unix: add Uname on dragonfly

The Utsname members are only 32 bytes on Dragonfly and the syscall
returns ENOMEM in case the value is longer than that. Like uname(3)
on Dragonfly, handle this case gracefully and just truncate the
value.

Change-Id: If617af1b6831cff6d4245f498dad9f264b8fd118
Reviewed-on: https://go-review.googlesource.com/82155
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_dragonfly.go

@@ -169,6 +169,69 @@ func IoctlGetTermios(fd int, req uint) (*Termios, error) {
 	return &value, err
 }
 
+func sysctlUname(mib []_C_int, old *byte, oldlen *uintptr) error {
+	err := sysctl(mib, old, oldlen, nil, 0)
+	if err != nil {
+		// Utsname members on Dragonfly are only 32 bytes and
+		// the syscall returns ENOMEM in case the actual value
+		// is longer.
+		if err == ENOMEM {
+			err = nil
+		}
+	}
+	return err
+}
+
+func Uname(uname *Utsname) error {
+	mib := []_C_int{CTL_KERN, KERN_OSTYPE}
+	n := unsafe.Sizeof(uname.Sysname)
+	if err := sysctlUname(mib, &uname.Sysname[0], &n); err != nil {
+		return err
+	}
+	uname.Sysname[unsafe.Sizeof(uname.Sysname)-1] = 0
+
+	mib = []_C_int{CTL_KERN, KERN_HOSTNAME}
+	n = unsafe.Sizeof(uname.Nodename)
+	if err := sysctlUname(mib, &uname.Nodename[0], &n); err != nil {
+		return err
+	}
+	uname.Nodename[unsafe.Sizeof(uname.Nodename)-1] = 0
+
+	mib = []_C_int{CTL_KERN, KERN_OSRELEASE}
+	n = unsafe.Sizeof(uname.Release)
+	if err := sysctlUname(mib, &uname.Release[0], &n); err != nil {
+		return err
+	}
+	uname.Release[unsafe.Sizeof(uname.Release)-1] = 0
+
+	mib = []_C_int{CTL_KERN, KERN_VERSION}
+	n = unsafe.Sizeof(uname.Version)
+	if err := sysctlUname(mib, &uname.Version[0], &n); err != nil {
+		return err
+	}
+
+	// The version might have newlines or tabs in it, convert them to
+	// spaces.
+	for i, b := range uname.Version {
+		if b == '\n' || b == '\t' {
+			if i == len(uname.Version)-1 {
+				uname.Version[i] = 0
+			} else {
+				uname.Version[i] = ' '
+			}
+		}
+	}
+
+	mib = []_C_int{CTL_HW, HW_MACHINE}
+	n = unsafe.Sizeof(uname.Machine)
+	if err := sysctlUname(mib, &uname.Machine[0], &n); err != nil {
+		return err
+	}
+	uname.Machine[unsafe.Sizeof(uname.Machine)-1] = 0
+
+	return nil
+}
+
 /*
  * Exposed directly
  */