IF-EXISTS :SUPERSEDE

["David A. Moon" <Moon%SCRC-TENEX@MIT-MC.ARPA>]

    Date: Tuesday, 6 December 1983, 10:30-EST
    From: Bernard S. Greenberg <BSG at SCRC-TENEX>

	Date: Monday, 5 December 1983, 23:12-EST
	From: David A. Moon <Moon at SCRC-TENEX>
	    Argument #2:
	      Given the primitives available to the file server in versionated
	      operating systems, it is impossible to implement this as proposed
	      without a window between determining the version number and opening
	      it for superseding.
    
	This argument is false.  The Tops-20/Tenex Chaosnet file server has
	no problem doing this without windows.  During the time between opening
	and closing, the old file is in an "open for reading" state that
	prevents anyone else from superseding it or renaming it.  Presumably other
	versionated operating systems may have comparable primitives.
    Could you detail the sequence of operations?  The above is not sufficiently
    clear.   When do you open the second file? Under what name?

Sure.  Here's the sequence of operations, translated from Midas to English:

Do a GTJFN to look up the file name specified.  The options specified
are one of the following cases:
 - direction is not output => tell the system to look up an existing
   version, complain if no file is found, and never create a new file.
 - direction is output (no bidirectional support yet);
   if append, overwrite, or truncate is set and create is not
   => tell the system to barf if no file is found and to look
   up an existing version if newest is specified.
 - if any of supersede, append, overwrite, truncate, rename, rename-and-delete,
   or if-exists-error is set
   => tell the system to look up an existing version if the version is
   newest, rather than creating a new version, and tell it to barf if
   an existing file is found.
 - otherwise => tell it to create a new file if the version is newest
   and to barf if an existing file is found.

The result of this GTJFN is # cases:

  (1) A file is found/created with no error => use it
  (2) An error other than file-already-exists, or any error if
      if-exists-error was specified => return error to user
  (3) File-already-exists error
      => GTJFN again, telling the system to look up the existing file.
      [An error here is just signalled to the user, and means someone
      changed something, e.g. deleted the existing file out from under
      you.  Probably should start over rather than erring out.]
      Now if append, overwrite, or truncate is set, use that file.

      Otherwise save that file, along with a flag saying whether
      rename, rename-and-delete, or supersede was specified.  Open
      the file for input with the "restricted" option so that no
      one else can mess with it; if this fails return an error;
      Tops-20 doesn't allow you to supersede a file that someone
      is using, unlike some other systems that will hold onto the
      file until all users of it close.
      Now open a temporary file in the same directory and use it.
      At non-abort close time, rename the temporary file on top of
      the other file, superseding it.  (For rename/rename-and-delete,
      the other file is renamed out of the way first).

Hopefully I haven't perpetrated too many typographical errors in the 
above.  I don't think the Tops-20 strangeness of having separate system calls
to get a handle on a file and to open a file is depended on in any
fundamental way.