[bug-inetutils] ftpd: Scanner and parser issues.

[Mats Erik Andersson]

Hello all,

I would like to document the reason behind two recent commits
that change code in the Bison parser and in the corresponding
handcrafted scanner in the file "ftpd/ftpcmd.y".

In commit 5f59063, "ftpd: Setting of idle time.", the old
misbehaviour

   ftp> idle
   200 Current IDLE time limit is 900 seconds; max 7200
   ftp> idle 800
   500 'SITE IDLE 800': command not understood.

was corrected to the expected

   ftp> idle
   200 Current IDLE time limit is 900 seconds; max 7200
   ftp> idle 800
   200 Maximum IDLE time set to 800 seconds

This malfunction seems to be there from the very beginning,
since the parser rule for "site idle number" was not written
properly to capture the intended syntax.

In commit 1d9298c, "ftpd: Capture syntax errors.", the old
unhelpful responses

   ftp> umask first
   500 'SITE UMASK first': command not understood.
   ftp> chmod second second
   500 'SITE UMASK first': command not understood.
   ftp> chmod third third
   500 'SITE CHMOD second second': command not understood.
   ftp> quote noop
   500 'SITE CHMOD third third': command not understood.
   ftp> reset
   200 NOOP command successful.

(observe the mismatched responses to commands; a "reset" is
needed to recover) have been corrected to produce

   ftp> umask first
   500 'SITE UMASK first': syntax error
   ftp> chmod second second
   500 'SITE UMASK second second': syntax error
   ftp> chmod third third
   500 'SITE CHMOD third third': syntax error
   ftp> quote noop
   200 NOOP command successful.

In addition, unknown commands see a new response:

   ftp> quote unknown
   500 'UNKNOWN': command not recognized

Also this misbehaviour on syntax errors is very old, in addition
to the fact that the user never got to see the hint that a syntax
error had indeed been identified.

The causes have been identified as too eagerly calling yyerror()
in the scanner, and also having the scanner to abort an evaluation
where continued evaluation would have enabled the parser to detect
the syntax error, thus avoiding to send two error messages to the
client. As said initially, this explanation is written for the record.

As a side effect, commit 88488eb, "ftpd: Implement FEAT and OPTS.",
should improve the response of our server program when it talks to
mechanized client software of more recent dates.

Best regards,
  Mats E A