Return a file size string when possible.

This takes the advice from
  http://files.stairways.com/other/ftp-list-specs-info.txt
further: search for the date by an explicit occurrence of a known month
name.  This means that we won't see files with bad names (they'd be
filtered out of the result), but the filtered out entries are ones that
would not be usable with `ftp-make-file-seconds'.

When the month is found, and the entry is a file, look for a number
preceding the month, and if found, return it as the file size string.
This is a minor change in the API.  (But it's probably better to either
revise it further, or eventually make it irrelevant by exposing the
interesting functionality via `net/url'.)

collects/net/ftp-unit.rkt

@@ -159,7 +159,9 @@
                       #"250" void (void)))
 
 (define re:dir-line
-  #rx"^(.)......... .* ([A-Z].* .* [0-9][0-9]:?[0-9][0-9]) (.*)$")
+  (regexp (string-append
+           "^(.)(.*) ((?i:jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)"
+           " .* [0-9][0-9]:?[0-9][0-9]) (.*)$")))
 
 (define (ftp-directory-list tcp-ports)
   (define tcp-data (establish-data-connection tcp-ports))
@@ -173,9 +175,15 @@
                       (ftp-connection-out tcp-ports)
                       #"226" print-msg (void))
   (for*/list ([l (in-list lines)]
-              [m (in-value (regexp-match re:dir-line l))]
+              [m (in-value (cond [(regexp-match re:dir-line l) => cdr]
+                                 [else #f]))]
               #:when m)
-    (cdr m)))
+    (define size (cond [(and (equal? "-" (car m))
+                             (regexp-match #rx"([0-9]+) *$" (cadr m)))
+                        => cadr]
+                       [else #f]))
+    (define r `(,(car m) ,@(cddr m)))
+    (if size `(,@r ,size) r)))
 
 (define (ftp-download-file tcp-ports folder filename)
   ;; Save the file under the name tmp.file, rename it once download is

collects/net/scribblings/ftp.scrbl

@@ -46,13 +46,16 @@ Returns a list of files and directories in the current directory of
 the server, assuming that the server provides directory information in
 the quasi-standard Unix format.
 
-Each file or directory is represented by a list of three strings. The
-first string is either @racket["-"], @racket["d"], or @racket["l"],
-depending on whether the items is a file, directory, or link,
-respectively.  The second item is the file's date; to convert this
+Each file or directory is represented by a list of three or four
+strings.  The first string is either @racket["-"], @racket["d"], or
+@racket["l"], depending on whether the items is a file, directory, or
+link, respectively.  The second item is the file's date; to convert this
 value to seconds consistent with @racket[file-seconds], pass the date
-string to @racket[ftp-make-file-seconds], below.  The third string is
-the name of the file or directory.
+string to @racket[ftp-make-file-seconds].  The third string is the name
+of the file or directory.  If the item is a file (the first string is
+@racket["-"]), and if the line that the server replied with has a size
+in the expected place, then a fourth string containing this size is
+included.
 
 Warning: the FTP protocol has no specification for the reply format, so
 this information can be unreliable.}