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.go

@@ -0,0 +1,53 @@
+// Exercise 06 — Retry with Exponential Backoff + Jitter
+//
+// ASSIGNMENT
+// ----------
+// Implement Retry(ctx, attempts, base, fn):
+//
+//  1. Calls fn up to `attempts` times; returns nil on first success.
+//  2. Retries ONLY errors matching ErrRetryable (errors.Is — fn may wrap it).
+//     Non-retryable errors are returned immediately.
+//  3. Between attempt i and i+1, sleeps a random duration in
+//     [0, base * 2^i)  — exponential backoff with FULL JITTER.
+//  4. The sleep must be interruptible by ctx: if ctx is cancelled mid-sleep,
+//     return promptly with an error that matches ctx.Err() (errors.Is).
+//  5. If all attempts fail, return an error that wraps the last error from fn.
+//  6. Never sleep after the final attempt.
+//
+// Run the tests:   go test -race ./06-retry-backoff/
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"time"
+)
+
+// ErrRetryable marks transient failures. fn returns errors wrapping this
+// to request a retry.
+var ErrRetryable = errors.New("retryable")
+
+// Retry runs fn with exponential backoff + full jitter.
+// TODO: implement me.
+func Retry(ctx context.Context, attempts int, base time.Duration, fn func(context.Context) error) error {
+	// TODO:
+	//   - loop attempts times, call fn
+	//   - errors.Is(err, ErrRetryable) to classify
+	//   - backoff := base << i ; sleep := rand in [0, backoff)
+	//   - select { time.After(sleep) / ctx.Done() }
+	//   - wrap the final error with fmt.Errorf("... %w", err)
+	return nil
+}
+
+func main() {
+	i := 0
+	err := Retry(context.Background(), 5, 50*time.Millisecond, func(ctx context.Context) error {
+		i++
+		if i < 3 {
+			return fmt.Errorf("attempt %d flaked: %w", i, ErrRetryable)
+		}
+		return nil
+	})
+	fmt.Println("attempts:", i, "err:", err)
+}