qga: Clean up unnecessarily dirty casts

qga_vss_fsfreeze() casts error_set_win32() from

    void (*)(Error **, int, ErrorClass, const char *, ...)

to

    void (*)(void **, int, int, const char *, ...)

The result is later called.  Since the two types are not compatible,
the call is undefined behavior.  It works in practice anyway.

However, there's no real need for trickery here.  Clean it up as
follows:

* Declare struct Error, and fix the first parameter.

* Switch to error_setg_win32().  This gets rid of the troublesome
  ErrorClass parameter.  Requires converting error_setg_win32() from
  macro to function, but that's trivially easy, because this is the
  only user of error_set_win32().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

include/qapi/error.h

@@ -44,8 +44,8 @@ void error_set_errno(Error **errp, int os_error, ErrorClass err_class,
  * printf-style human message, followed by a g_win32_error_message() string if
  * @win32_err is not zero.
  */
-void error_set_win32(Error **errp, int win32_err, ErrorClass err_class,
+void error_setg_win32(Error **errp, int win32_err, const char *fmt, ...)
-                     const char *fmt, ...) GCC_FMT_ATTR(4, 5);
+    GCC_FMT_ATTR(3, 4);
 #endif
 
 /**
@@ -56,11 +56,6 @@ void error_setg(Error **errp, const char *fmt, ...)
 #define error_setg_errno(errp, os_error, fmt, ...) \
     error_set_errno(errp, os_error, ERROR_CLASS_GENERIC_ERROR, \
                     fmt, ## __VA_ARGS__)
-#ifdef _WIN32
-#define error_setg_win32(errp, win32_err, fmt, ...) \
-    error_set_win32(errp, win32_err, ERROR_CLASS_GENERIC_ERROR, \
-                    fmt, ## __VA_ARGS__)
-#endif
 
 /**
  * Helper for open() errors

qga/vss-win32.c

@@ -150,9 +150,8 @@ void qga_vss_fsfreeze(int *nr_volume, Error **errp, bool freeze)
     const char *func_name = freeze ? "requester_freeze" : "requester_thaw";
     QGAVSSRequesterFunc func;
     ErrorSet errset = {
-        .error_set = (ErrorSetFunc)error_set_win32,
+        .error_setg_win32 = error_setg_win32,
-        .errp = (void **)errp,
+        .errp = errp,
-        .err_class = ERROR_CLASS_GENERIC_ERROR
     };
 
     func = (QGAVSSRequesterFunc)GetProcAddress(provider_lib, func_name);

qga/vss-win32/requester.cpp

@@ -24,7 +24,7 @@
 #define VSS_TIMEOUT_EVENT_MSEC 10
 
 #define err_set(e, err, fmt, ...) \
-    ((e)->error_set((e)->errp, err, (e)->err_class, fmt, ## __VA_ARGS__))
+    ((e)->error_setg_win32((e)->errp, err, fmt, ## __VA_ARGS__))
 #define err_is_set(e) ((e)->errp && *(e)->errp)
 
 

qga/vss-win32/requester.h

@@ -20,13 +20,14 @@
 extern "C" {
 #endif
 
+struct Error;
+
 /* Callback to set Error; used to avoid linking glib to the DLL */
-typedef void (*ErrorSetFunc)(void **errp, int win32_err, int err_class,
+typedef void (*ErrorSetFunc)(struct Error **errp, int win32_err,
-                             const char *fmt, ...) GCC_FMT_ATTR(4, 5);
+                             const char *fmt, ...) GCC_FMT_ATTR(3, 4);
 typedef struct ErrorSet {
-    ErrorSetFunc error_set;
+    ErrorSetFunc error_setg_win32;
-    void **errp;
+    struct Error **errp;
-    int err_class;
 } ErrorSet;
 
 STDAPI requester_init(void);

util/error.c

@@ -96,8 +96,7 @@ void error_setg_file_open(Error **errp, int os_errno, const char *filename)
 
 #ifdef _WIN32
 
-void error_set_win32(Error **errp, int win32_err, ErrorClass err_class,
+void error_setg_win32(Error **errp, int win32_err, const char *fmt, ...)
-                     const char *fmt, ...)
 {
     va_list ap;
     char *msg1, *msg2;
@@ -107,7 +106,7 @@ void error_set_win32(Error **errp, int win32_err, ErrorClass err_class,
     }
 
     va_start(ap, fmt);
-    error_setv(errp, err_class, fmt, ap);
+    error_setv(errp, ERROR_CLASS_GENERIC_ERROR, fmt, ap);
     va_end(ap);
 
     if (win32_err != 0) {