Windows: Implement a wrapper of the open() function.

The wrapper does two things:
- Requests to open /dev/null are redirected to open the nul pseudo file.
- A request to open a file that currently exists as a directory on
  Windows fails with EACCES; this is changed to EISDIR.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>

compat/mingw.c

@@ -2,6 +2,26 @@
 
 unsigned int _CRT_fmode = _O_BINARY;
 
+#undef open
+int mingw_open (const char *filename, int oflags, ...)
+{
+	va_list args;
+	unsigned mode;
+	va_start(args, oflags);
+	mode = va_arg(args, int);
+	va_end(args);
+
+	if (!strcmp(filename, "/dev/null"))
+		filename = "nul";
+	int fd = open(filename, oflags, mode);
+	if (fd < 0 && (oflags & O_CREAT) && errno == EACCES) {
+		DWORD attrs = GetFileAttributes(filename);
+		if (attrs != INVALID_FILE_ATTRIBUTES && (attrs & FILE_ATTRIBUTE_DIRECTORY))
+			errno = EISDIR;
+	}
+	return fd;
+}
+
 unsigned int sleep (unsigned int seconds)
 {
 	Sleep(seconds*1000);

compat/mingw.h

@@ -137,6 +137,9 @@ int sigaction(int sig, struct sigaction *in, struct sigaction *out);
  * replacements of existing functions
  */
 
+int mingw_open (const char *filename, int oflags, ...);
+#define open mingw_open
+
 char *mingw_getcwd(char *pointer, int len);
 #define getcwd mingw_getcwd