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Outline Instrumentation


Outline instrumentation is the non-invasive counterpart to inlining, where no changes are written to the program source code. Outlining externalises the extraction of, and analysis of trace events. It leverages the native tracing infrastructure the the EVM provides. EVM tracing is used by a multitude of other programs and utilities that target the Erlang ecosystem, such as the debugger dbg. The nice thing about Erlang tracing is that is supports any software component that is developed for the EVM. For instance, languages like Elixir and Clojerl can benefit from our type of runtime monitoring. Detailed information about the outline instrumentation algorithm detectEr uses can be found in the companion paper, and our preprint.

The calculator server program

For this demo, we will use an Elixir implementation of our buggy calculator server that subtracts numbers, rather than adding them. This implementation, shown below, is identical to the one in Erlang in every aspect, except for the programming language syntax. In particular, function invocations in Elixir may omit the parentheses (e.g., send, self), and atoms are prepended with a colon (e.g., :add). Elixir variables are specified in lowercase, as opposed to Erlang, which must be capitalised. Finally, messages in Elixir are sent using the function send, that accepts the PID of the recipient process and the message to be sent as arguments, and is the counterpart to ! in Erlang.

def start(n) do
  spawn __MODULE__, :loop, [n]

def loop(tot) do
  receive do
    {clt, {:add, a, b}} ->

      # Handle addition request from client.
      send clt, {:ok, a - b} # Bug!!
      loop tot + 1

    {clt, {:mul, a, b}} ->

      # Handle multiplication request from client.
      send clt, {:ok, a * b}
      loop tot + 1

    {clt, :stp} ->

      # Handle stop request. Server does not loop again.
      send clt, {:bye, tot}

The function Demo.CalcServerBug.start/1 returns the PID, as the case for the Erlang version, while the messages for addition, multiplication and termination requests are the same.

Let us familiarise ourselves with this implementation by executing it on the Elixir shell. Launch a new terminal emulator window, navigate to the root detectEr directory, and:

  1. Change the directory to example/elixir.

    [duncan@local]:/detecter$ cd examples/elixir
    [duncan@local]:/detecter/examples/elixir$ ls -l
    -rw-r--r--  1 duncan  duncan  4452 Jun 11 19:56 Makefile
    drwxr-xr-x@ 3 duncan  duncan    96 Apr 29 19:16 lib
    drwxr-xr-x@ 5 duncan  duncan   160 Jun 12 22:10 props
  2. Execute make to compile all of the Elixir source code modules located in examples/elixir/lib. A new ebin directory containing the compiled *.beam files is created.

  3. Launch the Elixir shell iex. As we did in the Erlang case, we add the detectEr binaries and the ones we just compiled to the shell code path using -pa. By contrast to what we did when launching the Erlang shell, -pa must be prepend each directory we wish to add separately.

    [duncan@local]:/detecter/examples/elixir$ iex -pa ../../detecter/ebin -pa ebin
    Erlang/OTP 23 [erts-11.2.1] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe] [dtrace]
    Interactive Elixir (1.11.4) - press Ctrl+C to exit (type h() ENTER for help)
  4. The buggy calculator server is started by invoking Demo.CalcServerBug.start/1 that accepts a single argument, n, that it uses to track the number of requests handled. Like its Erlang counterpart Demo.CalcServerBug.start/1 returns the server process PID, which we assign to variable pid.

    iex(1)> pid = Demo.CalcServerBug.start 0  
  5. The server is ready for client requests, which we submit using via send. To add two numbers:

    iex(2)> send pid, {self, {:add, 10, 97}}
    {#PID<0.106.0>, {:add, 10, 97}}

    Observe that the request we issued bears the same format as the one used in our Erlang implementation. Once again, note that {#PID<0.106.0>, {:add, 10, 97}} is not the server reply. Multiplying numbers is accomplished analogously.

  6. Viewing the server reply from the server is done using flush/0.

    iex(3)> flush
    {:ok, -87}
  7. To shutdown the server, we use stp.

    iex(4)> send pid, {self, :stp}
    {#PID<0.106.0>, :stp}
    iex(5)> flush
    {:bye, 1}

The correct implementation of the server in the Demo.CalcServer module can be launched by following the steps above.

Outline instrumentation in action

We need to synthesise the analysers from the prop_add_rec.hml in examples/elixir/props. These analysers designate the main calculator server loop functions Demo.CalcServer.loop/1 and Demo.CalcServerBug.loop/1 in their with statement.

iex(6)> :hml_eval.compile 'props/prop_add_rec.hml', [{:outdir, 'ebin'}, :v]

detectEr provides the monitor module that enables us to instrument the program we want analysed using outlining. This module offers two functions, start_online/3, and start_offline/4 for conducting outline and offline instrumentation. We use former function, and cover monitor:start_offline/4 in the next section on Offline Instrumentation.

start_online/3 accepts three arguments:

  1. the tuple specifying the main function of the program we want instrumented in the format {Mod, Fun, Args} where Mod is the module name, Fun is the main function to be launched, and Args, the list of arguments accepted by Fun,

  2. the function mfa_spec/1 of the analyser we want to use, and,

  3. an option list.

The options supported by start_online/3 are:

Option Description
parent Supervisor process that is linked to tracers, or self if no supervision is required. The supervisor is notified when tracers or analysers terminate their execution, via process linking. If left unspecified, defaults to self.
analysis Analysis mode, which can be either internal or external. Internal analysis means that the tracer process and analyser are one and the same; external analysis splits the tracing and analysers into separate processes. If left unspecified, defaults to internal.

start_online/3 returns the triple {ok, Pid, Return}, consisting of the atom ok, the PID of the root tracer Pid, and the Return result of the function specified in {Mod, Fun, Args}.

Let us instrument the buggy calculator server, Demo.CalcServerBug, to detect property violations.

  1. Launch the buggy server via start_online/3. As done above, we need to retain the PID of the calculator server process, which is the value returned by Demo.CalcServerBug.loop/1. We pattern match against the return value, and store the PID inside the variable pid. Demo.CalcServerBug.loop/1 is specified as the tuple {Demo.CalcServerBug, :start, [0]}, the first argument to start_online/3. The second argument is the analyser function :prop_add_rec.mfa_spec/1 we synthesised earlier, and the list of options is left empty.

    iex(7)> {:ok, _, pid} = :monitor.start_online {Demo.CalcServerBug, :start, [0]}, &:prop_add_rec.mfa_spec/1, []
    *** [<0.109.0>] Instrumenting monitor for MFA pattern '{'Elixir.Demo.CalcServerBug',loop,[0]}'.
    *** [<0.111.0>] Analyzing event {trace,<0.110.0>,spawned,<0.108.0>,{'Elixir.Demo.CalcServerBug',loop,[0]}}.
    {:ok, #PID<0.109.0>, #PID<0.110.0>}
  2. As soon as the calculator server process starts, the analyser immediately enters into action and analyses the first process event init. This is translated to the internal Erlang spawned event (see From specification to analyser).

  3. Now, let us try adding two numbers.

    iex(8)> send pid, {self, {:add, 10, 97}}
    *** [<0.111.0>] Analyzing event {trace,<0.110.0>,'receive',{<0.106.0>,{add,10,97}}}.
    *** [<0.111.0>] Analyzing event {trace,<0.110.0>,send,{ok,-87},<0.106.0>}.
    *** [<0.111.0>] Reached verdict 'no'.
    {#PID<0.106.0>, {:add, 10, 97}}

    This leads to a rejection verdict no, corresponding to a violation of our property P3.

  4. Stop the server.

    iex(9)> send pid, {self, {:add, 10, 97}}
    {#PID<0.106.0>, {:add, 10, 97}}
  5. Stop the outline instrumentation.


Irrevocable verdicts

By contrast to the inline case, the recv and send events that were exhibited by the calculator server do not appear when the server terminates in step 4 above. The question is why? In outlining, analysers are promptly terminated when a verdict is flagged, in an effort to make things efficient. This optimisation is possible because the verdicts analysers flag are irrevocable, so an analyser can terminate, safe in the knowledge that whatever verdict it has flagged, it will surely remain so. As a byproduct of analyser termination, no event extraction from the program is possible, which is why the recv and send in step 4 are never observed.

The program without analysis.

EVM tracing permits analysers to dynamically observe processes. A terminated analyser or one that is never started means that no events from the program are extracted. We can test this by launching the calculator server without going through monitor:start_online/3.

iex(10)> pid =  Demo.CalcServerBug.start 0
iex(11)> send pid, {self, {:mul, 10, 97}}
{#PID<0.106.0>, {:mul, 10, 97}}

No analysers are instrumented, nor trace events extracted, since outlining never modifies the program. Contrastingly, with inlining, a recompilation of the program is necessary should we want to run that program without instrumentation. The flexibility due to outlining enables us to execute the program with no instrumentation by a mere restart without the need of redeployments. In principle, analysers can also be attached while the program is operating, similar to how debuggers in Erlang work; this is an avenue left for near future work.

Testing the correct server implementation

Try running the correct server Demo.CalcServer as an exercise. You need to start it as follows.

iex(12)> {:ok, _, pid} = :monitor.start_online {Demo.CalcServer, :start, [0]}, &:prop_add_rec.mfa_spec/1, []

The benefits that outlining offers can be taken a step further with offline instrumentation.