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

07-fan-in/main_test.go

@@ -0,0 +1,105 @@
+package main
+
+import (
+	"context"
+	"runtime"
+	"sort"
+	"testing"
+	"time"
+)
+
+func gen(vals ...int) <-chan int {
+	ch := make(chan int)
+	go func() {
+		defer close(ch)
+		for _, v := range vals {
+			ch <- v
+		}
+	}()
+	return ch
+}
+
+func TestMergeAllValues(t *testing.T) {
+	out := Merge(context.Background(), gen(1, 2, 3), gen(4, 5), gen(6))
+	got := []int{}
+	done := make(chan struct{})
+	go func() {
+		for v := range out {
+			got = append(got, v)
+		}
+		close(done)
+	}()
+	select {
+	case <-done:
+	case <-time.After(2 * time.Second):
+		t.Fatal("output never closed after all inputs closed")
+	}
+	sort.Ints(got)
+	want := []int{1, 2, 3, 4, 5, 6}
+	if len(got) != len(want) {
+		t.Fatalf("want %v, got %v", want, got)
+	}
+	for i := range want {
+		if got[i] != want[i] {
+			t.Fatalf("want %v, got %v", want, got)
+		}
+	}
+}
+
+func TestNoInputs(t *testing.T) {
+	out := Merge[int](context.Background())
+	select {
+	case _, ok := <-out:
+		if ok {
+			t.Fatal("value from zero inputs")
+		}
+	case <-time.After(time.Second):
+		t.Fatal("output not closed with zero inputs")
+	}
+}
+
+func TestCancellationReleasesGoroutines(t *testing.T) {
+	before := runtime.NumGoroutine()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	// Inputs that never close and keep producing.
+	mk := func() <-chan int {
+		ch := make(chan int)
+		go func() {
+			for {
+				select {
+				case ch <- 1:
+				case <-ctx.Done():
+					return
+				}
+			}
+		}()
+		return ch
+	}
+	out := Merge(ctx, mk(), mk(), mk())
+
+	// Read a few values, then walk away AND cancel — forwarders must not
+	// stay blocked on `out <- v`.
+	for i := 0; i < 5; i++ {
+		select {
+		case <-out:
+		case <-time.After(2 * time.Second):
+			cancel()
+			t.Fatal("no values arriving on merged channel")
+		}
+	}
+	cancel()
+
+	deadline := time.After(2 * time.Second)
+	for {
+		runtime.GC()
+		if runtime.NumGoroutine() <= before+2 { // small slack
+			return
+		}
+		select {
+		case <-deadline:
+			t.Fatalf("goroutine leak after cancel: before=%d now=%d", before, runtime.NumGoroutine())
+		case <-time.After(20 * time.Millisecond):
+		}
+	}
+}