os/exec: build TestExtraFiles subprocess without cgo

Fixes #25628

Change-Id: I8b69e59f9c0123c4f65b5931d7c6d7ecc1c720e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/228639
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

1 files changed, 99 insertions, 0 deletions

diff --git a/src/os/exec/read3.go b/src/os/exec/read3.go
new file mode 100644
index 0000000000..25d732a991
--- /dev/null
+++ b/src/os/exec/read3.go
@@ -0,0 +1,99 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build ignore
+
+// This is a test program that verifies that it can read from
+// descriptor 3 and that no other descriptors are open.
+// This is not done via TestHelperProcess and GO_WANT_HELPER_PROCESS
+// because we want to ensure that this program does not use cgo,
+// because C libraries can open file descriptors behind our backs
+// and confuse the test. See issue 25628.
+package main
+
+import (
+	"fmt"
+	"internal/poll"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"runtime"
+	"strings"
+)
+
+func main() {
+	fd3 := os.NewFile(3, "fd3")
+	bs, err := ioutil.ReadAll(fd3)
+	if err != nil {
+		fmt.Printf("ReadAll from fd 3: %v\n", err)
+		os.Exit(1)
+	}
+
+	// Now verify that there are no other open fds.
+	// stdin == 0
+	// stdout == 1
+	// stderr == 2
+	// descriptor from parent == 3
+	// All descriptors 4 and up should be available,
+	// except for any used by the network poller.
+	var files []*os.File
+	for wantfd := uintptr(4); wantfd <= 100; wantfd++ {
+		if poll.IsPollDescriptor(wantfd) {
+			continue
+		}
+		f, err := os.Open(os.Args[0])
+		if err != nil {
+			fmt.Printf("error opening file with expected fd %d: %v", wantfd, err)
+			os.Exit(1)
+		}
+		if got := f.Fd(); got != wantfd {
+			fmt.Printf("leaked parent file. fd = %d; want %d\n", got, wantfd)
+			fdfile := fmt.Sprintf("/proc/self/fd/%d", wantfd)
+			link, err := os.Readlink(fdfile)
+			fmt.Printf("readlink(%q) = %q, %v\n", fdfile, link, err)
+			var args []string
+			switch runtime.GOOS {
+			case "plan9":
+				args = []string{fmt.Sprintf("/proc/%d/fd", os.Getpid())}
+			case "aix":
+				args = []string{fmt.Sprint(os.Getpid())}
+			default:
+				args = []string{"-p", fmt.Sprint(os.Getpid())}
+			}
+
+			// Determine which command to use to display open files.
+			ofcmd := "lsof"
+			switch runtime.GOOS {
+			case "dragonfly", "freebsd", "netbsd", "openbsd":
+				ofcmd = "fstat"
+			case "plan9":
+				ofcmd = "/bin/cat"
+			case "aix":
+				ofcmd = "procfiles"
+			}
+
+			cmd := exec.Command(ofcmd, args...)
+			out, err := cmd.CombinedOutput()
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "%s failed: %v\n", strings.Join(cmd.Args, " "), err)
+			}
+			fmt.Printf("%s", out)
+			os.Exit(1)
+		}
+		files = append(files, f)
+	}
+
+	for _, f := range files {
+		f.Close()
+	}
+
+	// Referring to fd3 here ensures that it is not
+	// garbage collected, and therefore closed, while
+	// executing the wantfd loop above. It doesn't matter
+	// what we do with fd3 as long as we refer to it;
+	// closing it is the easy choice.
+	fd3.Close()
+
+	os.Stdout.Write(bs)
+}