reflect: fix race condition on funcTypes

CL 425314 made creating funcTypes using StructOf, and using a mutex to
protect read+write to funcTypes. However, after initializing funcTypes,
it is accessed in FuncOf without holding lock, causing a race.

Fixing it by returning the n-th Type directly from initFuncTypes, so the
accessing funcTypes will always be guarded by a mutex.

Fixes #56011

Change-Id: I1b50d1ae342943f16f368b8606f2614076dc90fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/437997
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/reflect/all_test.go

@@ -15,6 +15,7 @@ import (
 	"io"
 	"math"
 	"math/rand"
+	"net"
 	"os"
 	. "reflect"
 	"reflect/internal/example1"
@@ -8240,3 +8241,20 @@ func TestValue_Equal(t *testing.T) {
 		}
 	}
 }
+
+func TestInitFuncTypes(t *testing.T) {
+	n := 100
+	var wg sync.WaitGroup
+
+	wg.Add(n)
+	for i := 0; i < n; i++ {
+		go func() {
+			defer wg.Done()
+			ipT := TypeOf(net.IP{})
+			for i := 0; i < ipT.NumMethod(); i++ {
+				_ = ipT.Method(i)
+			}
+		}()
+	}
+	wg.Wait()
+}