fscache: implement an FSCache-aware is_mount_point()

When FSCache is active, we can cache the reparse tag and use it directly
to determine whether a path refers to an NTFS junction, without any
additional, costly I/O.

Note: this change only makes a difference with the next commit, which
will make use of the FSCache in `git clean` (contingent on
`core.fscache` set, of course).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -2709,6 +2709,8 @@ pid_t waitpid(pid_t pid, int *status, int options)
 	return -1;
 }
 
+int (*win32_is_mount_point)(struct strbuf *path) = mingw_is_mount_point;
+
 int mingw_is_mount_point(struct strbuf *path)
 {
 	WIN32_FIND_DATAW findbuf = { 0 };

compat/mingw.h

@@ -471,7 +471,8 @@ static inline void convert_slashes(char *path)
 }
 struct strbuf;
 int mingw_is_mount_point(struct strbuf *path);
-#define is_mount_point mingw_is_mount_point
+extern int (*win32_is_mount_point)(struct strbuf *path);
+#define is_mount_point win32_is_mount_point
 #define CAN_UNLINK_MOUNT_POINTS 1
 #define PATH_SEP ';'
 char *mingw_query_user_email(void);

compat/win32/fscache.c

@@ -467,6 +467,7 @@ int fscache_enable(size_t initial_size)
 		/* redirect opendir and lstat to the fscache implementations */
 		opendir = fscache_opendir;
 		lstat = fscache_lstat;
+		win32_is_mount_point = fscache_is_mount_point;
 	}
 	initialized++;
 	LeaveCriticalSection(&fscache_cs);
@@ -527,6 +528,7 @@ void fscache_disable(void)
 		/* reset opendir and lstat to the original implementations */
 		opendir = dirent_opendir;
 		lstat = mingw_lstat;
+		win32_is_mount_point = mingw_is_mount_point;
 	}
 	LeaveCriticalSection(&fscache_cs);
 
@@ -597,6 +599,39 @@ int fscache_lstat(const char *filename, struct stat *st)
 	return 0;
 }
 
+/*
+ * is_mount_point() replacement, uses cache if enabled, otherwise falls
+ * back to mingw_is_mount_point().
+ */
+int fscache_is_mount_point(struct strbuf *path)
+{
+	int dirlen, base, len;
+	struct heap_fsentry key[2];
+	struct fsentry *fse;
+	struct fscache *cache = fscache_getcache();
+
+	if (!cache || !do_fscache_enabled(cache, path->buf))
+		return mingw_is_mount_point(path);
+
+	cache->lstat_requests++;
+	/* split path into path + name */
+	len = path->len;
+	if (len && is_dir_sep(path->buf[len - 1]))
+		len--;
+	base = len;
+	while (base && !is_dir_sep(path->buf[base - 1]))
+		base--;
+	dirlen = base ? base - 1 : 0;
+
+	/* lookup entry for path + name in cache */
+	fsentry_init(&key[0].u.ent, NULL, path->buf, dirlen);
+	fsentry_init(&key[1].u.ent, &key[0].u.ent, path->buf + base, len - base);
+	fse = fscache_get(cache, &key[1].u.ent);
+	if (!fse)
+		return mingw_is_mount_point(path);
+	return fse->reparse_tag == IO_REPARSE_TAG_MOUNT_POINT;
+}
+
 typedef struct fscache_DIR {
 	struct DIR base_dir; /* extend base struct DIR */
 	struct fsentry *pfsentry;

compat/win32/fscache.h

@@ -22,6 +22,7 @@ void fscache_flush(void);
 
 DIR *fscache_opendir(const char *dir);
 int fscache_lstat(const char *file_name, struct stat *buf);
+int fscache_is_mount_point(struct strbuf *path);
 
 /* opaque fscache structure */
 struct fscache;