syscall: support Getpid() on Windows

        Fixes #3379.

R=golang-dev, alex.brainman, rsc, bradfitz
CC=golang-dev
https://golang.org/cl/5909043

src/pkg/syscall/syscall_windows.go

@@ -199,6 +199,7 @@ func NewCallback(fn interface{}) uintptr
 //sys	RegQueryInfoKey(key Handle, class *uint16, classLen *uint32, reserved *uint32, subkeysLen *uint32, maxSubkeyLen *uint32, maxClassLen *uint32, valuesLen *uint32, maxValueNameLen *uint32, maxValueLen *uint32, saLen *uint32, lastWriteTime *Filetime) (regerrno error) = advapi32.RegQueryInfoKeyW
 //sys	RegEnumKeyEx(key Handle, index uint32, name *uint16, nameLen *uint32, reserved *uint32, class *uint16, classLen *uint32, lastWriteTime *Filetime) (regerrno error) = advapi32.RegEnumKeyExW
 //sys	RegQueryValueEx(key Handle, name *uint16, reserved *uint32, valtype *uint32, buf *byte, buflen *uint32) (regerrno error) = advapi32.RegQueryValueExW
+//sys	GetCurrentProcessId() (pid uint32) = kernel32.GetCurrentProcessId
 
 // syscall interface implementation for other packages
 
@@ -681,9 +682,9 @@ func SetsockoptIPMreq(fd Handle, level, opt int, mreq *IPMreq) (err error) {
 }
 func SetsockoptIPv6Mreq(fd Handle, level, opt int, mreq *IPv6Mreq) (err error) { return EWINDOWS }
 
-// TODO(brainman): fix all needed for os
+func Getpid() (pid int) { return int(GetCurrentProcessId()) }
 
-func Getpid() (pid int)   { return -1 }
+// TODO(brainman): fix all needed for os
 func Getppid() (ppid int) { return -1 }
 
 func Fchdir(fd Handle) (err error)                        { return EWINDOWS }

src/pkg/syscall/zsyscall_windows_386.go

@@ -142,6 +142,7 @@ var (
 	procOpenProcessToken                 = modadvapi32.NewProc("OpenProcessToken")
 	procGetTokenInformation              = modadvapi32.NewProc("GetTokenInformation")
 	procGetUserProfileDirectoryW         = moduserenv.NewProc("GetUserProfileDirectoryW")
+	procGetCurrentProcessId              = modkernel32.NewProc("GetCurrentProcessId")
 )
 
 func GetLastError() (lasterr error) {
@@ -1600,3 +1601,9 @@ func GetUserProfileDirectory(t Token, dir *uint16, dirLen *uint32) (err error) {
 	}
 	return
 }
+
+func GetCurrentProcessId() (pid uint32) {
+	r0, _, _ := Syscall(procGetCurrentProcessId.Addr(), 0, 0, 0, 0)
+	pid = uint32(r0)
+	return
+}

src/pkg/syscall/zsyscall_windows_amd64.go

@@ -142,6 +142,7 @@ var (
 	procOpenProcessToken                 = modadvapi32.NewProc("OpenProcessToken")
 	procGetTokenInformation              = modadvapi32.NewProc("GetTokenInformation")
 	procGetUserProfileDirectoryW         = moduserenv.NewProc("GetUserProfileDirectoryW")
+	procGetCurrentProcessId              = modkernel32.NewProc("GetCurrentProcessId")
 )
 
 func GetLastError() (lasterr error) {
@@ -1600,3 +1601,9 @@ func GetUserProfileDirectory(t Token, dir *uint16, dirLen *uint32) (err error) {
 	}
 	return
 }
+
+func GetCurrentProcessId() (pid uint32) {
+	r0, _, _ := Syscall(procGetCurrentProcessId.Addr(), 0, 0, 0, 0)
+	pid = uint32(r0)
+	return
+}