osdep/io: normalize relative or long paths on Windows and convert to UNC

Allow mp_open and mp_stat to work with paths which reach or exceed
MAX_PATH, by normalizing and changing them to UNC if needed. This
allows to open long absolute dos paths from CLI or the client interface.
Relative paths are also converted because CreateFileW fails when
cwd + path >= MAX_PATH. Using GetFullPathNameW has also nice side
effect of removing duplicated path separators and converting them to \.

osdep/io.c

@@ -140,6 +140,32 @@ char *mp_to_utf8(void *talloc_ctx, const wchar_t *s)
     return ret;
 }
 
+wchar_t *mp_get_absolute_path(void *talloc_ctx, const char *path, const wchar_t *prefix) {
+    size_t prefix_length = wcslen(prefix);
+    size_t buffer_size = strlen(path) + 1;
+    wchar_t *buffer = talloc_array(talloc_ctx, wchar_t, buffer_size + prefix_length);
+    wcscpy(buffer, prefix);
+    wchar_t *wpath = mp_from_utf8(talloc_ctx, path);
+    DWORD ret = GetFullPathNameW(wpath, buffer_size, buffer + prefix_length, NULL);
+    if (ret > buffer_size) {
+        buffer = talloc_realloc(talloc_ctx, buffer, wchar_t, ret + prefix_length);
+        GetFullPathNameW(wpath, ret, buffer + prefix_length, NULL);
+    }
+    return buffer;
+}
+
+wchar_t *mp_resolve_path(void *talloc_ctx, const char *path) {
+    bstr bpath = bstr0(path);
+    bool absolute = mp_path_is_absolute(bpath);
+    if (strncmp("\\\\", path, 2) == 0) {
+        return mp_get_absolute_path(talloc_ctx, path, L"");
+    } else if (absolute && bpath.len < MAX_PATH) {
+        return mp_from_utf8(talloc_ctx, path);
+    } else {
+        return mp_get_absolute_path(talloc_ctx, path, L"\\\\?\\");
+    }
+}
+
 #endif // _WIN32
 
 #ifdef __MINGW32__
@@ -270,12 +296,13 @@ static int hstat(HANDLE h, struct mp_stat *buf)
 
 int mp_stat(const char *path, struct mp_stat *buf)
 {
-    wchar_t *wpath = mp_from_utf8(NULL, path);
+    void *ctx = talloc_new(NULL);
+    wchar_t *wpath = mp_resolve_path(ctx, path);
     HANDLE h = CreateFileW(wpath, FILE_READ_ATTRIBUTES,
         FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE, NULL,
         OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS | SECURITY_SQOS_PRESENT |
         SECURITY_IDENTIFICATION, NULL);
-    talloc_free(wpath);
+    talloc_free(ctx);
     if (h == INVALID_HANDLE_VALUE) {
         set_errno_from_lasterror();
         return -1;
@@ -458,9 +485,10 @@ int mp_open(const char *filename, int oflag, ...)
     }
 
     // Open the Windows file handle
-    wchar_t *wpath = mp_from_utf8(NULL, filename);
+    void *ctx = talloc_new(NULL);
+    wchar_t *wpath = mp_resolve_path(ctx, filename);
     HANDLE h = CreateFileW(wpath, access, share, NULL, disposition, flags, NULL);
-    talloc_free(wpath);
+    talloc_free(ctx);
     if (h == INVALID_HANDLE_VALUE) {
         set_errno_from_lasterror();
         return -1;
@@ -562,9 +590,10 @@ struct mp_dir {
 
 DIR* mp_opendir(const char *path)
 {
-    wchar_t *wpath = mp_from_utf8(NULL, path);
+    void *ctx = talloc_new(NULL);
+    wchar_t *wpath = mp_resolve_path(ctx, path);
     _WDIR *wdir = _wopendir(wpath);
-    talloc_free(wpath);
+    talloc_free(ctx);
     if (!wdir)
         return NULL;
     struct mp_dir *mpdir = talloc(NULL, struct mp_dir);

osdep/io.h

@@ -79,8 +79,11 @@ void mp_flush_wakeup_pipe(int pipe_end);
 
 #ifdef _WIN32
 #include <wchar.h>
+#include "options/path.h"
 wchar_t *mp_from_utf8(void *talloc_ctx, const char *s);
 char *mp_to_utf8(void *talloc_ctx, const wchar_t *s);
+wchar_t *mp_get_absolute_path(void *talloc_ctx, const char *path, const wchar_t *prefix);
+wchar_t *mp_resolve_path(void *talloc_ctx, const char *path);
 #endif
 
 #ifdef __CYGWIN__