New oops.lua library

Loosely inspired by Rustlang's std::Result type:
https://doc.rust-lang.org/beta/std/result/index.html

This ought to be easy to use to replace uses of
`stdnse.format_output(false, ...)`

CHANGELOG

@@ -1,5 +1,8 @@
 #Nmap Changelog ($Id$); -*-text-*-
 
+o [NSE] New library, oops.lua, makes reporting errors easy, with plenty of
+  debugging detail when needed, and no clutter when not. [Daniel Miller]
+
 o [NSE][GH#1126] New script vulners.nse queries the Vulners CVE database API
   using CPE information from Nmap's service and application version detection.
   [GMedian, Daniel Miller]

nselib/comm.lua

@@ -25,6 +25,7 @@
 local nmap = require "nmap"
 local shortport
 local stdnse = require "stdnse"
+local oops = require "oops"
 _ENV = stdnse.module("comm", stdnse.seeall)
 
 -- This timeout value (in ms) is added to the connect timeout and represents
@@ -76,7 +77,7 @@ local setup_connect = function(host, port, opts)
 
   if not status then
     sock:close()
-    return status, err
+    return oops.raise("Could not connect", status, err)
   end
 
   sock:set_timeout(request_timeout)
@@ -85,20 +86,15 @@ local setup_connect = function(host, port, opts)
 end
 
 local read = function(sock, opts)
-  local response, status
-
   if opts.lines then
-    status, response = sock:receive_lines(opts.lines)
-    return status, response
+    return oops.raise("receive_lines failed", sock:receive_lines(opts.lines))
   end
 
   if opts.bytes then
-    status, response = sock:receive_bytes(opts.bytes)
-    return status, response
+    return oops.raise("receive_bytes failed", sock:receive_bytes(opts.bytes))
   end
 
-  status, response = sock:receive()
-  return status, response
+  return oops.raise("receive failed", sock:receive())
 end
 
 --- This function simply connects to the specified port number on the
@@ -115,12 +111,12 @@ end
 get_banner = function(host, port, opts)
   opts = opts or {}
   opts.recv_before = true
-  local socket, nothing, correct, banner = tryssl(host, port, "", opts)
+  local socket, errmsg, correct, banner = oops.raise("tryssl failed", tryssl(host, port, nil, opts))
   if socket then
     socket:close()
     return true, banner
   end
-  return false, banner
+  return false, errmsg
 end
 
 --- This function connects to the specified port number on the specified
@@ -143,21 +139,21 @@ exchange = function(host, port, data, opts)
 
   if not status then
     -- sock is an error message in this case
-    return status, sock
+    return oops.raise("Failed to connect", status, sock)
   end
 
   status, ret = sock:send(data)
 
   if not status then
     sock:close()
-    return status, ret
+    return oops.raise("Failed to send", status, ret)
   end
 
   status, ret = read(sock, opts)
 
   sock:close()
 
-  return status, ret
+  return oops.raise("Faield to read", status, ret)
 end
 
 --- This function uses shortport.ssl to check if the port is a likely SSL port
@@ -220,22 +216,21 @@ function opencon(host, port, data, opts)
   opts = opts or {}
   local status, sd = setup_connect(host, port, opts)
   if not status then
-    return nil, sd, nil
+    return oops.raise("Failed to connect", false, sd)
   end
 
-  local response, early_resp;
-  if opts.recv_before then status, early_resp = read(sd, opts) end
+  local response, early_resp
+  if opts.recv_before then status, early_resp = oops.raise("read failed", read(sd, opts)) end
   if data and #data > 0 then
     sd:send(data)
-    status, response = sd:receive()
+    status, response = oops.raise("receive failed", sd:receive())
   else
     response = early_resp
   end
   if not status then
     sd:close()
-    return nil, response, early_resp
   end
-  return sd, response, early_resp
+  return status and sd, response, early_resp
 end
 
 --- Opens a SSL connection if possible, with fallback to plain text.
@@ -275,11 +270,11 @@ function tryssl(host, port, data, opts)
     stdnse.debug2("DTLS (SSL over UDP) is not supported")
   end
   opts.proto = opt1
-  local sd, response, early_resp = opencon(host, port, data, opts)
+  local sd, response, early_resp = oops.raise(("%s failed"):format(opt1), opencon(host, port, data, opts))
   -- Try the second option (If udp, then both options are the same; skip it)
   if not sd and opt1 ~= "udp" then
     opts.proto = opt2
-    sd, response, early_resp = opencon(host, port, data, opts)
+    sd, response, early_resp = oops.raise(("%s failed"):format(opt2), opencon(host, port, data, opts))
     best = opt2
   end
   if not sd then best = "none" end

nselib/oops.lua

@@ -0,0 +1,129 @@
+--- Useful error stack objects
+--
+-- Many NSE library functions return a boolean status and an optional error
+-- message. The Oops library consists of several simple functions to accumulate
+-- these errors and pass them up the stack, resulting in a useful and verbose
+-- error message when debugging.
+--
+-- @author Daniel Miller
+-- @copyright Same as Nmap--See https://nmap.org/book/man-legal.html
+-- @class module
+-- @name oops
+
+local require = require
+local setmetatable = setmetatable
+local _ENV = require "strict" {}
+
+local nmap = require "nmap"
+local debugging = nmap.debugging
+local verbosity = nmap.verbosity
+
+local table = require "table"
+local concat = table.concat
+local insert = table.insert
+
+local Oops = {
+  new = function (self, message)
+    local o = {message}
+    setmetatable(o, self)
+    self.__index = self
+    return o
+  end,
+
+  push = function (self, message)
+    insert(self, 1, message)
+  end,
+
+  __tostring = function (self)
+    local banner = "The script encountered an error"
+    local sep = ":\n- "
+    if debugging() > 0 then
+      -- Print full error trace
+      return banner .. sep .. concat(self, sep)
+    end
+    if verbosity() > 0 then
+      -- Show just the top error
+      return banner .. ": " .. self[1]
+    end
+  end,
+}
+
+--- Add an error message to a stack of errors
+--
+-- @param message The error message to add to the stack.
+-- @param previous (Optional) Any error reported by other functions that failed.
+-- @return An Oops object representing the error stack.
+err = function (message, previous)
+  local result
+  if previous then
+    if previous.push then
+      result = previous
+    else
+      result = Oops:new(previous)
+    end
+    result:push(message)
+  elseif message.push then
+    result = message
+  else
+    result = Oops:new(message)
+  end
+  return result
+end
+local err = err
+
+--- Report an error or return a good value
+--
+-- If the status is true, just return the message. If it's false, return the
+-- message as an Oops object. This can be easily used as the final return value
+-- of a script.
+-- @param status The return status of the script.
+-- @param message The output of the script, or an error message if status is false.
+-- @return The message if status is true, or an error message if it is false.
+output = function (status, message)
+  if status then
+    return message
+  else
+    return err(message)
+  end
+end
+local output = output
+
+--- Report a status and error or return values
+--
+-- This is intended to wrap a function that returns a status and either an
+-- error or some value. If the status is false, the message is added to the
+-- stack of errors. Instead of this code:
+--
+-- <code>
+-- local status, value_or_error, value = somefunction(args)
+-- if not status then
+--   return status, "somefunction failed for some reason"
+-- end
+-- </code>
+--
+-- with this instead:
+--
+-- <code>
+-- local status, value_or_error, value = oops.raise("somefunction failed", somefunction(args))
+-- if not status then
+--   return status, value_or_error
+-- end
+-- </code>
+--
+-- but instead of just the one error, you get a stack of errors from
+-- <code>somefunction</code> with your own message at the top.
+--
+-- @param message The error message to report if status is false.
+-- @param status The first return value of the function. Treated as boolean, but returned unmodified.
+-- @param previous The second return value of the function, or error.
+-- @return The same status that was input.
+-- @return The rest of the return values, but on error, the message will be added to the stack.
+raise = function (message, status, previous, ...)
+  local r = previous
+  if not status then
+    r = err(message, previous)
+  end
+  return status, r, ...
+end
+
+return _ENV

scripts/daytime.nse

@@ -1,5 +1,6 @@
 local comm = require "comm"
 local shortport = require "shortport"
+local oops = require "oops"
 
 description = [[
 Retrieves the day and time from the Daytime service.
@@ -21,9 +22,5 @@ categories = {"discovery", "safe"}
 portrule = shortport.port_or_service(13, "daytime", {"tcp", "udp"})
 
 action = function(host, port)
-  local status, result = comm.exchange(host, port, "dummy", {lines=1})
-
-  if status then
-    return result
-  end
+  return oops.output(comm.exchange(host, port, "dummy", {lines=1}))
 end