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_test.go

@@ -0,0 +1,108 @@
+package main
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestAllURLsChecked(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer srv.Close()
+
+	urls := []string{srv.URL, srv.URL + "/a", srv.URL + "/b"}
+	res := CheckAll(context.Background(), urls, 2)
+
+	if len(res) != len(urls) {
+		t.Fatalf("want %d results, got %d", len(urls), len(res))
+	}
+	for i, r := range res {
+		if r.URL != urls[i] {
+			t.Errorf("result %d: URL order not preserved: want %s got %s", i, urls[i], r.URL)
+		}
+		if r.Err != nil {
+			t.Errorf("result %d: unexpected error: %v", i, r.Err)
+		}
+		if r.Status != http.StatusOK {
+			t.Errorf("result %d: want status 200, got %d", i, r.Status)
+		}
+	}
+}
+
+func TestErrorsAreReportedNotFatal(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusServiceUnavailable)
+	}))
+	defer srv.Close()
+
+	urls := []string{srv.URL, "http://127.0.0.1:1/unreachable"}
+	res := CheckAll(context.Background(), urls, 2)
+
+	if res[0].Status != http.StatusServiceUnavailable {
+		t.Errorf("want 503 for first url, got %d (a 5xx is a RESULT, not an error)", res[0].Status)
+	}
+	if res[1].Err == nil {
+		t.Errorf("want transport error for unreachable url, got nil")
+	}
+}
+
+func TestConcurrencyLimitRespected(t *testing.T) {
+	var inFlight, peak atomic.Int32
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		cur := inFlight.Add(1)
+		for {
+			p := peak.Load()
+			if cur <= p || peak.CompareAndSwap(p, cur) {
+				break
+			}
+		}
+		time.Sleep(50 * time.Millisecond)
+		inFlight.Add(-1)
+	}))
+	defer srv.Close()
+
+	urls := make([]string, 10)
+	for i := range urls {
+		urls[i] = srv.URL
+	}
+	const limit = 3
+	CheckAll(context.Background(), urls, limit)
+
+	if p := peak.Load(); p > limit {
+		t.Errorf("concurrency limit violated: peak in-flight = %d, limit = %d", p, limit)
+	} else if p == 0 {
+		t.Errorf("no requests observed — is anything running?")
+	}
+}
+
+func TestContextCancellation(t *testing.T) {
+	block := make(chan struct{})
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		<-block
+	}))
+	defer srv.Close()
+	defer close(block)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
+	defer cancel()
+
+	urls := []string{srv.URL, srv.URL, srv.URL}
+	done := make(chan []Result, 1)
+	go func() { done <- CheckAll(ctx, urls, 1) }()
+
+	select {
+	case res := <-done:
+		for i, r := range res {
+			if r.Err == nil {
+				t.Errorf("result %d: want error after ctx timeout, got nil", i)
+			}
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("CheckAll did not return after context cancellation — goroutines stuck?")
+	}
+}