Over several hours in the chat, this TIP was discussed. aspect tries to summarise the eventual consensus below. Any errors or misrepresentations are my own, so speak up :-).
While exposing mkdtemp(3) as file tempdir is desirable, the underlying issue can be addressed more usefully in a much simpler fashion.
file mkdir as implemented eats the EEXIST (or equivalent) error from the underlying syscall. This is desirable with mkdir -p style usage, where an arbitrary number of parent directories may or may not need to be created first, but makes it impossible for a script to safely generate a directory for its own use.
Presently, the best option a script writer has is to call file mkdir $dir after ensuring file exists $dir returns false. But here lies a race condition: in the short interval between file exists returning false and file mkdir being called, another process might have created the directory. file mkdir turns EEXIST into success, and the programmer is none the wiser.
In principle, it may be possible to recover security by checking the ownership and permissions of the directory after file mkdir, but this is nearly impossible to get correct. And there is no safe way to recover -- if we believe the directory has been tampered with, we cannot safely change it.
By contrast, the system call itself offers an "atomic create" which returns an error when the directory already exists. TclpObjCreateDirectory passes this error back to its caller, but TclFileMakeDirsCmd hides it.
Suggested behaviour for this command:
file createdir $dir Attempt to create the directory specified. Raises an error if: 1 - The immediate parent of $dir (ie, [file dirname $dir]) does not exist (POSIX ENOENT) 2 - A directory named $dir already exists (POSIX EEXIST) Otherwise, behaviour is equivalent to [file mkdir] called with one argument.
This corresponds to a simple call to Tcl_FSCreateDirectoryProc(), exposing the POSIX errors mentioned above. On Windows, these are translated from the native errors (ERROR_ALREADY_EXISTS, ERROR_PATH_NOT_FOUND) in tclWinError.c and this translation is tested in winFCmd-4.3 and 4.2.
A future enhancement might add platform-specific prefix options to file createdir, exposing permissions (on Unix) or security attributes (on Windows), but that's beyond scope for now.