syscall: only get parent PID if SysProcAttr.Pdeathsig is set

The value of the parent PID is only used to check against get value
returned by getppid(2) in case SysProcAttr.Pdeathsig is non-zero. Avoid
the useless getpid(2) system call otherwise.

Cq-Include-Trybots: luci.golang.try:gotip-freebsd-amd64
Change-Id: If89f9c7acc82016ec359c79bd861d41460f42218
Reviewed-on: https://go-review.googlesource.com/c/go/+/699175
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Auto-Submit: Tobias Klauser <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: tklauser

src/syscall/exec_freebsd.go

@@ -68,13 +68,15 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 		pgrp            _C_int
 		cred            *Credential
 		ngroups, groups uintptr
-		upid            uintptr
+		upid, ppid      uintptr
 	)
 
 	rlim := origRlimitNofile.Load()
 
 	// Record parent PID so child can test if it has died.
-	ppid, _, _ := RawSyscall(SYS_GETPID, 0, 0, 0)
+	if sys.Pdeathsig != 0 {
+		ppid, _, _ = RawSyscall(SYS_GETPID, 0, 0, 0)
+	}
 
 	// guard against side effects of shuffling fds below.
 	// Make sure that nextfd is beyond any currently open files so

src/syscall/exec_linux.go

@@ -244,7 +244,7 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 		nextfd                    int
 		i                         int
 		caps                      caps
-		fd1, flags                uintptr
+		fd1, flags, ppid          uintptr
 		puid, psetgroups, pgid    []byte
 		uidmap, setgroups, gidmap []byte
 		clone3                    *cloneArgs
@@ -278,7 +278,9 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 	}
 
 	// Record parent PID so child can test if it has died.
-	ppid, _ := rawSyscallNoError(SYS_GETPID, 0, 0, 0)
+	if sys.Pdeathsig != 0 {
+		ppid, _ = rawSyscallNoError(SYS_GETPID, 0, 0, 0)
+	}
 
 	// Guard against side effects of shuffling fds below.
 	// Make sure that nextfd is beyond any currently open files so