amend??? mingw: make the dirent implementation pluggable

mingw: make the dirent implementation pluggable

Emulating the POSIX `dirent` API on Windows via
`FindFirstFile()`/`FindNextFile()` is pretty staightforward, however,
most of the information provided in the `WIN32_FIND_DATA` structure is
thrown away in the process. A more sophisticated implementation may
cache this data, e.g. for later reuse in calls to `lstat()`.

Make the `dirent` implementation pluggable so that it can be switched at
runtime, e.g. based on a config option.

Define a base DIR structure with pointers to `readdir()`/`closedir()`
that match the `opendir()` implementation (similar to vtable pointers in
Object-Oriented Programming). Define `readdir()`/`closedir()` so that
they call the function pointers in the `DIR` structure. This allows to
choose the `opendir()` implementation on a call-by-call basis.

Make the fixed-size `dirent.d_name` buffer a flex array, as `d_name` may
be implementation specific (e.g. a caching implementation may allocate a
`struct dirent` with _just_ the size needed to hold the `d_name` in
question).

Note that we cannot use the `FLEX_ARRAY` macro here because it is
defined much later in `git-compat-util.h` than `dirent.h` is
(transitively) `#include`d.

Signed-off-by: Karsten Blees <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

compat/win32/dirent.h

@@ -8,7 +8,7 @@
 
 struct dirent {
 	unsigned char d_type; /* file type to prevent lstat after readdir */
-	char d_name[FLEX_ARRAY]; /* file name */
+	char d_name[/* FLEX_ARRAY */]; /* file name */
 };
 
 /*