Replace Open3 with POSIX::Spawn

Customers are experiencing capture3 returning a nil status in
production. It results in an engine error output of:

    NoMethodError: undefined method `success?' for nil:NilClass
        block in run at /usr/src/app/lib/cc/engine/analyzers/command_line_runner.rb:18

The theory is: the Open3 code internally uses Process.detach to get the
wait thread, which returns nil if the pid given to it is not a valid
pid. So my guess was that the parser ran & exited so quickly that by the
time detach was called it didn't exist anymore. But it may very well
have ran successfully.

Moving to POSIX::Spawn, which we typically us in all code anyway, should
behave more robustly in this sense.

Author: pbrisbin

Gemfile

@@ -6,6 +6,7 @@ gem 'ruby_parser'
 gem 'pry'
 gem 'sexp_processor'
 gem 'json'
+gem 'posix-spawn'
 
 group :test do
   gem 'rake'

Gemfile.lock

@@ -12,6 +12,7 @@ GEM
     diff-lcs (1.2.5)
     json (1.8.3)
     method_source (0.8.2)
+    posix-spawn (0.3.11)
     pry (0.10.3)
       coderay (~> 1.1.0)
       method_source (~> 0.8.1)
@@ -42,6 +43,7 @@ DEPENDENCIES
   concurrent-ruby (~> 1.0.0)
   flay!
   json
+  posix-spawn
   pry
   rake
   rspec

lib/cc/engine/analyzers/command_line_runner.rb

@@ -1,4 +1,4 @@
-require "open3"
+require "posix/spawn"
 require "timeout"
 
 module CC
@@ -13,13 +13,14 @@ def initialize(command, timeout = DEFAULT_TIMEOUT)
         end
 
         def run(input)
-          Timeout.timeout(timeout) do
-            out, err, status = Open3.capture3(command, stdin_data: input)
-            if status.success?
-              yield out
-            else
-              raise ::CC::Engine::Analyzers::ParserError, "`#{command}` exited with code #{status.exitstatus}:\n#{err}"
-            end
+          child = POSIX::Spawn::Child.build(command, input: input, timeout: timeout)
+          child.exec!
+
+          if child.success?
+            yield child.out
+          else
+            exitstatus = child.status.exitstatus
+            raise ::CC::Engine::Analyzers::ParserError, "`#{command}` exited with code #{exitstatus}:\n#{child.err}"
           end
         end
 

spec/cc/engine/analyzers/command_line_runner_spec.rb

@@ -0,0 +1,31 @@
+require "spec_helper"
+require "cc/engine/duplication"
+
+module CC::Engine::Analyzers
+  RSpec.describe CommandLineRunner do
+    describe "#run" do
+      it "runs the command on the input and yields the output" do
+        runner = CommandLineRunner.new("cat; echo hi")
+
+        output = runner.run("oh ") { |o| o }
+
+        expect(output).to eq "oh hi\n"
+      end
+
+
+      it "raises on errors" do
+        runner = CommandLineRunner.new("echo error output >&2; false")
+
+        expect { runner.run("") }.to raise_error(
+          ParserError, /code 1:\nerror output/
+        )
+      end
+
+      it "times out commands" do
+        runner = CommandLineRunner.new("sleep 3", 0.01)
+
+        expect { runner.run("") }.to raise_error(POSIX::Spawn::TimeoutExceeded)
+      end
+    end
+  end
+end