interleaving to be had. The cost is the same one a single-file watch always had: a commit that touches nothing this source reads still moves the ref, so on_change may be called with an identical document. Spurious, never torn.

A host this machine does not already trust

An enterprise GitLab behind a private certificate authority, or a host that wants a client certificate before it will say hello:

use dynamic_config_git::TlsConfig;

GitSource::builder("https://gitlab.internal/acme/config.git")
    .path(Keys::prefix("conf"))
    .format(Format::Yaml)
    .tls(
        TlsConfig::new()
            .with_ca_certificate_file("/etc/ssl/certs/acme-root.pem")
            .with_client_certificate_files("/etc/ssl/app.crt", "/etc/ssl/app.key"),
    )
    .build()?;

TlsConfig is dynamic-config-store-core's, shared with the other store crates, so a deployment configuring two stores writes the same calls for both. The platform's own trust store still applies, so one configuration reaches both a private host and github.com. There is a bytes spelling of each — with_ca_certificate_pem, with_client_certificate_pem — for a program that already has the material and should not have to put a private key on a disk for a client to read back.

Which knob applies to which transport:

They do not overlap, and a .tls(..) on an ssh:// url is refused at build() rather than quietly doing nothing — a caller who wrote it believes a certificate authority is what authenticates an SSH host, and it is not.

There is no way to turn verification off, and the absence is a decision. The two situations anybody reaches for that in — a development server with a self-signed certificate, an enterprise private CA — are both trusting one more certificate, which is one call above and keeps the server authenticated. Turning verification off does not make TLS weaker in the way a checklist means; it makes it absent. And git sharpens it: a fetch presents its credential before it has received anything, so an unverified connection is one that hands a personal access token to whoever is on the path. There is no name frightening enough to make that a reasonable thing to offer.

What gix allowed, measured. gix is configured here with the pure-Rust reqwest transport, whose HTTP options type carries ssl_ca_info and ssl_verify — and whose reqwest backend reads neither; only the curl backend does. Its client is built inside a worker thread from a builder with no root store, no identity and no hook to reach either, so setting http.sslCAInfo through this crate would do nothing at all. gix does take a transport of the caller's own, and that is what .tls(..) installs: gix keeps the whole git half — handshake, protocol version, credential header, packet framing — and only the client construction is this crate's. Without .tls(..) nothing changes and gix's own transport is used. Adding a C TLS stack to a workspace that has none was the other way, and it is not worth what this crate's front page claims.

Where the objects live

A private directory, 0700 from the moment it exists, because it holds the contents of a private repository. A temporary one by default, removed with the source; name your own with .cache_dir(..) to survive restarts.

Either one would otherwise grow: a shallow fetch of a moving branch writes a pack every time the branch moves and nothing in git removes it until a gc. So every thirty-second transfer — .compact_after(n), and 0 turns it off — the object database is emptied and refilled by the fetch that emptied it. The rule it deletes under is narrow enough for one line: only what it wrote, only in a directory it created for itself, and only on a trigger you can see and turn off. A .cache_dir(..) pointing at a repository that already existed carries no marker file and is never touched, whatever compact_after says.

Two sources in one program may not share a directory: the second is refused at build() rather than allowed to corrupt the first. Two processes sharing one are not detected, and this crate will not grow a lock file to change that — a lock left by a killed process turns a fresh start into a hang, and the deployment that shares a directory is one volume in two containers, where pid liveness cannot see across a namespace and two nodes do not agree on a clock. What is done instead is to bound the damage: concurrent fetches into one object database are what git's own write-and-rename is built for, and a program that empties a directory another is reading costs that program one failed fetch — a failure this crate already survives. Give each program its own directory anyway.

When a fetch fails

The program stays up. A failed fetch leaves the previously fetched document installed and the previous configuration serving, and this crate's job is to be accurate enough for that to work:

• a host that refuses the credential is ErrorKind::Auth — waiting will not fix a wrong token, and a watch loop should stop rather than hammer;
• everything else is ErrorKind::Remote, which a watch loop waits out.

A refused credential that can be replaced is refreshed and retried once. One retry, not a loop: if a fresh credential is refused too, the grant is wrong.

Untrusted input

Everything the remote sends is untrusted, and each of these is an error rather than a surprise:

• a symlink in the tree is refused, never followed — where it points is the repository's choice, not this program's;
• a directory or a submodule where a file was expected;
• a tree entry, under a Keys::prefix walk, whose name leaves the directory — no porcelain writes one called .., but git mktree does and so can a pack from a host nobody controls, so every discovered path is checked again;
• a blob over max_bytes (a megabyte by default), refused from the object header before any of it is loaded — and max_bytes bounds the whole read as well as each file in it, so naming a directory does not multiply it by the file budget;
• a blob that is not UTF-8;
• a configured path with a . or .. component, refused at build().

Nothing is ever checked out, so no name in the tree can name a place outside the working directory in the first place.

Watching

git has no watch, so watch polls — and says so. Each tick is one ref advertisement; only a ref that moved costs a transfer. The push half needs nothing from this crate: whoever terminates a GitHub or GitLab webhook calls remote_sink().apply(..).

MSRV

1.85, higher than the workspace's 1.71 because gix is edition 2024. The core crate stays where it is; a companion pays for what it pulls in.

What it deliberately does not do

Shelling out to the system git. It would reach every credential helper on the host for free, and it would also be a second implementation of every decision above. Credential::ssh_command(..) reaches the one method the pure-Rust path cannot, without a second code path.