syscall: fix and document uses of RegEnumKeyEx

RegEnumKeyEx has an undocumented requirement that subsequent calls need
to be made from the same thread. This change documents that requirement
and fixes uses of it in std.

Fixes #49320.

Change-Id: I6d182227e22dd437996a878b3a978943f01b2d61
Reviewed-on: https://go-review.googlesource.com/c/go/+/361154
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/syscall/windows/registry/key.go

@@ -25,7 +25,10 @@
 //
 package registry
 
-import "syscall"
+import (
+	"runtime"
+	"syscall"
+)
 
 const (
 	// Registry key security and access rights.
@@ -88,6 +91,12 @@ func OpenKey(k Key, path string, access uint32) (Key, error) {
 
 // ReadSubKeyNames returns the names of subkeys of key k.
 func (k Key) ReadSubKeyNames() ([]string, error) {
+	// RegEnumKeyEx must be called repeatedly and to completion.
+	// During this time, this goroutine cannot migrate away from
+	// its current thread. See #49320.
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
 	names := make([]string, 0)
 	// Registry key size limit is 255 bytes and described there:
 	// https://msdn.microsoft.com/library/windows/desktop/ms724872.aspx

src/syscall/syscall_windows.go

@@ -279,7 +279,7 @@ func NewCallbackCDecl(fn interface{}) uintptr {
 //sys	RegOpenKeyEx(key Handle, subkey *uint16, options uint32, desiredAccess uint32, result *Handle) (regerrno error) = advapi32.RegOpenKeyExW
 //sys	RegCloseKey(key Handle) (regerrno error) = advapi32.RegCloseKey
 //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	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
 //sys	GetConsoleMode(console Handle, mode *uint32) (err error) = kernel32.GetConsoleMode
@@ -1301,3 +1301,31 @@ func newProcThreadAttributeList(maxAttrCount uint32) (*_PROC_THREAD_ATTRIBUTE_LI
 	}
 	return al, nil
 }
+
+// RegEnumKeyEx enumerates the subkeys of an open registry key.
+// Each call retrieves information about one subkey. name is
+// a buffer that should be large enough to hold the name of the
+// subkey plus a null terminating character. nameLen is its
+// length. On return, nameLen will contain the actual length of the
+// subkey.
+//
+// Should name not be large enough to hold the subkey, this function
+// will return ERROR_MORE_DATA, and must be called again with an
+// appropriately sized buffer.
+//
+// reserved must be nil. class and classLen behave like name and nameLen
+// but for the class of the subkey, except that they are optional.
+// lastWriteTime, if not nil, will be populated with the time the subkey
+// was last written.
+//
+// The caller must enumerate all subkeys in order. That is
+// RegEnumKeyEx must be called with index starting at 0, incrementing
+// the index until the function returns ERROR_NO_MORE_ITEMS, or with
+// the index of the last subkey (obtainable from RegQueryInfoKey),
+// decrementing until index 0 is enumerated.
+//
+// Successive calls to this API must happen on the same OS thread,
+// so call runtime.LockOSThread before calling this function.
+func RegEnumKeyEx(key Handle, index uint32, name *uint16, nameLen *uint32, reserved *uint32, class *uint16, classLen *uint32, lastWriteTime *Filetime) (regerrno error) {
+	return regEnumKeyEx(key, index, name, nameLen, reserved, class, classLen, lastWriteTime)
+}

src/syscall/zsyscall_windows.go

@@ -305,7 +305,7 @@ func RegCloseKey(key Handle) (regerrno error) {
 	return
 }
 
-func RegEnumKeyEx(key Handle, index uint32, name *uint16, nameLen *uint32, reserved *uint32, class *uint16, classLen *uint32, lastWriteTime *Filetime) (regerrno error) {
+func regEnumKeyEx(key Handle, index uint32, name *uint16, nameLen *uint32, reserved *uint32, class *uint16, classLen *uint32, lastWriteTime *Filetime) (regerrno error) {
 	r0, _, _ := Syscall9(procRegEnumKeyExW.Addr(), 8, uintptr(key), uintptr(index), uintptr(unsafe.Pointer(name)), uintptr(unsafe.Pointer(nameLen)), uintptr(unsafe.Pointer(reserved)), uintptr(unsafe.Pointer(class)), uintptr(unsafe.Pointer(classLen)), uintptr(unsafe.Pointer(lastWriteTime)), 0)
 	if r0 != 0 {
 		regerrno = Errno(r0)