runtime: make tidExists more robust

The LockThreadExit tests in the runtime have been observed to fail after
reading /proc/self/task/<tid>/stat and blindly assuming its contents
followed a specific format. The parsing code is also wrong, because
splitting by spaces doesn't work when the comm name contains a space.
It also ignores errors without reporting them, which isn't great.

This change rewrites tidExists to be more robust by using
/proc/self/task/<tid>/status instead. It also modifies tidExists'
signature to report an error to its caller. Its caller then prints that
error.

Ignoring a non-not-exist error with opening this file is the likely but
unconfirmed cause of #65736 (ESRCH). This change also checks for that
error explicitly as an optimistic fix.

Fixes #65736.

Change-Id: Iea560b457d514426da2781b7eb7b8616a91ec23b
Reviewed-on: https://go-review.googlesource.com/c/go/+/567938
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/testdata/testprog/lockosthread.go

@@ -90,7 +90,11 @@ func LockOSThreadAlt() {
 			println("locked thread reused")
 			os.Exit(1)
 		}
-		exists, supported := tidExists(subTID)
+		exists, supported, err := tidExists(subTID)
+		if err != nil {
+			println("error:", err.Error())
+			return
+		}
 		if !supported || !exists {
 			goto ok
 		}

src/runtime/testdata/testprog/syscalls_linux.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"fmt"
 	"internal/testenv"
+	"io"
 	"os"
 	"syscall"
 )
@@ -16,14 +17,43 @@ func gettid() int {
 	return syscall.Gettid()
 }
 
-func tidExists(tid int) (exists, supported bool) {
-	stat, err := os.ReadFile(fmt.Sprintf("/proc/self/task/%d/stat", tid))
-	if os.IsNotExist(err) {
-		return false, true
+func tidExists(tid int) (exists, supported bool, err error) {
+	// Open the magic proc status file for reading with the syscall package.
+	// We want to identify certain valid errors very precisely.
+	statusFile := fmt.Sprintf("/proc/self/task/%d/status", tid)
+	fd, err := syscall.Open(statusFile, syscall.O_RDONLY, 0)
+	if errno, ok := err.(syscall.Errno); ok {
+		if errno == syscall.ENOENT || errno == syscall.ESRCH {
+			return false, true, nil
+		}
+	}
+	if err != nil {
+		return false, false, err
+	}
+	status, err := io.ReadAll(os.NewFile(uintptr(fd), statusFile))
+	if err != nil {
+		return false, false, err
+	}
+	lines := bytes.Split(status, []byte{'\n'})
+	// Find the State line.
+	stateLineIdx := -1
+	for i, line := range lines {
+		if bytes.HasPrefix(line, []byte("State:")) {
+			stateLineIdx = i
+			break
+		}
+	}
+	if stateLineIdx < 0 {
+		// Malformed status file?
+		return false, false, fmt.Errorf("unexpected status file format: %s:\n%s", statusFile, status)
+	}
+	stateLine := bytes.SplitN(lines[stateLineIdx], []byte{':'}, 2)
+	if len(stateLine) != 2 {
+		// Malformed status file?
+		return false, false, fmt.Errorf("unexpected status file format: %s:\n%s", statusFile, status)
 	}
 	// Check if it's a zombie thread.
-	state := bytes.Fields(stat)[2]
-	return !(len(state) == 1 && state[0] == 'Z'), true
+	return !bytes.Contains(stateLine[1], []byte{'Z'}), true, nil
 }
 
 func getcwd() (string, error) {

src/runtime/testdata/testprog/syscalls_none.go

@@ -11,8 +11,8 @@ func gettid() int {
 	return 0
 }
 
-func tidExists(tid int) (exists, supported bool) {
-	return false, false
+func tidExists(tid int) (exists, supported bool, err error) {
+	return false, false, nil
 }
 
 func getcwd() (string, error) {