unix: suppress ENOMEM errors from sysctl's implementing Uname(uname *Utsname) on FreeBSD due to truncated fields.

Fixes golang/go#65585.

Change-Id: I7e529eaef224be977e3b0c20d071802f49b297e7
Reviewed-on: https://go-review.googlesource.com/c/sys/+/562617
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

Author: paulzhol

unix/syscall_freebsd.go

@@ -13,6 +13,7 @@
 package unix
 
 import (
+	"errors"
 	"sync"
 	"unsafe"
 )
@@ -169,25 +170,26 @@ func Getfsstat(buf []Statfs_t, flags int) (n int, err error) {
 func Uname(uname *Utsname) error {
 	mib := []_C_int{CTL_KERN, KERN_OSTYPE}
 	n := unsafe.Sizeof(uname.Sysname)
-	if err := sysctl(mib, &uname.Sysname[0], &n, nil, 0); err != nil {
+	// Suppress ENOMEM errors to be compatible with the C library __xuname() implementation.
+	if err := sysctl(mib, &uname.Sysname[0], &n, nil, 0); err != nil && !errors.Is(err, ENOMEM) {
 		return err
 	}
 
 	mib = []_C_int{CTL_KERN, KERN_HOSTNAME}
 	n = unsafe.Sizeof(uname.Nodename)
-	if err := sysctl(mib, &uname.Nodename[0], &n, nil, 0); err != nil {
+	if err := sysctl(mib, &uname.Nodename[0], &n, nil, 0); err != nil && !errors.Is(err, ENOMEM) {
 		return err
 	}
 
 	mib = []_C_int{CTL_KERN, KERN_OSRELEASE}
 	n = unsafe.Sizeof(uname.Release)
-	if err := sysctl(mib, &uname.Release[0], &n, nil, 0); err != nil {
+	if err := sysctl(mib, &uname.Release[0], &n, nil, 0); err != nil && !errors.Is(err, ENOMEM) {
 		return err
 	}
 
 	mib = []_C_int{CTL_KERN, KERN_VERSION}
 	n = unsafe.Sizeof(uname.Version)
-	if err := sysctl(mib, &uname.Version[0], &n, nil, 0); err != nil {
+	if err := sysctl(mib, &uname.Version[0], &n, nil, 0); err != nil && !errors.Is(err, ENOMEM) {
 		return err
 	}
 
@@ -205,7 +207,7 @@ func Uname(uname *Utsname) error {
 
 	mib = []_C_int{CTL_HW, HW_MACHINE}
 	n = unsafe.Sizeof(uname.Machine)
-	if err := sysctl(mib, &uname.Machine[0], &n, nil, 0); err != nil {
+	if err := sysctl(mib, &uname.Machine[0], &n, nil, 0); err != nil && !errors.Is(err, ENOMEM) {
 		return err
 	}