Add coverage reports to py.test runner

Summary: Of note this now forces the pytest-cov plugin to be installed unless they turn it off.

Test Plan:
```
../arcanist/bin/arc unit

COVERAGE REPORT
     97%     changes/models/test.py
    100%     tests/changes/api/serializer/models/test_testcase.py
    100%     changes/api/serializer/models/testcase.py
 UNIT OKAY  No unit test failures.
Updated an existing Differential revision:
        Revision URI: https://tails.corp.dropbox.com/D43387

Included changes:
  M       changes/api/serializer/models/testcase.py
  M       changes/models/test.py
  M       tests/changes/api/serializer/models/test_testcase.py
```

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8667

src/unit/engine/PytestTestEngine.php

@@ -8,16 +8,128 @@
 final class PytestTestEngine extends ArcanistBaseUnitTestEngine {
 
   public function run() {
+    $working_copy = $this->getWorkingCopy();
+    $this->project_root = $working_copy->getProjectRoot();
+
     $junit_tmp = new TempFile();
+    $cover_tmp = new TempFile();
 
-    $cmd_line = csprintf('py.test --junitxml=%s',
+    $future = $this->buildTestFuture($junit_tmp, $cover_tmp);
+    $future->resolvex();
+
+    $future = new ExecFuture('coverage xml -o %s', $cover_tmp);
+    $future->setCWD($this->project_root);
+    $future->resolvex();
+
+    return $this->parseTestResults($junit_tmp, $cover_tmp);
+  }
+
+  public function buildTestFuture($junit_tmp, $cover_tmp) {
+    $paths = $this->getPaths();
+
+    $cmd_line = csprintf('py.test --junitxml %s',
                          $junit_tmp);
-    $future = new ExecFuture('%C', $cmd_line);
-    list($stdout, $stderr) = $future->resolvex();
 
+  if ($this->getEnableCoverage() !== false) {
+      $cmd_line = csprintf('coverage run --source %s -m %C',
+                           $this->project_root, $cmd_line);
+    }
+
+    return new ExecFuture('%C', $cmd_line);
+  }
+
+  public function parseTestResults($junit_tmp, $cover_tmp) {
     $parser = new ArcanistXUnitTestResultParser();
+    $results = $parser->parseTestResults(
+      Filesystem::readFile($junit_tmp));
 
-    return $parser->parseTestResults(Filesystem::readFile($junit_tmp));
+    if ($this->getEnableCoverage() !== false) {
+      $coverage_report = $this->readCoverage($cover_tmp);
+      foreach ($results as $result) {
+          $result->setCoverage($coverage_report);
+      }
     }
 
+    return $results;
+  }
+
+  public function readCoverage($path) {
+    $coverage_data = Filesystem::readFile($path);
+    if (empty($coverage_data)) {
+       return array();
+    }
+
+    $coverage_dom = new DOMDocument();
+    $coverage_dom->loadXML($coverage_data);
+
+    $paths = $this->getPaths();
+    $reports = array();
+    $classes = $coverage_dom->getElementsByTagName("class");
+
+    foreach ($classes as $class) {
+      // filename is actually python module path with ".py" at the end,
+      // e.g.: tornado.web.py
+      $relative_path = explode(".", $class->getAttribute("filename"));
+      array_pop($relative_path);
+      $relative_path = implode("/", $relative_path);
+
+      // first we check if the path is a directory (a Python package), if it is
+      // set relative and absolute paths to have __init__.py at the end.
+      $absolute_path = Filesystem::resolvePath($relative_path);
+      if (is_dir($absolute_path)) {
+        $relative_path .= "/__init__.py";
+        $absolute_path .= "/__init__.py";
+      }
+
+      // then we check if the path with ".py" at the end is file (a Python
+      // submodule), if it is - set relative and absolute paths to have
+      // ".py" at the end.
+      if (is_file($absolute_path.".py")) {
+        $relative_path .= ".py";
+        $absolute_path .= ".py";
+      }
+
+      if (!file_exists($absolute_path)) {
+        continue;
+      }
+
+      if (!in_array($relative_path, $paths)) {
+        continue;
+      }
+
+      // get total line count in file
+      $line_count = count(file($absolute_path));
+
+      $coverage = "";
+      $start_line = 1;
+      $lines = $class->getElementsByTagName("line");
+      for ($ii = 0; $ii < $lines->length; $ii++) {
+        $line = $lines->item($ii);
+
+        $next_line = intval($line->getAttribute("number"));
+        for ($start_line; $start_line < $next_line; $start_line++) {
+            $coverage .= "N";
+        }
+
+        if (intval($line->getAttribute("hits")) == 0) {
+            $coverage .= "U";
+        }
+        else if (intval($line->getAttribute("hits")) > 0) {
+            $coverage .= "C";
+        }
+
+        $start_line++;
+      }
+
+      if ($start_line < $line_count) {
+        foreach (range($start_line, $line_count) as $line_num) {
+          $coverage .= "N";
+        }
+      }
+
+      $reports[$relative_path] = $coverage;
+    }
+
+    return $reports;
+  }
 }