Gopher Server

Thursday, October 27, 2016

A few days ago, I noticed a post about building a Gopher Server in Perl 6. I had already implemented a Gopher Client in Factor, and thought it might be fun to show a simple Gopher Server in Factor in around 50 lines of code.

Using the io.servers vocabulary, we will define a new multi-threaded server that has a directory to serve content from and hostname that it can be accessed at:

TUPLE: gopher-server < threaded-server
    { serving-hostname string }
    { serving-directory string } ;

When a file is requested, it can be streamed back to clients:

: send-file ( path -- )
    binary [ [ write ] each-block ] with-file-reader ;

The Gopher protocol is defined in RFC 1436 and lists a few differentiated file types. We use the mime.types vocabulary to return the correct one.

: gopher-type ( entry -- type )
    dup directory? [
        drop "1"
    ] [
        name>> mime-type {
            { [ dup "text/" head? ] [ drop "0" ] }
            { [ dup "image/gif" = ] [ drop "g" ] }
            { [ dup "image/" head? ] [ drop "I" ] }
            [ drop "9" ]
        } cond
    ] if ;

When a directory is requested, we can send a listing of all the sub-directories and files it contains, sending their relative path to the root directory being served so they can be requested properly by the client:

:: send-directory ( server path -- )
    path [
        [
            [ gopher-type ] [ name>> ] bi
            dup path prepend-path
            server serving-directory>> ?head drop
            server serving-hostname>
            server insecure>
            "%s%s\t%s\t%s\t%d\r\n" sprintf utf8 encode write
        ] each
    ] with-directory-entries ;

To know which path was requested, we read the line, split on the first tab, carriage return, or newline character we see:

: read-gopher-path ( -- path )
    readln [ "\t\r\n" member? ] split1-when drop ;

With all of that built, we can now implement a word to handle a client request:

M: gopher-server handle-client*
    dup serving-directory>> read-gopher-path append-path
    dup file-info directory? [
        send-directory
    ] [
        send-file drop
    ] if flush ;

Initializing a gopher-server instance and providing a convenience word to start one:

: <gopher-server> ( directory port -- server )
    utf8 gopher-server new-threaded-server
        "gopher.server" >>name
        swap >>insecure
        binary >>encoding
        "localhost" >>serving-hostname
        swap resolve-symlinks >>serving-directory ;

: start-gopher-server ( directory port -- server )
    <gopher-server> start-server ;

This is available in the gopher.server vocabulary with a few improvements such as:

Support for .gophermap files for alternate results when content is requested.
Support for .gopherhead files to print headers above directory listings.
Navigation to parent directories using .. links.
Display file modified timestamp and file sizes.
Improved error handling.