os: limit temp file randomness to uint32

CL 516860 accidentally changed the randomness
used in TempFile from 32 to 64 bits on 64-bit platforms,
meaning from 10 to 20 decimal bytes.
This is enough to cause problems in a few tests
because it makes temporary directory names just
a little bit longer.

Limit back down to 32 bits of randomness, which is fine,
and add a test to avoid repeating the mistake.

Fixes #64605.

Change-Id: I17b8c063d11d5c0a96a68b5e5f83c889a13bca77
Reviewed-on: https://go-review.googlesource.com/c/go/+/548635
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Russ Cox <rsc@golang.org>

src/os/os_test.go

@@ -3330,3 +3330,27 @@ func TestPipeCloseRace(t *testing.T) {
 		t.Errorf("got nils %d errs %d, want 2 2", nils, errs)
 	}
 }
+
+func TestRandomLen(t *testing.T) {
+	for range 5 {
+		dir, err := MkdirTemp(t.TempDir(), "*")
+		if err != nil {
+			t.Fatal(err)
+		}
+		base := filepath.Base(dir)
+		if len(base) > 10 {
+			t.Errorf("MkdirTemp returned len %d: %s", len(base), base)
+		}
+	}
+	for range 5 {
+		f, err := CreateTemp(t.TempDir(), "*")
+		if err != nil {
+			t.Fatal(err)
+		}
+		base := filepath.Base(f.Name())
+		f.Close()
+		if len(base) > 10 {
+			t.Errorf("CreateTemp returned len %d: %s", len(base), base)
+		}
+	}
+}

src/os/tempfile.go

@@ -19,7 +19,7 @@ import (
 func runtime_rand() uint64
 
 func nextRandom() string {
-	return itoa.Uitoa(uint(runtime_rand()))
+	return itoa.Uitoa(uint(uint32(runtime_rand())))
 }
 
 // CreateTemp creates a new temporary file in the directory dir,