FIX: READ %/ doesn't return the list of drives on Windows

fixes: metaeducation/rebol-issues#2031

This is not a perfect fix. It would be probably better to have `read %""` to get list of drives instead and implement `read %/` for reading root of the current drive (for which is now used `read %//`)

src/core/s-file.c

@@ -143,6 +143,13 @@
  out = UNI_HEAD(dst);
 #ifdef TO_WINDOWS
  i++;
+ if (len == 1) {
+ // special case: reading drive letters -> read %/
+ // https://github.com/rebol/rebol-issues/issues/2031
+ out[0] = 0;
+ SERIES_TAIL(dst) = 0;
+ return dst;
+ }
  if (i < len) {
  c = GET_CHAR_UNI(uni, bp, i);
  i++;

src/os/win32/dev-file.c

@@ -134,21 +134,51 @@ static BOOL Seek_File_64(REBREQ *file)
  WIN32_FIND_DATA info;
  HANDLE h= (HANDLE)(dir->handle);
  REBCHR *cp = 0;
+ REBCNT len;
 
  if (!h) {
-
- // Read first file entry:
- h = FindFirstFile(dir->file.path, &info);
- if (h == INVALID_HANDLE_VALUE) {
- dir->error = -RFE_OPEN_FAIL;
- return DR_ERROR;
+ file->modes = 0; 
+ if (dir->file.path[0] == 0) {
+ // Reading drives.. Rebol code: read %/
+ len = (1 + GetLogicalDriveStrings(0, NULL)) << 1;
+ h = MAKE_MEM(len);
+ GetLogicalDriveStrings(len, h);
+ dir->length = len;
+ dir->actual = 0;
+ file->modes = 0;
+ SET_FLAG(file->modes, RFM_DIR);
+ } else {
+ // Read first file entry:
+ h = FindFirstFile(dir->file.path, &info);
+ if (h == INVALID_HANDLE_VALUE) {
+ dir->error = -RFE_OPEN_FAIL;
+ return DR_ERROR;
+ }
+ cp = info.cFileName;
  }
  dir->handle = h;
  CLR_FLAG(dir->flags, RRF_DONE);
- cp = info.cFileName;
-
  }
 
+ if (dir->length > 0) {
+ // Drive names processing...
+ cp = (REBCHR*)h + dir->actual;
+ file->file.path = cp;
+ if (cp[0] == 0 || dir->length <= dir->actual) {
+ // end
+ FREE_MEM(dir->handle);
+ dir->handle = NULL;
+ SET_FLAG(dir->flags, RRF_DONE); // no more files
+ return DR_DONE;
+ }
+ while (cp[0] != 0 && dir->length > dir->actual) {
+ cp++; dir->actual++;
+ }
+ cp[-2] = 0; // the names are as: "C:\" but we want just "C", so just terminate the string sooner
+ dir->actual++; // skip the null after current drive name
+ return DR_DONE;
+ }
+ // regular DIR processing follows...
  // Skip over the . and .. dir cases:
  while (cp == 0 || (cp[0] == '.' && (cp[1] == 0 || (cp[1] == '.' && cp[2] == 0)))) {