learning-go-for-sre-interview/01-url-health-checker/main_test.go

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?")
	}
}