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feature.go
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feature.go
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package internal
import (
"errors"
"fmt"
"sync"
)
// ErrNotSupported indicates that a feature is not supported by the current kernel.
var ErrNotSupported = errors.New("not supported")
// UnsupportedFeatureError is returned by FeatureTest() functions.
type UnsupportedFeatureError struct {
// The minimum Linux mainline version required for this feature.
// Used for the error string, and for sanity checking during testing.
MinimumVersion Version
// The name of the feature that isn't supported.
Name string
}
func (ufe *UnsupportedFeatureError) Error() string {
if ufe.MinimumVersion.Unspecified() {
return fmt.Sprintf("%s not supported", ufe.Name)
}
return fmt.Sprintf("%s not supported (requires >= %s)", ufe.Name, ufe.MinimumVersion)
}
// Is indicates that UnsupportedFeatureError is ErrNotSupported.
func (ufe *UnsupportedFeatureError) Is(target error) bool {
return target == ErrNotSupported
}
// FeatureTest caches the result of a [FeatureTestFn].
//
// Fields should not be modified after creation.
type FeatureTest struct {
// The name of the feature being detected.
Name string
// Version in in the form Major.Minor[.Patch].
Version string
// The feature test itself.
Fn FeatureTestFn
mu sync.RWMutex
done bool
result error
}
// FeatureTestFn is used to determine whether the kernel supports
// a certain feature.
//
// The return values have the following semantics:
//
// err == ErrNotSupported: the feature is not available
// err == nil: the feature is available
// err != nil: the test couldn't be executed
type FeatureTestFn func() error
// NewFeatureTest is a convenient way to create a single [FeatureTest].
func NewFeatureTest(name, version string, fn FeatureTestFn) func() error {
ft := &FeatureTest{
Name: name,
Version: version,
Fn: fn,
}
return ft.execute
}
// execute the feature test.
//
// The result is cached if the test is conclusive.
//
// See [FeatureTestFn] for the meaning of the returned error.
func (ft *FeatureTest) execute() error {
ft.mu.RLock()
result, done := ft.result, ft.done
ft.mu.RUnlock()
if done {
return result
}
ft.mu.Lock()
defer ft.mu.Unlock()
// The test may have been executed by another caller while we were
// waiting to acquire ft.mu.
if ft.done {
return ft.result
}
err := ft.Fn()
if err == nil {
ft.done = true
return nil
}
if errors.Is(err, ErrNotSupported) {
var v Version
if ft.Version != "" {
v, err = NewVersion(ft.Version)
if err != nil {
return fmt.Errorf("feature %s: %w", ft.Name, err)
}
}
ft.done = true
ft.result = &UnsupportedFeatureError{
MinimumVersion: v,
Name: ft.Name,
}
return ft.result
}
// We couldn't execute the feature test to a point
// where it could make a determination.
// Don't cache the result, just return it.
return fmt.Errorf("detect support for %s: %w", ft.Name, err)
}
// FeatureMatrix groups multiple related feature tests into a map.
//
// Useful when there is a small number of discrete features which are known
// at compile time.
//
// It must not be modified concurrently with calling [FeatureMatrix.Result].
type FeatureMatrix[K comparable] map[K]*FeatureTest
// Result returns the outcome of the feature test for the given key.
//
// It's safe to call this function concurrently.
func (fm FeatureMatrix[K]) Result(key K) error {
ft, ok := fm[key]
if !ok {
return fmt.Errorf("no feature probe for %v", key)
}
return ft.execute()
}
// FeatureCache caches a potentially unlimited number of feature probes.
//
// Useful when there is a high cardinality for a feature test.
type FeatureCache[K comparable] struct {
mu sync.RWMutex
newTest func(K) *FeatureTest
features map[K]*FeatureTest
}
func NewFeatureCache[K comparable](newTest func(K) *FeatureTest) *FeatureCache[K] {
return &FeatureCache[K]{
newTest: newTest,
features: make(map[K]*FeatureTest),
}
}
func (fc *FeatureCache[K]) Result(key K) error {
// NB: Executing the feature test happens without fc.mu taken.
return fc.retrieve(key).execute()
}
func (fc *FeatureCache[K]) retrieve(key K) *FeatureTest {
fc.mu.RLock()
ft := fc.features[key]
fc.mu.RUnlock()
if ft != nil {
return ft
}
fc.mu.Lock()
defer fc.mu.Unlock()
if ft := fc.features[key]; ft != nil {
return ft
}
ft = fc.newTest(key)
fc.features[key] = ft
return ft
}