Win32: simplify loading of DLL functions

Dynamic loading of DLL functions is duplicated in several places in Git
for Windows' source code.

This patch adds a pair of macros to simplify the process: the
DECLARE_PROC_ADDR(<dll>, <return-type>, <function-name>,
...<function-parameter-types>...) macro to be used at the beginning of a
code block, and the INIT_PROC_ADDR(<function-name>) macro to call before
using the declared function. The return value of the INIT_PROC_ADDR()
call has to be checked; If it is NULL, the function was not found in the
specified DLL.

Example:

        DECLARE_PROC_ADDR(kernel32.dll, BOOL, CreateHardLinkW,
                          LPCWSTR, LPCWSTR, LPSECURITY_ATTRIBUTES);

        if (!INIT_PROC_ADDR(CreateHardLinkW))
                return error("Could not find CreateHardLinkW() function";

	if (!CreateHardLinkW(source, target, NULL))
		return error("could not create hardlink from %S to %S",
			     source, target);
	return 0;
Signed-off-by: NKarsten Blees <blees@dcon.de>
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

compat/win32/lazyload.h

+#ifndef LAZYLOAD_H
+#define LAZYLOAD_H
+
+/*
+ * A pair of macros to simplify loading of DLL functions. Example:
+ *
+ *   DECLARE_PROC_ADDR(kernel32.dll, BOOL, CreateHardLinkW,
+ *                     LPCWSTR, LPCWSTR, LPSECURITY_ATTRIBUTES);
+ *
+ *   if (!INIT_PROC_ADDR(CreateHardLinkW))
+ *           return error("Could not find CreateHardLinkW() function";
+ *
+ *   if (!CreateHardLinkW(source, target, NULL))
+ *           return error("could not create hardlink from %S to %S",
+ *                        source, target);
+ */
+
+struct proc_addr {
+	const char *const dll;
+	const char *const function;
+	FARPROC pfunction;
+	unsigned initialized : 1;
+};
+
+/* Declares a function to be loaded dynamically from a DLL. */
+#define DECLARE_PROC_ADDR(dll, rettype, function, ...) \
+	static struct proc_addr proc_addr_##function = \
+	{ #dll, #function, NULL, 0 }; \
+	static rettype (WINAPI *function)(__VA_ARGS__)
+
+/*
+ * Loads a function from a DLL (once-only).
+ * Returns non-NULL function pointer on success.
+ * Returns NULL + errno == ENOSYS on failure.
+ * This function is not thread-safe.
+ */
+#define INIT_PROC_ADDR(function) \
+	(function = get_proc_addr(&proc_addr_##function))
+
+static inline void *get_proc_addr(struct proc_addr *proc)
+{
+	/* only do this once */
+	if (!proc->initialized) {
+		HANDLE hnd;
+		proc->initialized = 1;
+		hnd = LoadLibraryExA(proc->dll, NULL,
+				     LOAD_LIBRARY_SEARCH_SYSTEM32);
+		if (hnd)
+			proc->pfunction = GetProcAddress(hnd, proc->function);
+	}
+	/* set ENOSYS if DLL or function was not found */
+	if (!proc->pfunction)
+		errno = ENOSYS;
+	return proc->pfunction;
+}
+
+#endif