Add comments to main.rs

Cargo.toml

@@ -3,6 +3,7 @@ name = "youki"
 version = "0.0.1"
 authors = ["utam0k <k0ma@utam0k.jp>"]
 edition = "2018"
+description = "A container runtime written in Rust"
 
 [dependencies]
 clap = "3.0.0-beta.2"

docs/doc-draft.md

@@ -0,0 +1,53 @@
+_This is a draft for a high level documentation of Youki. After finished this is intended to provide how control flow and high level functioning of Youki happens for development purposes._
+
+## Some reference links
+
+These are references to various documentations and specifications, which can be useful to understand commands and constraints.
+
+- [OCI runtime specification] : The specification for a container runtime. Any OCI complaisant runtime must follow this.
+- [runc man pages] : has information on various commandline options supported by runc, can be used to understand commands and their options.
+- [cgroups man page](https://man7.org/linux/man-pages/man7/cgroups.7.html) : contains information about cgroups, their creation, deletion etc.
+
+---
+
+## Control flow diagram
+
+This is diagram as given in #14, which is not actually how this works, but helpful to understand overall flow. Someone needs to check and correct.
+
+```mermaid
+sequenceDiagram
+participant U as User
+participant D as Docker
+participant YP as Youki(Parent Process)
+participant YC as Youki(Child Process)
+participant YI as Youki(Init Process)
+
+
+U ->> D : $ docker run --rm -it --runtime youki $image
+D ->> YP : youki create $container_id
+YP ->> YC : fork(2)
+YC ->> YC : create new namespace
+YC ->> YI : fork(2)
+YI ->> YI : Mount the device
+YI -->> YP : ready message (Unix domain socket)
+YP ->> D : exit $code
+D ->> YP : $ youki start $container_id
+YP -->> YI : start message (Unix domain socket)
+YI ->> YI : run the commands in dockerfile
+D ->> D : monitor pid written in pid file
+D ->> U : exit $code
+
+```
+
+---
+
+## Control flow
+
+### main invocation
+
+On invoking Youki, main function parses args passed to it, which contains directory path to store container state (check runc . 8 . md in [runc man pages]), optional log path and log format string and a subcommand such as create, delete etc.
+
+From there it matches subcommand arg with possible subcommand and takes appropriate actions, such as creating a new container, deleting a container erc.
+
+[oci runtime specification]: https://github.com/opencontainers/runtime-spec/blob/master/runtime.md
+[runc man pages]: (https://github.com/opencontainers/runc/blob/master/man/runc.8.md)

src/main.rs

@@ -1,3 +1,7 @@
+//! # Youki
+//! Container Runtime written in Rust, inspired by [railcar](https://github.com/oracle/railcar)
+//! This crate provides a container runtime which can be used by a high-level container runtime to run containers.
+
 use std::fs;
 use std::path::PathBuf;
 
@@ -12,15 +16,20 @@ use youki::spec;
 use youki::start;
 use youki::{cgroups::Manager, command::linux::LinuxCommand};
 
+/// High-level commandline option definition
+/// This takes global options as well as individual commands as specified in [OCI runtime-spec](https://github.com/opencontainers/runtime-spec/blob/master/runtime.md)
+/// Also check [runc commandline documentation](https://github.com/opencontainers/runc/blob/master/man/runc.8.md) for more explanation
 #[derive(Clap, Debug)]
 #[clap(version = "1.0", author = "utam0k <k0ma@utam0k.jp>")]
 struct Opts {
+    /// root directory to store container state
     #[clap(short, long, default_value = "/run/youki")]
     root: PathBuf,
     #[clap(short, long)]
     log: Option<PathBuf>,
     #[clap(long)]
     log_format: Option<String>,
+    /// command to actually manage container
     #[clap(subcommand)]
     subcmd: SubCommand,
 }
@@ -40,6 +49,9 @@ pub struct Delete {
 pub struct StateArgs {
     pub container_id: String,
 }
+
+/// Subcommands accepted by Youki, confirming with [OCI runtime-spec](https://github.com/opencontainers/runtime-spec/blob/master/runtime.md)
+/// Also for a short information, check [runc commandline documentation](https://github.com/opencontainers/runc/blob/master/man/runc.8.md)
 #[derive(Clap, Debug)]
 enum SubCommand {
     #[clap(version = "0.0.1", author = "utam0k <k0ma@utam0k.jp>")]
@@ -66,6 +78,8 @@ impl SubCommand {
     }
 }
 
+/// This is the entry point in the container runtime. The binary is run by a high-level container runtime,
+/// with various flags passed. This parses the flags, creates and manages appropriate resources.
 fn main() -> Result<()> {
     let opts = Opts::parse();
 
@@ -88,11 +102,17 @@ fn main() -> Result<()> {
         SubCommand::Create(create) => create.exec(root_path, LinuxCommand),
         SubCommand::Start(start) => start.exec(root_path),
         SubCommand::Kill(kill) => {
+            // resolves relative paths, symbolic links etc. and get complete path
             let root_path = fs::canonicalize(root_path)?;
+            // state of container is stored in a directory named as container id inside
+            // root directory given in commandline options
             let container_root = root_path.join(&kill.container_id);
             if !container_root.exists() {
                 bail!("{} doesn't exists.", kill.container_id)
             }
+
+            // load container state from json file, and check status of the container
+            // it might be possible that kill is invoked on a already stopped container etc.
             let container = Container::load(container_root)?.refresh_status()?;
             if container.can_kill() {
                 let sig = signal::from_str(kill.signal.as_str())?;
@@ -109,15 +129,23 @@ fn main() -> Result<()> {
             }
         }
         SubCommand::Delete(delete) => {
+            // state of container is stored in a directory named as container id inside
+            // root directory given in commandline options
             let container_root = root_path.join(&delete.container_id);
             if !container_root.exists() {
                 bail!("{} doesn't exists.", delete.container_id)
             }
+            // load container state from json file, and check status of the container
+            // it might be possible that delete is invoked on a running container.
             let container = Container::load(container_root)?.refresh_status()?;
             if container.can_delete() {
                 if container.root.exists() {
+                    // remove the directory storing container state
                     fs::remove_dir_all(&container.root)?;
                     let spec = spec::Spec::load("config.json")?;
+                    // remove the cgroup created for the container
+                    // check https://man7.org/linux/man-pages/man7/cgroups.7.html
+                    // creating and removing cgroups section for more information on cgroups
                     let cmanager = Manager::new(spec.linux.unwrap().cgroups_path)?;
                     cmanager.remove()?;
                 }