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>

4 files changed, 3 insertions, 40 deletions

diff --git a/src/os/exec.go b/src/os/exec.go
index 7ef1fee595..42e8a399a9 100644
--- a/src/os/exec.go
+++ b/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)
 }
diff --git a/src/os/exec_unix.go b/src/os/exec_unix.go
index 21d03da48e..2c66a8be20 100644
--- a/src/os/exec_unix.go
+++ b/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 {
diff --git a/src/os/pidfd_linux.go b/src/os/pidfd_linux.go
index cc67dfa05f..d6e1d53eee 100644
--- a/src/os/pidfd_linux.go
+++ b/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)
diff --git a/src/os/pidfd_other.go b/src/os/pidfd_other.go
index bb38c72404..1918acbec5 100644
--- a/src/os/pidfd_other.go
+++ b/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) {