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refcount.go
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refcount.go
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package refcount
import (
"context"
"sync"
"sync/atomic"
"github.com/aperturerobotics/util/broadcast"
"github.com/aperturerobotics/util/ccontainer"
"github.com/aperturerobotics/util/promise"
)
// RefCountResolver resolves a value within a RefCount container.
//
// Accepts a released callback which can be called when the returned value is invalid.
// Returns a release function which will be called when the returned value is no longer needed.
type RefCountResolver[T comparable] func(ctx context.Context, released func()) (T, func(), error)
// RefCount is a refcount driven object container.
// Wraps a ccontainer with a ref count mechanism.
// When there are no references, the container contents are released.
type RefCount[T comparable] struct {
// ctx contains the root context
// can be nil
ctx context.Context
// keepUnref sets if the value should be kept if there are zero references.
keepUnref bool
// target is the target ccontainer
target *ccontainer.CContainer[T]
// targetErr is the destination for resolution errors
targetErr *ccontainer.CContainer[*error]
// resolver is the resolver function
// returns the value and a release function
// call the released callback if the value is no longer valid.
resolver RefCountResolver[T]
// mtx guards below fields
mtx sync.Mutex
// refs is the list of references.
refs map[*Ref[T]]struct{}
// resolveCtx is the resolution context.
resolveCtx context.Context
// resolveCtxCancel cancels resolveCtx
resolveCtxCancel context.CancelFunc
// nonce is incremented when starting/stopping the resolver
nonce uint32
// waitCh is a channel to wait before starting next resolve
// may be nil
waitCh chan struct{}
// resolved indicates the value is set
resolved bool
// value is the current value
value T
// valueErr is the current value error.
valueErr error
// valueRel releases the current value.
valueRel func()
}
// RefLike is an interface implemented by Ref.
type RefLike interface {
// Release releases the reference.
Release()
}
// Ref is a reference to a RefCount.
type Ref[T comparable] struct {
rc *RefCount[T]
rel atomic.Bool
cb func(resolved bool, val T, err error)
}
// Release releases the reference.
func (k *Ref[T]) Release() {
if k.rel.Swap(true) {
return
}
k.rc.removeRef(k)
}
// NewRefCount builds a new RefCount.
//
// ctx, target and targetErr can be empty
//
// keepUnref sets if the value should be kept if there are zero references.
// resolver is the resolver function
// returns the value and a release function
// call the released callback if the value is no longer valid.
func NewRefCount[T comparable](
ctx context.Context,
keepUnref bool,
target *ccontainer.CContainer[T],
targetErr *ccontainer.CContainer[*error],
resolver RefCountResolver[T],
) *RefCount[T] {
return &RefCount[T]{
ctx: ctx,
keepUnref: keepUnref,
target: target,
targetErr: targetErr,
resolver: resolver,
refs: make(map[*Ref[T]]struct{}),
}
}
// WaitRefCountContainer waits for a RefCount container handling errors.
// targetErr can be nil
func WaitRefCountContainer[T comparable](
ctx context.Context,
target *ccontainer.CContainer[T],
targetErr *ccontainer.CContainer[*error],
) (T, error) {
var errCh chan error
if targetErr != nil {
errCh = make(chan error, 1)
go func() {
outErr, _ := targetErr.WaitValue(ctx, errCh)
if outErr != nil && *outErr != nil {
select {
case errCh <- *outErr:
default:
}
}
}()
}
return target.WaitValue(ctx, errCh)
}
// SetContext updates the context to use for the RefCount container resolution.
// If ctx=nil the RefCount will wait until ctx != nil to start.
// This also restarts resolution, if there are any refs.
// Returns if the context was updated.
func (r *RefCount[T]) SetContext(ctx context.Context) bool {
var updated bool
r.mtx.Lock()
if r.ctx != ctx {
r.ctx = ctx
r.startResolveLocked()
updated = true
}
r.mtx.Unlock()
return updated
}
// SetContextIfCanceled updates the context to use for the RefCount container resolution.
// If the current r.ctx is not nil and not canceled, does nothing.
// If the passed ctx is nil or canceled, does nothing.
// This also restarts resolution if there are any refs.
// Returns if the context was updated.
func (r *RefCount[T]) SetContextIfCanceled(ctx context.Context) bool {
var updated bool
r.mtx.Lock()
if r.ctx == nil || r.ctx.Err() != nil {
if ctx != nil && ctx.Err() == nil {
r.ctx = ctx
r.startResolveLocked()
updated = true
}
}
r.mtx.Unlock()
return updated
}
// ClearContext clears the context and shuts down all routines.
func (r *RefCount[T]) ClearContext() {
_ = r.SetContext(nil)
}
// AddRef adds a reference to the RefCount container.
// cb is an optional callback to call when the value changes.
// the callback will be called with an empty value when the value becomes empty.
func (r *RefCount[T]) AddRef(cb func(resolved bool, val T, err error)) *Ref[T] {
r.mtx.Lock()
nref := &Ref[T]{rc: r, cb: cb}
r.refs[nref] = struct{}{}
if len(r.refs) == 1 && !r.resolved {
r.startResolveLocked()
} else if r.resolved {
nref.cb(true, r.value, r.valueErr)
}
r.mtx.Unlock()
return nref
}
// AddRefPromise adds a reference and returns a promise with the value.
func (r *RefCount[T]) AddRefPromise() (promise.PromiseLike[T], *Ref[T]) {
promCtr := promise.NewPromiseContainer[T]()
ref := r.AddRef(func(resolved bool, val T, err error) {
if !resolved {
promCtr.SetPromise(nil)
} else {
promCtr.SetResult(val, err)
}
})
return promCtr, ref
}
// Wait adds a reference and waits for a value.
// Returns the value, reference, and any error.
// If err != nil, value and reference will be nil.
func (r *RefCount[T]) Wait(ctx context.Context) (T, *Ref[T], error) {
prom, ref := r.AddRefPromise()
val, err := prom.Await(ctx)
if err != nil {
ref.Release()
return val, nil, err
}
return val, ref, nil
}
// WaitWithReleased adds a reference, waits for a value, returns the value and a release function.
// Calls the released callback (if set) when the value or reference is released.
// Note: it's very unlikely, but still possible, that released will be called before the promise resolves.
// Note: released will always be called from a new goroutine.
func (r *RefCount[T]) WaitWithReleased(ctx context.Context, released func()) (promise.PromiseLike[T], *Ref[T]) {
prom := promise.NewPromise[T]()
// fields guarded by r.mtx
var currResolved bool
var currNonce uint32
var callReleasedOnce sync.Once
var ref *Ref[T]
ref = r.AddRef(func(resolved bool, val T, err error) {
// note: r.mtx is held while calling this function.
// check if state is different, if we returned already.
if currResolved {
if !resolved || r.nonce != currNonce {
callReleasedOnce.Do(func() {
go func() {
ref.Release()
if released != nil {
released()
}
}()
})
}
return
}
if resolved || err != nil {
currResolved = true
currNonce = r.nonce
prom.SetResult(val, err)
}
})
return prom, ref
}
// Resolve adds a reference and waits for a value.
// Returns the value, release function, and any error.
// If err != nil, value and reference will be nil.
func (r *RefCount[T]) Resolve(ctx context.Context) (T, func(), error) {
val, ref, err := r.Wait(ctx)
if err != nil {
return val, nil, err
}
return val, ref.Release, nil
}
// ResolveWithReleased adds a reference, waits for a value, returns the value and a release function.
// Calls the released callback (if set) when the value or reference is released.
// Note: it's very unlikely, but still possible, that released will be called before the promise resolves.
// Note: released will always be called from a new goroutine.
// Note: this matches the signature of the refcount resolver function.
func (r *RefCount[T]) ResolveWithReleased(ctx context.Context, released func()) (T, func(), error) {
prom, ref := r.WaitWithReleased(ctx, released)
val, err := prom.Await(ctx)
if err != nil {
ref.Release()
return val, nil, err
}
return val, ref.Release, nil
}
// Access adds a reference, waits for a value, and calls the callback.
// Releases the reference once the callback has returned.
// The context will be canceled if the value is removed / changed.
// Return context.Canceled if the context is canceled.
// The callback may be restarted if the context is canceled and a new value is resolved.
// If useCtx=true and the current context is canceled, updates r to use ctx instead.
func (r *RefCount[T]) Access(ctx context.Context, useCtx bool, cb func(ctx context.Context, val T) error) error {
var bcast broadcast.Broadcast
var currVal T
var currErr error
var currResolved bool
var currNonce uint32
var currComplete bool
ref := r.AddRef(func(nowResolved bool, nowVal T, nowErr error) {
bcast.HoldLock(func(broadcast func(), getWaitCh func() <-chan struct{}) {
if nowResolved != currResolved || nowVal != currVal || nowErr != currErr {
currVal = nowVal
currErr = nowErr
currResolved = nowResolved
broadcast()
}
})
})
defer ref.Release()
var prevCancel context.CancelFunc
var prevWait chan struct{}
for {
if useCtx {
r.SetContextIfCanceled(ctx)
}
// mark the nonce has increased to ensure we release the correct value
bcast.HoldLock(func(broadcast func(), getWaitCh func() <-chan struct{}) {
currNonce++
})
if prevCancel != nil {
prevCancel()
prevCancel = nil
}
if prevWait != nil {
select {
case <-ctx.Done():
return context.Canceled
case <-prevWait:
prevWait = nil
}
}
var val T
var err error
var resolved bool
var nonce uint32
var complete bool
var waitCh <-chan struct{}
bcast.HoldLock(func(broadcast func(), getWaitCh func() <-chan struct{}) {
val, err, resolved, nonce, complete = currVal, currErr, currResolved, currNonce, currComplete
waitCh = getWaitCh()
})
if err != nil || complete {
return err
}
if resolved {
var cbCtx context.Context
cbCtx, prevCancel = context.WithCancel(ctx)
prevWait = make(chan struct{})
go func(cbCtx context.Context, doneCh chan struct{}, nonce uint32, val T) {
defer close(doneCh)
cbErr := cb(cbCtx, val)
bcast.HoldLock(func(broadcast func(), getWaitCh func() <-chan struct{}) {
if currNonce == nonce {
if currErr == nil {
currErr = cbErr
}
currComplete = currErr == nil
currResolved = false
broadcast()
}
})
}(cbCtx, prevWait, nonce, val)
}
select {
case <-ctx.Done():
if prevCancel != nil {
prevCancel()
}
return context.Canceled
case <-waitCh:
}
}
}
// removeRef removes a reference and shuts down if no refs remain.
func (r *RefCount[T]) removeRef(ref *Ref[T]) {
r.mtx.Lock()
lenBefore := len(r.refs)
delete(r.refs, ref)
lenAfter := len(r.refs)
if lenAfter < lenBefore && lenAfter == 0 {
if !r.keepUnref || !r.resolved || r.valueErr != nil {
r.shutdown()
}
}
r.mtx.Unlock()
}
// shutdown shuts down the resolver and clears state.
// expects mtx is locked by caller
func (r *RefCount[T]) shutdown() {
r.nonce++
r.clearResolvedState()
}
// clearResolvedState clears the resolved state.
// expects mtx is locked by caller
func (r *RefCount[T]) clearResolvedState() {
if r.resolved {
r.resolved = false
if r.valueErr != nil {
r.valueErr = nil
if r.targetErr != nil {
r.targetErr.SetValue(nil)
}
}
var empty T
if r.value != empty {
r.value = empty
if r.target != nil {
r.target.SetValue(empty)
}
}
r.callRefCbsLocked(false, empty, nil)
}
if r.resolveCtxCancel != nil {
r.resolveCtxCancel()
r.resolveCtx, r.resolveCtxCancel = nil, nil
}
if r.valueRel != nil {
r.valueRel()
r.valueRel = nil
}
}
// startResolveLocked starts the resolve goroutine.
// expects caller to lock mutex.
func (r *RefCount[T]) startResolveLocked() {
r.shutdown()
if r.ctx == nil || len(r.refs) == 0 {
return
}
waitCh := r.waitCh
doneCh := make(chan struct{})
r.waitCh = doneCh
r.resolveCtx, r.resolveCtxCancel = context.WithCancel(r.ctx)
nonce := r.nonce
go r.resolve(r.resolveCtx, waitCh, doneCh, nonce)
}
// resolve is the goroutine to resolve the value to the container.
func (r *RefCount[T]) resolve(ctx context.Context, waitCh, doneCh chan struct{}, nonce uint32) {
defer close(doneCh)
if waitCh != nil {
select {
case <-ctx.Done():
return
case <-waitCh:
}
}
released := func() {
resolveAfterRelease := func(lock bool) {
if lock {
r.mtx.Lock()
}
defer r.mtx.Unlock()
if r.nonce == nonce {
// calls shutdown internally
r.startResolveLocked()
}
}
if r.mtx.TryLock() {
resolveAfterRelease(false)
} else {
go resolveAfterRelease(true)
}
}
val, valRel, err := r.resolver(ctx, released)
r.mtx.Lock()
defer r.mtx.Unlock()
// assert we are still the resolver
if r.nonce != nonce {
if valRel != nil {
defer valRel()
}
return
}
// store the value and/or error
r.resolved = true
r.value, r.valueErr = val, err
r.valueRel = valRel
if err != nil {
if r.targetErr != nil {
r.targetErr.SetValue(&err)
}
} else {
if r.targetErr != nil {
r.targetErr.SetValue(nil)
}
if r.target != nil {
r.target.SetValue(val)
}
}
r.callRefCbsLocked(true, val, err)
}
// callRefCbsLocked calls the reference callbacks.
func (r *RefCount[T]) callRefCbsLocked(resolved bool, val T, err error) {
for ref := range r.refs {
if ref.cb != nil {
ref.cb(resolved, val, err)
}
}
}
// _ is a type assertion
var _ RefLike = (*Ref[*struct{}])(nil)