scsh-0.6/scsh/filesys.scm

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;;; Ops that create objects in the file system:
;;; create-{directory,fifo,hard-link,symlink}
;;; Copyright (c) 1993 by Olin Shivers.
;;; This procedure nukes FNAME, whatever it may be: directory, file, fifo,
;;; symlink.
;;;
;;; We can't probe FNAME to find out what it is and then do the right
;;; delete operation because there's a window in-between the probe and the
;;; delete where the file system can be altered -- the probe and delete
;;; aren't atomic. In order to deliver on our contract, we have to spin
;;; in a funny loop until we win. In practice, the loop will probably never
;;; execute more than once.
(define (delete-filesys-object fname)
(let loop ()
(or (with-errno-handler ; Assume it's a file and try.
((err data)
((errno/perm) #f) ; Return #f if directory
((errno/isdir) #f)
((errno/noent) #t))
(delete-file fname)
#t)
(with-errno-handler ; Assume it's a directory and try.
((err data)
((errno/notdir) #f) ; Return #f if fname is not a directory.
((errno/noent) #t))
(delete-directory fname)
#t)
(loop)))) ; Strange things are happening. Try again.
;;; For similar reasons, all of these ops must loop.
;;; Abstract out common code for create-{directory,fifo,hard-link,symlink}:
(define (create-file-thing fname makeit override? op-name syscall)
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(with-cwd-aligned
(with-umask-aligned
(let ((query (lambda ()
(y-or-n? (string-append op-name ": " fname
" already exists. Delete")))))
(let ((result
(let loop ((override? override?))
(with-errno-handler
((err data)
((errno/exist)
(cond ((if (eq? override? 'query)
(query)
override?)
(delete-filesys-object fname)
(loop #t))
;;; raising an error here won't work due to S48's
;;; broken exception system
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(else (list err syscall fname)))))
(makeit fname)
#f))))
(if (list? result)
(apply errno-error result)
(if #f #f)))))))
;;;;;;;
(define (create-directory dir . rest)
(let ((perms (if (null? rest) #o777 (car rest)))
(override? (if (or (null? rest) (null? (cdr rest))) #f
(cadr rest))))
(create-file-thing dir
(lambda (dir) (%create-directory dir perms))
override?
"create-directory"
create-directory)))
(define (create-fifo fifo . rest)
(let ((perms (if (null? rest) #o777 (car rest)))
(override? (if (or (null? rest) (null? (cdr rest))) #f
(cadr rest))))
(create-file-thing fifo
(lambda (fifo) (%create-fifo fifo perms))
override?
"create-fifo"
create-fifo)))
(define (create-hard-link old-fname new-fname . maybe-override?)
(create-file-thing new-fname
(lambda (new-fname)
(%create-hard-link old-fname new-fname))
(:optional maybe-override? #f)
"create-hard-link"
create-hard-link))
(define (create-symlink old-fname new-fname . maybe-override?)
(create-file-thing new-fname
(lambda (symlink)
(%create-symlink old-fname symlink))
(:optional maybe-override? #f)
"create-symlink"
create-symlink))
;;; Unix rename() works backwards from mkdir(), mkfifo(), link(), and
;;; symlink() -- it overrides by default, (though insisting on a type
;;; match between the old and new object). So we can't use create-file-thing.
;;; Note that this loop has a tiny atomicity problem -- if someone
;;; creates a file *after* we do our existence check, but *before* we
;;; do the rename, we could end up overriding it, when the user asked
;;; us not to. That's life in the food chain.
(define (rename-file old-fname new-fname . maybe-override?)
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(with-cwd-aligned
(let ((override? (:optional maybe-override? #f)))
(if (or (and override? (not (eq? override? 'query)))
(file-not-exists? new-fname)
(and override?
(y-or-n? (string-append "rename-file:" new-fname
" already exists. Delete"))))
(%rename-file old-fname new-fname)))))