ociman - OCI Manager

A Rust library providing a unified API for OCI container runtimes (Docker, Podman).

Status: Pre-1.0 - exists to serve mbj/mrs monorepo, expect breaking changes without notice.

Goals

• Unified API: Single interface for OCI-compliant container runtimes
• Auto-detection: Automatically detects available container runtime
• Environment override: Control backend selection via OCIMAN_BACKEND environment variable
• Container lifecycle management: Run, execute commands, inspect, and manage containers
• Image building: Build images from Dockerfiles or inline instructions
• Content-based hashing: Automatic tag generation based on SHA256 of build context/instructions for deterministic builds

Executing Commands in Containers

Use container.exec() to run commands inside a running container:

// Simple command execution
container.exec("echo")
    .argument("hello")
    .status()?;

// Capture stdout
let output = container.exec("cat")
    .argument("/etc/os-release")
    .build()
    .capture_stdout()
    .string()?;

// With environment variables and stdin
let result = container.exec("psql")
    .argument("--dbname=mydb")
    .environment_variable(PG_PASSWORD, "secret")
    .stdin(b"SELECT 1;")
    .build()
    .capture_stdout()
    .bytes()?;

The ExecCommand builder focuses on container exec configuration. For stream capture, use .build() to get a cmd_proc::Command, then use its stream methods.

Container Stopping and Removal

Containers must be stopped and removed explicitly by the caller. ociman::Container intentionally does not implement Drop for automatic cleanup. This is a deliberate design decision:

• Blocking I/O in Drop is unsafe: Stopping/removing a container shells out to docker/podman. If the subprocess fails, an unwrap() inside Drop causes a panic, which aborts the process when unwinding from another panic.
• --rm is the correct cleanup mechanism: Use .remove() on the Definition to pass --rm to the container runtime. This ensures the runtime removes the container when it stops, even if the Rust process is killed.
• Explicit lifecycle is clearer: Callers always know when stop/remove happens. There are no hidden side effects on scope exit.

Typical usage patterns:

// Pattern 1: Use --rm flag + with_container (most common)
// Container is explicitly stopped after the closure, and --rm handles removal.
let definition = Definition::new(backend, image).remove();
definition.with_container(|container| {
    // use container
});

// Pattern 2: Stop, commit, then remove (for snapshotting)
// Cannot use --rm here because the container must survive stop for commit.
let mut container = definition.run_detached();
container.stop();
container.commit(&snapshot_image, false)?;
container.remove();

Content-Based Image Hashing

ociman supports automatic tag generation based on content hashing (SHA256). This ensures deterministic builds where the same content always produces the same image tag.

Benefits:

• Deterministic: Same content always produces the same tag
• Automatic cache invalidation: Content changes automatically produce a new tag
• No manual tag management: Hash is computed automatically
• Reproducibility: Easy to verify if an image matches its source

Important: Content-based hashing only captures the Dockerfile and build context, not the base images. Using unspecific tags like FROM alpine:latest reduces reproducibility since latest can point to different images over time. For fully reproducible builds, use specific base image digests:

# Less reproducible - tag can change
FROM alpine:latest

# More reproducible - specific version tag
FROM alpine:3.19

# Most reproducible - pinned to specific digest
FROM alpine@sha256:6457d53fb065d6f250e1504b9bc42d5b6c65941d57532c072d929dd0628977d0