os: Use GetComputerNameEx to get Hostname on win32

The existing Hostname function uses the GetComputerName system
function in windows to determine the hostname. It has some downsides:

  - The name is limited to 15 characters.
  - The name returned is for NetBIOS, other OS's return a DNS name

This change adds to the internal/syscall/windows package a
GetComputerNameEx function, and related enum constants. They are used
instead of the syscall.ComputerName function to implement os.Hostname
on windows.

Fixes #9982

Change-Id: Idc8782785eb1eea37e64022bd201699ce9c4b39c
Reviewed-on: https://go-review.googlesource.com/5852
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Carlos Castillo <cookieo9@gmail.com>
Reviewed-by: Yasuhiro MATSUMOTO <mattn.jp@gmail.com>

2 files changed, 31 insertions, 3 deletions

diff --git a/src/internal/syscall/windows/syscall_windows.go b/src/internal/syscall/windows/syscall_windows.go
index 2541a83440..49bfeea1f4 100644
--- a/src/internal/syscall/windows/syscall_windows.go
+++ b/src/internal/syscall/windows/syscall_windows.go
@@ -4,9 +4,7 @@
 
 package windows
 
-import (
-	"syscall"
-)
+import "syscall"
 
 //go:generate go run ../../../syscall/mksyscall_windows.go -output zsyscall_windows.go syscall_windows.go
 
@@ -97,3 +95,17 @@ const (
 )
 
 //sys GetAdaptersAddresses(family uint32, flags uint32, reserved uintptr, adapterAddresses *IpAdapterAddresses, sizeOfPointer *uint32) (errcode error) = iphlpapi.GetAdaptersAddresses
+
+//sys	GetComputerNameEx(nameformat uint32, buf *uint16, n *uint32) (err error) = GetComputerNameExW
+
+const (
+	ComputerNameNetBIOS                   = 0
+	ComputerNameDnsHostname               = 1
+	ComputerNameDnsDomain                 = 2
+	ComputerNameDnsFullyQualified         = 3
+	ComputerNamePhysicalNetBIOS           = 4
+	ComputerNamePhysicalDnsHostname       = 5
+	ComputerNamePhysicalDnsDomain         = 6
+	ComputerNamePhysicalDnsFullyQualified = 7
+	ComputerNameMax                       = 8
+)
diff --git a/src/internal/syscall/windows/zsyscall_windows.go b/src/internal/syscall/windows/zsyscall_windows.go
index 90e2034641..50c7c5165b 100644
--- a/src/internal/syscall/windows/zsyscall_windows.go
+++ b/src/internal/syscall/windows/zsyscall_windows.go
@@ -5,10 +5,14 @@ package windows
 import "unsafe"
 import "syscall"
 
+var _ unsafe.Pointer
+
 var (
 	modiphlpapi = syscall.NewLazyDLL("iphlpapi.dll")
+	modkernel32 = syscall.NewLazyDLL("kernel32.dll")
 
 	procGetAdaptersAddresses = modiphlpapi.NewProc("GetAdaptersAddresses")
+	procGetComputerNameExW   = modkernel32.NewProc("GetComputerNameExW")
 )
 
 func GetAdaptersAddresses(family uint32, flags uint32, reserved uintptr, adapterAddresses *IpAdapterAddresses, sizeOfPointer *uint32) (errcode error) {
@@ -18,3 +22,15 @@ func GetAdaptersAddresses(family uint32, flags uint32, reserved uintptr, adapter
 	}
 	return
 }
+
+func GetComputerNameEx(nameformat uint32, buf *uint16, n *uint32) (err error) {
+	r1, _, e1 := syscall.Syscall(procGetComputerNameExW.Addr(), 3, uintptr(nameformat), uintptr(unsafe.Pointer(buf)), uintptr(unsafe.Pointer(n)))
+	if r1 == 0 {
+		if e1 != 0 {
+			err = error(e1)
+		} else {
+			err = syscall.EINVAL
+		}
+	}
+	return
+}