Implement MergeContext

that has multiple contexts as parent and chooses the earliest deadline.

pkg/util/merge_context.go

@@ -0,0 +1,67 @@
+package util
+
+import (
+	"context"
+	"fmt"
+	"reflect"
+	"time"
+)
+
+// mergeContext combines multiple contexts.
+type mergeContext struct {
+	contexts []context.Context
+}
+
+func NewMergeContext(contexts []context.Context) context.Context {
+	return mergeContext{contexts: contexts}
+}
+
+// Deadline returns the earliest Deadline of all contexts.
+func (m mergeContext) Deadline() (deadline time.Time, ok bool) {
+	for _, ctx := range m.contexts {
+		if anotherDeadline, anotherOk := ctx.Deadline(); anotherOk {
+			if ok && anotherDeadline.After(deadline) {
+				continue
+			}
+			deadline = anotherDeadline
+			ok = anotherOk
+		}
+	}
+	return deadline, ok
+}
+
+// Done notifies when the first context is done.
+func (m mergeContext) Done() <-chan struct{} {
+	ch := make(chan struct{})
+	cases := make([]reflect.SelectCase, 0, len(m.contexts))
+	for _, ctx := range m.contexts {
+		cases = append(cases, reflect.SelectCase{Dir: reflect.SelectRecv, Chan: reflect.ValueOf(ctx.Done())})
+	}
+	go func(cases []reflect.SelectCase, ch chan struct{}) {
+		_, _, _ = reflect.Select(cases)
+		ch <- struct{}{}
+		close(ch)
+	}(cases, ch)
+	return ch
+}
+
+// Err returns the error of any (random) context and nil iff no context has an error.
+func (m mergeContext) Err() error {
+	for _, ctx := range m.contexts {
+		if ctx.Err() != nil {
+			return fmt.Errorf("mergeContext wrapped: %w", ctx.Err())
+		}
+	}
+	return nil
+}
+
+// Value returns the value for the key if any context has it.
+// If multiple contexts have a value for the key, the result is any (random) of them.
+func (m mergeContext) Value(key any) any {
+	for _, ctx := range m.contexts {
+		if value := ctx.Value(key); value != nil {
+			return value
+		}
+	}
+	return nil
+}