Revert "os: make FindProcess use pidfd on Linux"

This reverts CL 542699.

Reason for revert: Some applications assume FindProcess does not return
errors.

For #62654.
Fixes #65866.

Change-Id: Ic185a6253c8e508b08150b618c39a9905f6cdd60
Reviewed-on: https://go-review.googlesource.com/c/go/+/566476
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

doc/godebug.md

@@ -126,11 +126,6 @@ for example,
 see the [runtime documentation](/pkg/runtime#hdr-Environment_Variables)
 and the [go command documentation](/cmd/go#hdr-Build_and_test_caching).
 
-### Go 1.23
-
-Go 1.23 enabled Linux pidfd support for process lookup. This feature can be
-disabled by using the [`osfinderr` setting](/pkg/os#FindProcess).
-
 ### Go 1.22
 
 Go 1.22 adds a configurable limit to control the maximum acceptable RSA key size

src/internal/godebugs/table.go

@@ -42,7 +42,6 @@ var All = []Info{
 	{Name: "multipartmaxparts", Package: "mime/multipart"},
 	{Name: "multipathtcp", Package: "net"},
 	{Name: "netdns", Package: "net", Opaque: true},
-	{Name: "osfinderr", Package: "os"},
 	{Name: "panicnil", Package: "runtime", Changed: 21, Old: "1"},
 	{Name: "randautoseed", Package: "math/rand"},
 	{Name: "tarinsecurepath", Package: "archive/tar"},

src/os/exec.go

@@ -86,17 +86,10 @@ func Getppid() int { return syscall.Getppid() }
 // The Process it returns can be used to obtain information
 // about the underlying operating system process.
 //
-// On Unix systems other than Linux, FindProcess always succeeds and returns a Process
+// On Unix systems, FindProcess always succeeds and returns a Process
 // for the given pid, regardless of whether the process exists. To test whether
 // the process actually exists, see whether p.Signal(syscall.Signal(0)) reports
 // an error.
-//
-// On Linux, FindProcess may either return ErrProcessGone for a non-existing
-// process (thus eliminating the need to use a signal to check if the process
-// exists), or work the same way as for other Unix systems, described above,
-// depending on the kernel version used and the system configuration. The old
-// behavior (of always succeeding) can be enforced by using GODEBUG setting
-// osfinderr=0.
 func FindProcess(pid int) (*Process, error) {
 	return findProcess(pid)
 }

src/os/exec_unix.go

@@ -106,14 +106,8 @@ func (p *Process) release() error {
 }
 
 func findProcess(pid int) (p *Process, err error) {
-	h, err := pidfdFind(pid)
-	if err == ErrProcessDone {
-		return nil, err
-	}
-	// Ignore all other errors from pidfdFind,
-	// as the callers do not expect them, and
-	// we can use pid anyway.
-	return newProcess(pid, h), nil
+	// NOOP for unix.
+	return newProcess(pid, unsetHandle), nil
 }
 
 func (p *ProcessState) userTime() time.Duration {

src/os/pidfd_linux.go

@@ -14,7 +14,6 @@
 package os
 
 import (
-	"internal/godebug"
 	"internal/syscall/unix"
 	"sync"
 	"syscall"
@@ -50,25 +49,6 @@ func getPidfd(sysAttr *syscall.SysProcAttr) uintptr {
 	return uintptr(*sysAttr.PidFD)
 }
 
-var osfinderr = godebug.New("osfinderr")
-
-func pidfdFind(pid int) (uintptr, error) {
-	if !pidfdWorks() {
-		return unsetHandle, syscall.ENOSYS
-	}
-	if osfinderr.Value() == "0" {
-		osfinderr.IncNonDefault()
-		return unsetHandle, syscall.ENOSYS
-
-	}
-
-	h, err := unix.PidFDOpen(pid, 0)
-	if err == nil {
-		return h, nil
-	}
-	return unsetHandle, convertESRCH(err)
-}
-
 func (p *Process) pidfdRelease() {
 	// Release pidfd unconditionally.
 	handle := p.handle.Swap(unsetHandle)

src/os/pidfd_other.go

@@ -16,10 +16,6 @@ func getPidfd(_ *syscall.SysProcAttr) uintptr {
 	return unsetHandle
 }
 
-func pidfdFind(_ int) (uintptr, error) {
-	return unsetHandle, syscall.ENOSYS
-}
-
 func (p *Process) pidfdRelease() {}
 
 func (_ *Process) pidfdWait() (*ProcessState, error) {

src/runtime/metrics/doc.go

@@ -290,10 +290,6 @@ Below is the full list of supported metrics, ordered lexicographically.
 		The number of non-default behaviors executed by the net package
 		due to a non-default GODEBUG=multipathtcp=... setting.
 
-	/godebug/non-default-behavior/osfinderr:events
-		The number of non-default behaviors executed by the os package
-		due to a non-default GODEBUG=osfinderr=... setting.
-
 	/godebug/non-default-behavior/panicnil:events
 		The number of non-default behaviors executed by the runtime
 		package due to a non-default GODEBUG=panicnil=... setting.