Drive circuit breaker state transitions via internal/clock

The Open->HalfOpen promotion used time.Now/time.Since directly, forcing tests
to use real time.Sleep and diverging from the project's clock convention. Add
an unexported withClock option (default clock.System) and replace the real
sleeps in tests with mock-clock Advance, making the transitions deterministic
and the package faster.

circuitbreaker/breaker.go

@@ -72,7 +72,7 @@ func (b *Breaker) State() State {
 // stateLocked returns the effective state, promoting Open → HalfOpen when the
 // open duration has elapsed. Caller must hold b.mu.
 func (b *Breaker) stateLocked() State {
-	if b.state == StateOpen && time.Since(b.openedAt) >= b.opts.openDuration {
+	if b.state == StateOpen && b.opts.clk.Since(b.openedAt) >= b.opts.openDuration {
 		b.state = StateHalfOpen
 		b.halfOpenCur = 0
 	}
@@ -142,7 +142,7 @@ func (b *Breaker) record(success bool) {
 // tripLocked transitions to the Open state and records the timestamp.
 func (b *Breaker) tripLocked() {
 	b.state = StateOpen
-	b.openedAt = time.Now()
+	b.openedAt = b.opts.clk.Now()
 	b.halfOpenCur = 0
 }
 

circuitbreaker/breaker_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 	"time"
 
+	"git.codelab.vc/pkg/httpx/internal/clock"
 	"git.codelab.vc/pkg/httpx/middleware"
 )
 
@@ -80,9 +81,11 @@ func TestBreaker_OpenRejectsRequests(t *testing.T) {
 
 func TestBreaker_TransitionsToHalfOpenAfterDuration(t *testing.T) {
 	const openDuration = 50 * time.Millisecond
+	clk := clock.Mock(time.Now())
 	b := NewBreaker(
 		WithFailureThreshold(1),
 		WithOpenDuration(openDuration),
+		withClock(clk),
 	)
 
 	// Trip the breaker.
@@ -96,8 +99,8 @@ func TestBreaker_TransitionsToHalfOpenAfterDuration(t *testing.T) {
 		t.Fatalf("state = %v, want %v", s, StateOpen)
 	}
 
-	// Wait for the open duration to elapse.
-	time.Sleep(openDuration + 10*time.Millisecond)
+	// Advance past the open duration.
+	clk.Advance(openDuration + time.Millisecond)
 
 	if s := b.State(); s != StateHalfOpen {
 		t.Fatalf("state = %v, want %v", s, StateHalfOpen)
@@ -106,9 +109,11 @@ func TestBreaker_TransitionsToHalfOpenAfterDuration(t *testing.T) {
 
 func TestBreaker_HalfOpenToClosedOnSuccess(t *testing.T) {
 	const openDuration = 50 * time.Millisecond
+	clk := clock.Mock(time.Now())
 	b := NewBreaker(
 		WithFailureThreshold(1),
 		WithOpenDuration(openDuration),
+		withClock(clk),
 	)
 
 	// Trip the breaker.
@@ -118,8 +123,8 @@ func TestBreaker_HalfOpenToClosedOnSuccess(t *testing.T) {
 	}
 	done(false)
 
-	// Wait for half-open.
-	time.Sleep(openDuration + 10*time.Millisecond)
+	// Advance into half-open.
+	clk.Advance(openDuration + time.Millisecond)
 
 	// A successful request in half-open should close the breaker.
 	done, err = b.Allow()
@@ -135,9 +140,11 @@ func TestBreaker_HalfOpenToClosedOnSuccess(t *testing.T) {
 
 func TestBreaker_HalfOpenToOpenOnFailure(t *testing.T) {
 	const openDuration = 50 * time.Millisecond
+	clk := clock.Mock(time.Now())
 	b := NewBreaker(
 		WithFailureThreshold(1),
 		WithOpenDuration(openDuration),
+		withClock(clk),
 	)
 
 	// Trip the breaker.
@@ -147,8 +154,8 @@ func TestBreaker_HalfOpenToOpenOnFailure(t *testing.T) {
 	}
 	done(false)
 
-	// Wait for half-open.
-	time.Sleep(openDuration + 10*time.Millisecond)
+	// Advance into half-open.
+	clk.Advance(openDuration + time.Millisecond)
 
 	// A failed request in half-open should re-open the breaker.
 	done, err = b.Allow()

circuitbreaker/options.go

@@ -1,11 +1,16 @@
 package circuitbreaker
 
-import "time"
+import (
+	"time"
+
+	"git.codelab.vc/pkg/httpx/internal/clock"
+)
 
 type options struct {
 	failureThreshold int           // consecutive failures to trip
 	openDuration     time.Duration // how long to stay open before half-open
 	halfOpenMax      int           // max concurrent requests in half-open
+	clk              clock.Clock   // time source (real by default)
 }
 
 func defaults() options {
@@ -13,12 +18,23 @@ func defaults() options {
 		failureThreshold: 5,
 		openDuration:     30 * time.Second,
 		halfOpenMax:      1,
+		clk:              clock.System(),
 	}
 }
 
 // Option configures a Breaker.
 type Option func(*options)
 
+// withClock sets the clock used for state-transition timing. Unexported; for
+// deterministic tests.
+func withClock(c clock.Clock) Option {
+	return func(o *options) {
+		if c != nil {
+			o.clk = c
+		}
+	}
+}
+
 // WithFailureThreshold sets the number of consecutive failures required to
 // trip the breaker from Closed to Open. Default is 5.
 func WithFailureThreshold(n int) Option {