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// Package system provides helpers to apply and revert groups of operations to the system.
package system
import (
"context"
"errors"
"strings"
"hakurei.app/hst"
"hakurei.app/message"
)
const (
// User type is reverted at final instance exit.
User = hst.EM << iota
// Process type is unconditionally reverted on exit.
Process
CM
)
// Criteria specifies types of Op to revert.
type Criteria hst.Enablement
func (ec *Criteria) hasType(t hst.Enablement) bool {
// nil criteria: revert everything except User
if ec == nil {
return t != User
}
return hst.Enablement(*ec)&t != 0
}
// Op is a reversible system operation.
type Op interface {
// Type returns [Op]'s enablement type, for matching a revert criteria.
Type() hst.Enablement
apply(sys *I) error
revert(sys *I, ec *Criteria) error
Is(o Op) bool
Path() string
String() string
}
// TypeString extends [hst.Enablement.String] to support [User] and [Process].
func TypeString(e hst.Enablement) string {
switch e {
case User:
return "user"
case Process:
return "process"
default:
buf := new(strings.Builder)
buf.Grow(48)
if v := e &^ User &^ Process; v != 0 {
buf.WriteString(v.String())
}
for i := User; i < CM; i <<= 1 {
if e&i != 0 {
buf.WriteString(", " + TypeString(i))
}
}
return strings.TrimPrefix(buf.String(), ", ")
}
}
// New returns the address of a new [I] targeting uid.
func New(ctx context.Context, msg message.Msg, uid int) (sys *I) {
if ctx == nil || msg == nil || uid < 0 {
panic("invalid call to New")
}
return &I{ctx: ctx, msg: msg, uid: uid, syscallDispatcher: direct{}}
}
// An I provides deferred operating system interaction. [I] must not be copied.
// Methods of [I] must not be used concurrently.
type I struct {
_ noCopy
uid int
ops []Op
ctx context.Context
// the behaviour of Commit is only defined for up to one call
committed bool
// the behaviour of Revert is only defined for up to one call
reverted bool
msg message.Msg
syscallDispatcher
}
func (sys *I) UID() int { return sys.uid }
// Equal returns whether all [Op] instances held by sys matches that of target.
func (sys *I) Equal(target *I) bool {
if sys == nil || target == nil || sys.uid != target.uid || len(sys.ops) != len(target.ops) {
return false
}
for i, o := range sys.ops {
if !o.Is(target.ops[i]) {
return false
}
}
return true
}
// Commit applies all [Op] held by [I] and reverts all successful [Op] on first error encountered.
// Commit must not be called more than once.
func (sys *I) Commit() error {
if sys.committed {
panic("attempting to commit twice")
}
sys.committed = true
sp := New(sys.ctx, sys.msg, sys.uid)
sp.syscallDispatcher = sys.syscallDispatcher
sp.ops = make([]Op, 0, len(sys.ops)) // prevent copies during commits
defer func() {
// sp is set to nil when all ops are applied
if sp != nil {
// rollback partial commit
sys.msg.Verbosef("commit faulted after %d ops, rolling back partial commit", len(sp.ops))
if err := sp.Revert(nil); err != nil {
printJoinedError(sys.println, "cannot revert partial commit:", err)
}
}
}()
for _, o := range sys.ops {
if err := o.apply(sys); err != nil {
return err
} else {
// register partial commit
sp.ops = append(sp.ops, o)
}
}
// disarm partial commit rollback
sp = nil
return nil
}
// Revert reverts all [Op] meeting [Criteria] held by [I].
func (sys *I) Revert(ec *Criteria) error {
if sys.reverted {
panic("attempting to revert twice")
}
sys.reverted = true
// collect errors
errs := make([]error, len(sys.ops))
for i := range sys.ops {
errs[i] = sys.ops[len(sys.ops)-i-1].revert(sys, ec)
}
// errors.Join filters nils
return errors.Join(errs...)
}
// noCopy may be added to structs which must not be copied
// after the first use.
//
// See https://golang.org/issues/8005#issuecomment-190753527
// for details.
//
// Note that it must not be embedded, due to the Lock and Unlock methods.
type noCopy struct{}
// Lock is a no-op used by -copylocks checker from `go vet`.
func (*noCopy) Lock() {}
func (*noCopy) Unlock() {}
|
