feat: handling panic in mapreduce, panic in calling goroutine, not inside goroutines (#1490)

* feat: handle panic

* chore: update fuzz test

* chore: optimize square sum algorithm

core/mr/mapreduce_fuzz_test.go

@@ -0,0 +1,78 @@
+//go:build go1.18
+// +build go1.18
+
+package mr
+
+import (
+	"fmt"
+	"math/rand"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"go.uber.org/goleak"
+)
+
+func FuzzMapReduce(f *testing.F) {
+	rand.Seed(time.Now().UnixNano())
+
+	f.Add(int64(10), runtime.NumCPU())
+	f.Fuzz(func(t *testing.T, n int64, workers int) {
+		n = n%5000 + 5000
+		genPanic := rand.Intn(100) == 0
+		mapperPanic := rand.Intn(100) == 0
+		reducerPanic := rand.Intn(100) == 0
+		genIdx := rand.Int63n(n)
+		mapperIdx := rand.Int63n(n)
+		reducerIdx := rand.Int63n(n)
+		squareSum := (n - 1) * n * (2*n - 1) / 6
+
+		fn := func() (interface{}, error) {
+			defer goleak.VerifyNone(t, goleak.IgnoreCurrent())
+
+			return MapReduce(func(source chan<- interface{}) {
+				for i := int64(0); i < n; i++ {
+					source <- i
+					if genPanic && i == genIdx {
+						panic("foo")
+					}
+				}
+			}, func(item interface{}, writer Writer, cancel func(error)) {
+				v := item.(int64)
+				if mapperPanic && v == mapperIdx {
+					panic("bar")
+				}
+				writer.Write(v * v)
+			}, func(pipe <-chan interface{}, writer Writer, cancel func(error)) {
+				var idx int64
+				var total int64
+				for v := range pipe {
+					if reducerPanic && idx == reducerIdx {
+						panic("baz")
+					}
+					total += v.(int64)
+					idx++
+				}
+				writer.Write(total)
+			}, WithWorkers(workers%50+runtime.NumCPU()))
+		}
+
+		if genPanic || mapperPanic || reducerPanic {
+			var buf strings.Builder
+			buf.WriteString(fmt.Sprintf("n: %d", n))
+			buf.WriteString(fmt.Sprintf(", genPanic: %t", genPanic))
+			buf.WriteString(fmt.Sprintf(", mapperPanic: %t", mapperPanic))
+			buf.WriteString(fmt.Sprintf(", reducerPanic: %t", reducerPanic))
+			buf.WriteString(fmt.Sprintf(", genIdx: %d", genIdx))
+			buf.WriteString(fmt.Sprintf(", mapperIdx: %d", mapperIdx))
+			buf.WriteString(fmt.Sprintf(", reducerIdx: %d", reducerIdx))
+			assert.Panicsf(t, func() { fn() }, buf.String())
+		} else {
+			val, err := fn()
+			assert.Nil(t, err)
+			assert.Equal(t, squareSum, val.(int64))
+		}
+	})
+}