Win32: 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 (i.e. similar to vtable pointers in OOP).
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.

Move the fixed sized dirent.d_name buffer to the dirent-specific DIR
structure, as d_name may be implementation specific (e.g. a caching
implementation may just set d_name to point into the cache instead of
copying the entire file name string).

Signed-off-by: Karsten Blees <blees@dcon.de>

Author: kblees

compat/win32/dirent.c

@@ -1,15 +1,19 @@
 #include "../../git-compat-util.h"
 
-struct DIR {
+typedef struct dirent_DIR {
+	struct DIR base_dir;  /* extend base struct DIR */
 	struct dirent dd_dir; /* includes d_type */
 	HANDLE dd_handle;     /* FindFirstFile handle */
 	int dd_stat;          /* 0-based index */
-};
+	char dd_name[MAX_PATH * 3]; /* file name (* 3 for UTF-8 conversion) */
+} dirent_DIR;
+
+DIR *(*opendir)(const char *dirname) = dirent_opendir;
 
 static inline void finddata2dirent(struct dirent *ent, WIN32_FIND_DATAW *fdata)
 {
-	/* convert UTF-16 name to UTF-8 */
-	xwcstoutf(ent->d_name, fdata->cFileName, sizeof(ent->d_name));
+	/* convert UTF-16 name to UTF-8 (d_name points to dirent_DIR.dd_name) */
+	xwcstoutf(ent->d_name, fdata->cFileName, MAX_PATH * 3);
 
 	/* Set file type, based on WIN32_FIND_DATA */
 	if (fdata->dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY)
@@ -18,7 +22,7 @@ static inline void finddata2dirent(struct dirent *ent, WIN32_FIND_DATAW *fdata)
 		ent->d_type = DT_REG;
 }
 
-struct dirent *readdir(DIR *dir)
+static struct dirent *dirent_readdir(dirent_DIR *dir)
 {
 	if (!dir) {
 		errno = EBADF; /* No set_errno for mingw */
@@ -45,7 +49,7 @@ struct dirent *readdir(DIR *dir)
 	return &dir->dd_dir;
 }
 
-int closedir(DIR *dir)
+static int dirent_closedir(dirent_DIR *dir)
 {
 	if (!dir) {
 		errno = EBADF;
@@ -57,13 +61,13 @@ int closedir(DIR *dir)
 	return 0;
 }
 
-DIR *opendir(const char *name)
+DIR *dirent_opendir(const char *name)
 {
 	wchar_t pattern[MAX_PATH + 2]; /* + 2 for '/' '*' */
 	WIN32_FIND_DATAW fdata;
 	HANDLE h;
 	int len;
-	DIR *dir;
+	dirent_DIR *dir;
 
 	/* convert name to UTF-16 and check length < MAX_PATH */
 	if ((len = xutftowcs_path(pattern, name)) < 0)
@@ -84,9 +88,12 @@ DIR *opendir(const char *name)
 	}
 
 	/* initialize DIR structure and copy first dir entry */
-	dir = xmalloc(sizeof(DIR));
+	dir = xmalloc(sizeof(dirent_DIR));
+	dir->base_dir.preaddir = (struct dirent *(*)(DIR *dir)) dirent_readdir;
+	dir->base_dir.pclosedir = (int (*)(DIR *dir)) dirent_closedir;
+	dir->dd_dir.d_name = dir->dd_name;
 	dir->dd_handle = h;
 	dir->dd_stat = 0;
 	finddata2dirent(&dir->dd_dir, &fdata);
-	return dir;
+	return (DIR*) dir;
 }

compat/win32/dirent.h

@@ -1,20 +1,32 @@
 #ifndef DIRENT_H
 #define DIRENT_H
 
-typedef struct DIR DIR;
-
 #define DT_UNKNOWN 0
 #define DT_DIR     1
 #define DT_REG     2
 #define DT_LNK     3
 
 struct dirent {
-	unsigned char d_type;      /* file type to prevent lstat after readdir */
-	char d_name[MAX_PATH * 3]; /* file name (* 3 for UTF-8 conversion) */
+	unsigned char d_type; /* file type to prevent lstat after readdir */
+	char *d_name;         /* file name */
 };
 
-DIR *opendir(const char *dirname);
-struct dirent *readdir(DIR *dir);
-int closedir(DIR *dir);
+/*
+ * Base DIR structure, contains pointers to readdir/closedir implementations so
+ * that opendir may choose a concrete implementation on a call-by-call basis.
+ */
+typedef struct DIR {
+	struct dirent *(*preaddir)(struct DIR *dir);
+	int (*pclosedir)(struct DIR *dir);
+} DIR;
+
+/* default dirent implementation */
+extern DIR *dirent_opendir(const char *dirname);
+
+/* current dirent implementation */
+extern DIR *(*opendir)(const char *dirname);
+
+#define readdir(dir) (dir->preaddir(dir))
+#define closedir(dir) (dir->pclosedir(dir))
 
 #endif /* DIRENT_H */