rework digwatch event output handling

the high-level change is that events matching a rule are now send into a
lua "on_event" function for handling, rather than doing the handling
down in c++.

more specifics:

before, the lua "load_rule" function registered formatters with
associated IDs with the c++ side, which later used this state to
reconcile events with formats and print output accordingly.

now, no such state is kept on the c++ side. the lua "load_rule" function
maintains the id->formatters map, and uses it to print outputs when it
receives events.

this change simplifies the existing flow and will also make the forthcoming
implementation of function outputs far simpler than it would have been
in the current setup.

userspace/digwatch/lua/compiler.lua

@@ -173,7 +173,7 @@ end
 
 local function rule(filter, output)
    if not output then
-      output = outputformat("")
+      output = outputformat(nil)
    end
    return {type = "Rule", filter = filter, output = output}
 end

userspace/digwatch/lua/rule_loader.lua

@@ -5,6 +5,8 @@
 
 --]]
 
+local DEFAULT_OUTPUT_FORMAT = "%evt.num %evt.time %evt.cpu %proc.name (%thread.tid) %evt.dir %evt.type %evt.args"
+
 local compiler = require "compiler"
 
 --[[
@@ -72,7 +74,7 @@ local state
    to the line-oriented compiler.
 --]]
 local function init()
-   return {macros={}, filter_ast=nil, n_rules=0}
+   return {macros={}, filter_ast=nil, n_rules=0, outputs={}}
 end
 
 
@@ -90,8 +92,16 @@ function load_rule(r)
       error ("Unexpected type in load_rule: "..line_ast.type)
    end
 
-   -- Register a formatter with the output string from this rule
-   digwatch.set_formatter(state.n_rules, line_ast.output.value)
+   state.n_rules = state.n_rules + 1
+
+   local format
+   if line_ast.output.value==nil then
+      format = DEFAULT_OUTPUT_FORMAT
+   else
+      format = line_ast.output.value
+   end
+
+   state.outputs[state.n_rules] = digwatch.formatter(format)
 
    -- Store the index of this formatter in each relational expression that
    -- this rule contains.
@@ -100,8 +110,6 @@ function load_rule(r)
    -- event.
    mark_relational_nodes(line_ast.filter.value, state.n_rules)
 
-   state.n_rules = state.n_rules + 1
-
    -- Rule ASTs are merged together into one big AST, with "OR" between each
    -- rule.
    if (state.filter_ast == nil) then
@@ -114,3 +122,8 @@ end
 function on_done()
    install_filter(state.filter_ast)
 end
+
+function on_event(evt, rule_id)
+   print(digwatch.format_event(evt, state.outputs[rule_id]))
+end
+