Initial commit: Go SRE interview exercises

Includes solved 02-worker-pool and 09-safe-counter (mutex, atomic, sharded).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GbnJj9ZqEx9nUvzkjdwfMq

06-retry-backoff/main_test.go

@@ -0,0 +1,88 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"testing"
+	"time"
+)
+
+func TestSucceedsAfterTransientFailures(t *testing.T) {
+	calls := 0
+	err := Retry(context.Background(), 5, time.Millisecond, func(ctx context.Context) error {
+		calls++
+		if calls < 3 {
+			return fmt.Errorf("flaky: %w", ErrRetryable)
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatalf("want success, got %v", err)
+	}
+	if calls != 3 {
+		t.Fatalf("want 3 calls, got %d", calls)
+	}
+}
+
+func TestNonRetryableStopsImmediately(t *testing.T) {
+	fatal := errors.New("permission denied")
+	calls := 0
+	err := Retry(context.Background(), 5, time.Millisecond, func(ctx context.Context) error {
+		calls++
+		return fatal
+	})
+	if calls != 1 {
+		t.Fatalf("non-retryable error must stop retries: %d calls", calls)
+	}
+	if !errors.Is(err, fatal) {
+		t.Fatalf("returned error should match/wrap the fatal error, got %v", err)
+	}
+}
+
+func TestExhaustionWrapsLastError(t *testing.T) {
+	calls := 0
+	err := Retry(context.Background(), 3, time.Millisecond, func(ctx context.Context) error {
+		calls++
+		return fmt.Errorf("boom %d: %w", calls, ErrRetryable)
+	})
+	if calls != 3 {
+		t.Fatalf("want 3 attempts, got %d", calls)
+	}
+	if err == nil || !errors.Is(err, ErrRetryable) {
+		t.Fatalf("exhaustion error must wrap the last attempt error, got %v", err)
+	}
+}
+
+func TestContextCancelsSleep(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	go func() {
+		time.Sleep(30 * time.Millisecond)
+		cancel()
+	}()
+
+	start := time.Now()
+	err := Retry(ctx, 10, 10*time.Second, func(ctx context.Context) error {
+		return fmt.Errorf("always: %w", ErrRetryable)
+	})
+	elapsed := time.Since(start)
+
+	if elapsed > time.Second {
+		t.Fatalf("Retry ignored ctx during backoff sleep: took %v (time.Sleep instead of select?)", elapsed)
+	}
+	if err == nil || !errors.Is(err, context.Canceled) {
+		t.Fatalf("want error matching context.Canceled, got %v", err)
+	}
+}
+
+func TestBackoffActuallyWaits(t *testing.T) {
+	start := time.Now()
+	_ = Retry(context.Background(), 3, 40*time.Millisecond, func(ctx context.Context) error {
+		return fmt.Errorf("nope: %w", ErrRetryable)
+	})
+	// Full jitter lower bound is 0, so we can't assert a minimum reliably —
+	// but the loop must finish in bounded time: < base*(2^0+2^1+2^2) + slack.
+	if elapsed := time.Since(start); elapsed > 500*time.Millisecond {
+		t.Fatalf("backoff far too long (sleeping after last attempt?): %v", elapsed)
+	}
+}