plumb – format of plumb messages and rules

#include <plumb.h>


Message format
The messages formed by the plumb(2) library are formatted for transmission between processes into textual form, using newlines to separate the fields. Only the data field may contain embedded newlines. The fields occur in a specified order, and each has a name, corresponding to the elements of the Plumbmsg structure, that is used in the plumbing rules. The fields, in order, are:
src     application/service generating message
dst     destination `port' for message
wdir    working directory (used if data is a file name)
type    form of the data, e.g. text
attributes of the message, in name=value pairs separated by white space (the value must follow the usual quoting convention if it contains white space or quote characters or equal signs; it cannot contain a newline)
ndata   number of bytes of data
data    the data itself
At the moment, only textual data (type=text) is supported.

All fields are optional, but type should usually be set since it describes the form of the data, and ndata must be an accurate count (possibly zero) of the number of bytes of data. A missing field is represented by an empty line.

Plumbing rules
The plumber (see plumb(2)) receives messages on its send port (applications send messages there), interprets and reformats them, and (typically) emits them from a destination port. Its behavior is determined by a plumbing rules file, default /usr/$user/lib/plumbing, which defines a set of pattern/action rules with which to analyze, rewrite, and dispatch received messages.

The file is a sequence of rule sets, each of which is a set of one–line rules called patterns and actions. There must be at least one pattern and one action in each rule set. (The only exception is that a rule set may contain nothing but plumb to rules; such a rule set declares the named ports but has no other effect.) A blank line terminates a rule set. Lines beginning with a # character are commentary and are regarded as blank lines.

A line of the form
include file
substitutes the contents of file for the line, much as in a C #include statement. Unlike in C, the file name is not quoted. If file is not an absolute path name, or one beginning ./ or ../, file is looked for first in the directory in which the plumber is executing, and then in /sys/lib/plumb.

When a message is received by the plumber, the rule sets are examined in order. For each rule set, if the message matches all the patterns in the rule set, the actions associated with the rule set are triggered to dispose of the message. If a rule set is triggered, the rest are ignored for this message. If none is triggered, the message is discarded (giving a write error to the sender) unless it has a dst field that specifies an existing port, in which case the message is emitted, unchanged, from there.

Patterns and actions all consist of three components: an object, a verb, and arguments. These are separated by white space on the line. The arguments may contain quoted strings and variable substitutions, described below, and in some cases contain multiple words. The object and verb are single words from a pre– defined set.

The object in a pattern is the name of an element of the message, such as src or data, or the special case arg, which refers to the argument component of the current rule. The object in an action is always the word plumb.

The verbs in the pattern rules describe how the objects and arguments are to be interpreted. Within a rule set, the patterns are evaluated in sequence; if one fails, the rule set fails. Some verbs are predicates that check properties of the message; others rewrite components of the message and implicitly always succeed. Such rewritings are permanent, so rules that specify them should be placed after all pattern–matching rules in the rule set.
add      The object must be attr. Append the argument, which must be a sequence of name=value pairs, to the list of attributes of the message.
delete   The object must be attr. If the message has an attribute whose name is the argument, delete it from the list of attributes of the message. (Even if the message does not, the rule matches the message.)
is       If the text of the object is identical to the text of the argument, the rule matches.
isdir    If the text of the object is the name of an existing directory, the rule matches and sets the variable $dir to that directory name.
isfile   If the text of the object is the name of an existing file (not a directory), the rule matches and sets the variable $file to that file name.
matchesIf the entire text of the object matches the regular expression specified in the argument, the rule matches. This verb is described in more detail below.
set      The value of the object is set to the value of the argument.

The matches verb has special properties that enable the rules to select which portion of the data is to be sent to the destination. By default, a data matches rule requires that the entire text matches the regular expression. If, however, the message has an attribute named click, that reports that the message was produced by a mouse click within the text and that the regular expressions in the rule set should be used to identify what portion of the data the user intended. Typically, a program such as an editor will send a white–space delimited block of text containing the mouse click, using the value of the click attribute (a number starting from 0) to indicate where in the textual data the user pointed.

When the message has a click attribute, the data matches rules extract the longest leftmost match to the regular expression that contains or abuts the textual location identified by the click. For a sequence of such rules within a given rule set, each regular expression, evaluated by this specification, must match the same subset of the data for the rule set to match the message. For example, here is a pair of patterns that identify a message whose data contains the name of an existing file with a conventional ending for an encoded picture file:
data matches '[a–zA–Z0–9_–./]+'
data matches '([a–zA–Z0–9_–./]+).(jpe?g|gif|bit|ps|pdf)'
The first expression extracts the largest subset of the data around the click that contains file name characters; the second sees if it ends with, for example, .jpeg. If only the second pattern were present, a piece of text could be misinterpreted as an image file named horse.gif.

If a click attribute is specified in a message, it will be deleted by the plumber before sending the message if the data matches rules expand the selection.

The action rules all have the object plumb. There are only three verbs for action rules:
to       The argument is the name of the port to which the message will be sent. If the message has a destination specified, it must match the to port of the rule set or the entire rule set will be skipped. (This is the only rule that is evaluated out of order.)
client   If no application has the port open, the arguments to a plumb start rule specify a shell program to run in response to the message. The message will be held, with the supposition that the program will eventually open the port to retrieve it.
start    Like client, but the message is discarded. Only one start or client rule should be specified in a rule set.

The arguments to all rules may contain quoted strings, exactly as in rc(1). They may also contain simple string variables, identified by a leading dollar sign $. Variables may be set, between rule sets, by assignment statements in the style of rc. Only one variable assignment may appear on a line. The plumber also maintains some built–in variables:
$0      The text that matched the entire regular expression in a previous data matches rule. $1, $2, etc. refer to text matching the first, second, etc. parenthesized subexpression.
$attr   The textual representation of the attributes of the message.
$data   The contents of the data field of the message.
$dir    The directory name resulting from a successful isdir rule. If no such rule has been applied, it is the string constructed syntactically by interpreting data as a file name in wdir.
$dst    The contents of the dst field of the message.
$file   The file name resulting from a successful isfile rule. If no such rule has been applied, it is the string constructed syntactically by interpreting data as a file name in wdir.
$type   The contents of the type field of the message.
$src    The contents of the src field of the message.
$wdir   The contents of the wdir field of the message.

The following is a modest, representative file of plumbing rules.
# these are generally in order from most specific to least,
# since first rule that fires wins.
# image files go to page
type is text
data matches '[a–zA–Z0–9_\–./]+'
data matches '([a–zA–Z0–9_\–./]+).(jpe?g|gif|bit)'
arg isfile $0
plumb to image
plumb start page –w $file
# URLs go to web browser
type is text
data matches $protocol://$domain$file
plumb to web
plumb start window webbrowser $0
# existing files, possibly tagged by line number, go to edit/sam
type is text
data matches '([.a–zA–Z0–9_/–]+[a–zA–Z0–9_/\–])('$addr')?'
arg isfile $1
data set $file
attr add addr=$3
plumb to edit
plumb start window sam $file
# .h files are looked up in /sys/include and passed to edit/sam
type is text
data matches '([a–zA–Z0–9]+\.h)('$addr')?'
arg isfile /sys/include/$1
data set $file
attr add addr=$3
plumb to edit
plumb start window sam $file

The following simple plumbing rules file is a good beginning set of rules.
# to update: cp /usr/$user/lib/plumbing /mnt/plumb/rules
editor = acme
# or editor = sam
include basic

/usr/$user/lib/plumbing   default rules file.
/mnt/plumb               mount point for plumber(4).
/sys/lib/plumb           directory for include files.
/sys/lib/plumb/fileaddr   public macro definitions.
/sys/lib/plumb/basic      basic rule set.

plumb(1), plumb(2), plumber(4), regexp(6)
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