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

02-worker-pool/main_test.go

@@ -0,0 +1,100 @@
+package main
+
+import (
+	"sort"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestAllJobsProcessedAndOutputCloses(t *testing.T) {
+	jobs := make(chan int)
+	go func() {
+		for i := 0; i < 100; i++ {
+			jobs <- i
+		}
+		close(jobs)
+	}()
+
+	out := Pool(jobs, 5, func(n int) int { return n * 2 })
+
+	got := []int{}
+	done := make(chan struct{})
+	go func() {
+		for v := range out { // terminates only if out is closed
+			got = append(got, v)
+		}
+		close(done)
+	}()
+
+	select {
+	case <-done:
+	case <-time.After(2 * time.Second):
+		t.Fatal("output channel never closed — deadlock or missing close(out)")
+	}
+
+	if len(got) != 100 {
+		t.Fatalf("want 100 results, got %d", len(got))
+	}
+	sort.Ints(got)
+	for i, v := range got {
+		if v != i*2 {
+			t.Fatalf("missing/wrong result: index %d = %d", i, v)
+		}
+	}
+}
+
+func TestActuallyConcurrent(t *testing.T) {
+	var inFlight, peak atomic.Int32
+	jobs := make(chan int)
+	go func() {
+		for i := 0; i < 12; i++ {
+			jobs <- i
+		}
+		close(jobs)
+	}()
+
+	out := Pool(jobs, 4, func(n int) int {
+		cur := inFlight.Add(1)
+		for {
+			p := peak.Load()
+			if cur <= p || peak.CompareAndSwap(p, cur) {
+				break
+			}
+		}
+		time.Sleep(30 * time.Millisecond)
+		inFlight.Add(-1)
+		return n
+	})
+	drained := make(chan struct{})
+	go func() {
+		for range out {
+		}
+		close(drained)
+	}()
+	select {
+	case <-drained:
+	case <-time.After(3 * time.Second):
+		t.Fatal("output never closed")
+	}
+
+	if p := peak.Load(); p < 2 {
+		t.Errorf("expected concurrent execution, peak parallelism was %d", p)
+	} else if p > 4 {
+		t.Errorf("more than `workers` goroutines running fn: peak %d > 4", p)
+	}
+}
+
+func TestEmptyJobs(t *testing.T) {
+	jobs := make(chan int)
+	close(jobs)
+	out := Pool(jobs, 3, func(n int) int { return n })
+	select {
+	case _, ok := <-out:
+		if ok {
+			t.Fatal("got a value from empty job stream")
+		}
+	case <-time.After(time.Second):
+		t.Fatal("output not closed for empty job stream")
+	}
+}