cmd/go/internal/par: actually make par.Work run things in parallel

This was an unfortunate debugging print introduced
while working on the unfortunately large CL 123576.
At least now we're done with that awfulness.

Change-Id: Ib83db59382a799f649832d22d3c6f039d2ef9d2c
Reviewed-on: https://go-review.googlesource.com/125015
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/par/work.go

@@ -55,7 +55,6 @@ func (w *Work) Do(n int, f func(item interface{})) {
 	if n < 1 {
 		panic("par.Work.Do: n < 1")
 	}
-	n = 1
 	if w.running >= 1 {
 		panic("par.Work.Do: already called Do")
 	}

src/cmd/go/internal/par/work_test.go

@@ -7,6 +7,7 @@ package par
 import (
 	"sync/atomic"
 	"testing"
+	"time"
 )
 
 func TestWork(t *testing.T) {
@@ -30,6 +31,30 @@ func TestWork(t *testing.T) {
 	}
 }
 
+func TestWorkParallel(t *testing.T) {
+	var w Work
+
+	for tries := 0; tries < 10; tries++ {
+		const N = 100
+		for i := 0; i < N; i++ {
+			w.Add(i)
+		}
+		start := time.Now()
+		var n int32
+		w.Do(N, func(x interface{}) {
+			time.Sleep(1 * time.Millisecond)
+			atomic.AddInt32(&n, +1)
+		})
+		if n != N {
+			t.Fatalf("par.Work.Do did not do all the work")
+		}
+		if time.Since(start) < N/2*time.Millisecond {
+			return
+		}
+	}
+	t.Fatalf("par.Work.Do does not seem to be parallel")
+}
+
 func TestCache(t *testing.T) {
 	var cache Cache