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

01-url-health-checker/main.go

@@ -0,0 +1,53 @@
+// Exercise 01 — Concurrent URL Health Checker
+//
+// ASSIGNMENT
+// ----------
+// Implement CheckAll so that it:
+//
+//  1. Checks every URL in `urls` with an HTTP GET.
+//  2. Runs AT MOST `limit` checks concurrently (bounded concurrency).
+//  3. Uses a per-request timeout of 5s AND respects the global `ctx`
+//     (if ctx is cancelled, pending/unstarted checks must return ctx.Err()).
+//  4. Returns one Result per URL, at the SAME index as the input URL.
+//  5. Reuses a single http.Client and properly drains+closes response bodies
+//     so connections are reused.
+//
+// Run the tests:   go test -race ./01-url-health-checker/
+// Run the demo:    go run ./01-url-health-checker/
+package main
+
+import (
+	"context"
+	"fmt"
+	"time"
+)
+
+type Result struct {
+	URL     string
+	Status  int
+	Latency time.Duration
+	Err     error
+}
+
+// CheckAll checks all URLs with at most `limit` in flight at once.
+// TODO: implement me.
+func CheckAll(ctx context.Context, urls []string, limit int) []Result {
+	results := make([]Result, len(urls))
+	// TODO:
+	//   - semaphore (buffered channel) or worker pool for the concurrency limit
+	//   - sync.WaitGroup to wait for completion
+	//   - http.NewRequestWithContext for cancellation
+	//   - write results[i] from goroutine i (why is this safe without a mutex?)
+	_ = ctx
+	_ = limit
+	return results
+}
+
+func main() {
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+	urls := []string{"https://example.com", "https://httpbin.org/status/500", "https://nope.invalid"}
+	for _, r := range CheckAll(ctx, urls, 2) {
+		fmt.Printf("%-40s status=%d latency=%v err=%v\n", r.URL, r.Status, r.Latency, r.Err)
+	}
+}