container: improve documentation

This change removes inconsistencies collected over time in this package.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/container.go

@@ -1,4 +1,5 @@
-// Package container implements unprivileged Linux containers with built-in support for syscall filtering.
+// Package container implements unprivileged Linux containers with built-in
+// support for syscall filtering.
 package container
 
 import (
@@ -42,22 +43,25 @@ type (
 		SchedPolicy int
 		// Cgroup fd, nil to disable.
 		Cgroup *int
-		// ExtraFiles passed through to initial process in the container,
-		// with behaviour identical to its [exec.Cmd] counterpart.
+		// ExtraFiles passed through to initial process in the container, with
+		// behaviour identical to its [exec.Cmd] counterpart.
 		ExtraFiles []*os.File
 
-		// param pipe for shim and init
+		// Write end of a pipe connected to the init to deliver [Params].
 		setup *os.File
-		// cancels cmd
+		// Cancels the context passed to the underlying cmd.
 		cancel context.CancelFunc
-		// closed after Wait returns
+		// Closed after Wait returns. Keeps the spawning thread alive.
 		wait chan struct{}
 
 		Stdin  io.Reader
 		Stdout io.Writer
 		Stderr io.Writer
 
-		Cancel    func(cmd *exec.Cmd) error
+		// Custom cancellation behaviour for the underlying [exec.Cmd]. Must
+		// deliver [CancelSignal] before returning.
+		Cancel func(cmd *exec.Cmd) error
+		// Copied to the underlying [exec.Cmd].
 		WaitDelay time.Duration
 
 		cmd *exec.Cmd
@@ -286,7 +290,11 @@ func (p *Container) Start() error {
 
 	// place setup pipe before user supplied extra files, this is later restored by init
 	if fd, f, err := Setup(&p.cmd.ExtraFiles); err != nil {
-		return &StartError{true, "set up params stream", err, false, false}
+		return &StartError{
+			Fatal: true,
+			Step:  "set up params stream",
+			Err:   err,
+		}
 	} else {
 		p.setup = f
 		p.cmd.Env = []string{setupEnv + "=" + strconv.Itoa(fd)}
@@ -298,10 +306,16 @@ func (p *Container) Start() error {
 		runtime.LockOSThread()
 		p.wait = make(chan struct{})
 
-		done <- func() error { // setup depending on per-thread state must happen here
-			// PR_SET_NO_NEW_PRIVS: depends on per-thread state but acts on all processes created from that thread
+		// setup depending on per-thread state must happen here
+		done <- func() error {
+			// PR_SET_NO_NEW_PRIVS: thread-directed but acts on all processes
+			// created from the calling thread
 			if err := SetNoNewPrivs(); err != nil {
-				return &StartError{true, "prctl(PR_SET_NO_NEW_PRIVS)", err, false, false}
+				return &StartError{
+					Fatal: true,
+					Step:  "prctl(PR_SET_NO_NEW_PRIVS)",
+					Err:   err,
+				}
 			}
 
 			// landlock: depends on per-thread state but acts on a process group
@@ -313,28 +327,40 @@ func (p *Container) Start() error {
 
 				if abi, err := LandlockGetABI(); err != nil {
 					if p.HostAbstract {
-						// landlock can be skipped here as it restricts access to resources
-						// already covered by namespaces (pid)
+						// landlock can be skipped here as it restricts access
+						// to resources already covered by namespaces (pid)
 						goto landlockOut
 					}
-					return &StartError{false, "get landlock ABI", err, false, false}
+					return &StartError{Step: "get landlock ABI", Err: err}
 				} else if abi < 6 {
 					if p.HostAbstract {
 						// see above comment
 						goto landlockOut
 					}
-					return &StartError{false, "kernel version too old for LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET", ENOSYS, true, false}
+					return &StartError{
+						Step:   "kernel too old for LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET",
+						Err:    ENOSYS,
+						Origin: true,
+					}
 				} else {
 					p.msg.Verbosef("landlock abi version %d", abi)
 				}
 
 				if rulesetFd, err := rulesetAttr.Create(0); err != nil {
-					return &StartError{true, "create landlock ruleset", err, false, false}
+					return &StartError{
+						Fatal: true,
+						Step:  "create landlock ruleset",
+						Err:   err,
+					}
 				} else {
 					p.msg.Verbosef("enforcing landlock ruleset %s", rulesetAttr)
 					if err = LandlockRestrictSelf(rulesetFd, 0); err != nil {
 						_ = Close(rulesetFd)
-						return &StartError{true, "enforce landlock ruleset", err, false, false}
+						return &StartError{
+							Fatal: true,
+							Step:  "enforce landlock ruleset",
+							Err:   err,
+						}
 					}
 					if err = Close(rulesetFd); err != nil {
 						p.msg.Verbosef("cannot close landlock ruleset: %v", err)
@@ -346,7 +372,7 @@ func (p *Container) Start() error {
 			}
 
 			// sched_setscheduler: thread-directed but acts on all processes
-			// created from that thread
+			// created from the calling thread
 			if p.SchedPolicy > 0 {
 				p.msg.Verbosef("setting scheduling policy %d", p.SchedPolicy)
 				if err := schedSetscheduler(
@@ -364,7 +390,11 @@ func (p *Container) Start() error {
 
 			p.msg.Verbose("starting container init")
 			if err := p.cmd.Start(); err != nil {
-				return &StartError{false, "start container init", err, false, true}
+				return &StartError{
+					Step:        "start container init",
+					Err:         err,
+					Passthrough: true,
+				}
 			}
 			return nil
 		}()
@@ -376,6 +406,7 @@ func (p *Container) Start() error {
 }
 
 // Serve serves [Container.Params] to the container init.
+//
 // Serve must only be called once.
 func (p *Container) Serve() error {
 	if p.setup == nil {
@@ -385,12 +416,21 @@ func (p *Container) Serve() error {
 	setup := p.setup
 	p.setup = nil
 	if err := setup.SetDeadline(time.Now().Add(initSetupTimeout)); err != nil {
-		return &StartError{true, "set init pipe deadline", err, false, true}
+		return &StartError{
+			Fatal:       true,
+			Step:        "set init pipe deadline",
+			Err:         err,
+			Passthrough: true,
+		}
 	}
 
 	if p.Path == nil {
 		p.cancel()
-		return &StartError{false, "invalid executable pathname", EINVAL, true, false}
+		return &StartError{
+			Step:   "invalid executable pathname",
+			Err:    EINVAL,
+			Origin: true,
+		}
 	}
 
 	// do not transmit nil
@@ -415,7 +455,8 @@ func (p *Container) Serve() error {
 	return err
 }
 
-// Wait waits for the container init process to exit and releases any resources associated with the [Container].
+// Wait blocks until the container init process to exit and releases any
+// resources associated with the [Container].
 func (p *Container) Wait() error {
 	if p.cmd == nil || p.cmd.Process == nil {
 		return EINVAL
@@ -460,11 +501,13 @@ func (p *Container) StderrPipe() (r io.ReadCloser, err error) {
 }
 
 func (p *Container) String() string {
-	return fmt.Sprintf("argv: %q, filter: %v, rules: %d, flags: %#x, presets: %#x",
-		p.Args, !p.SeccompDisable, len(p.SeccompRules), int(p.SeccompFlags), int(p.SeccompPresets))
+	return fmt.Sprintf(
+		"argv: %q, filter: %v, rules: %d, flags: %#x, presets: %#x",
+		p.Args, !p.SeccompDisable, len(p.SeccompRules), int(p.SeccompFlags), int(p.SeccompPresets),
+	)
 }
 
-// ProcessState returns the address to os.ProcessState held by the underlying [exec.Cmd].
+// ProcessState returns the address of os.ProcessState held by the underlying [exec.Cmd].
 func (p *Container) ProcessState() *os.ProcessState {
 	if p.cmd == nil {
 		return nil
@@ -472,7 +515,8 @@ func (p *Container) ProcessState() *os.ProcessState {
 	return p.cmd.ProcessState
 }
 
-// New returns the address to a new instance of [Container] that requires further initialisation before use.
+// New returns the address to a new instance of [Container]. This value requires
+// further initialisation before use.
 func New(ctx context.Context, msg message.Msg) *Container {
 	if msg == nil {
 		msg = message.New(nil)
@@ -486,7 +530,13 @@ func New(ctx context.Context, msg message.Msg) *Container {
 }
 
 // NewCommand calls [New] and initialises the [Params.Path] and [Params.Args] fields.
-func NewCommand(ctx context.Context, msg message.Msg, pathname *check.Absolute, name string, args ...string) *Container {
+func NewCommand(
+	ctx context.Context,
+	msg message.Msg,
+	pathname *check.Absolute,
+	name string,
+	args ...string,
+) *Container {
 	z := New(ctx, msg)
 	z.Path = pathname
 	z.Args = append([]string{name}, args...)