node tests: Allow --coverage for individual files.

We now allow you to run --coverage on individual files.  This helps
when you want to make sure a file is being covered directly and not
just getting incidental coverage from higher level tests.

Before this commit, we were conflating wanting coverage reports with
wanting coverage checks.  For individual files, we now solve that by
simply eliminating the coverage checks.  This required some minor
refactoring to extract some functions.

tools/test-js-with-node

@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 import argparse
 import os
+import shutil
 import subprocess
 import sys
 from typing import Dict, Any
@@ -102,22 +103,24 @@ for index, arg in enumerate(options.args):
     if not arg.endswith('.js'):
         options.args[index] = arg + '.js'
 
+individual_files = options.args
+
 # The index.js test runner is the real "driver" here, and we launch
 # with either istanbul or node, depending on whether we want coverage
 # reports.  Running under istanbul is slower and creates funny
 # tracebacks, so you generally want to get coverage reports only
 # after making sure tests will pass.
 if options.coverage:
-    if options.args:
-        print('BAD ARGS! Coverage reports run against all files.')
-        sys.exit(1)
-
+    coverage_dir = os.path.join(ROOT_DIR, 'var/node-coverage')
+    shutil.rmtree(coverage_dir, True)
     istanbul = os.path.join(ROOT_DIR, 'node_modules/.bin/istanbul')
-    command = [istanbul, 'cover', INDEX_JS, '--dir', 'var/node-coverage']
+    command = [istanbul, 'cover', '--dir', coverage_dir, INDEX_JS]
 else:
     # Normal testing, no coverage analysis.
     # Run the index.js test runner, which runs all the other tests.
-    command = ['node', '--stack-trace-limit=100', INDEX_JS] + options.args
+    command = ['node', '--stack-trace-limit=100', INDEX_JS]
+
+command += individual_files
 
 print('Starting node tests...')
 
@@ -128,8 +131,8 @@ except OSError:
     print('Bad command: %s' % (command,))
     raise
 
-def check_line_coverage(line_coverage, line_mapping, log=True):
-    # type: (Dict[Any, Any], Dict[Any, Any], bool) -> bool
+def check_line_coverage(fn, line_coverage, line_mapping, log=True):
+    # type: (str, Dict[Any, Any], Dict[Any, Any], bool) -> bool
     missing_lines = []
     for line in line_coverage:
         if line_coverage[line] == 0:
@@ -137,7 +140,7 @@ def check_line_coverage(line_coverage, line_mapping, log=True):
             missing_lines.append(str(actual_line["start"]["line"]))
     if missing_lines:
         if log:
-            print("ERROR: %s no longer has complete node test coverage" % (relative_path,))
+            print("ERROR: %s no longer has complete node test coverage" % (fn,))
             print("  Lines missing coverage: %s" % (", ".join(sorted(missing_lines, key=int)),))
             print()
         return False
@@ -145,7 +148,7 @@ def check_line_coverage(line_coverage, line_mapping, log=True):
 
 NODE_COVERAGE_PATH = 'var/node-coverage/coverage.json'
 
-if options.coverage and ret == 0:
+def read_coverage() -> Any:
     coverage_json = None
     try:
         with open(NODE_COVERAGE_PATH, 'r') as f:
@@ -153,6 +156,10 @@ if options.coverage and ret == 0:
     except IOError:
         print(NODE_COVERAGE_PATH + " doesn't exist. Cannot enforce fully covered files.")
         raise
+    return coverage_json
+
+def enforce_proper_coverage(coverage_json: Any) -> bool:
+    coverage_lost = False
     for relative_path in enforce_fully_covered:
         path = ROOT_DIR + "/" + relative_path
         if not (path in coverage_json):
@@ -160,29 +167,39 @@ if options.coverage and ret == 0:
             continue
         line_coverage = coverage_json[path]['s']
         line_mapping = coverage_json[path]['statementMap']
-        if not check_line_coverage(line_coverage, line_mapping):
-            ret = 1
-    if ret:
+        if not check_line_coverage(relative_path, line_coverage, line_mapping):
+            coverage_lost = True
+    if coverage_lost:
         print()
         print("It looks like your changes lost 100% test coverage in one or more files.")
         print("Usually, the right fix for this is to add some tests.")
         print("But also check out the include/exclude lists in `tools/test-js-with-node`.")
         print("To run this check locally, use `test-js-with-node --coverage`.")
 
-    ok = True
+    coverage_not_enforced = False
     for path in coverage_json:
         if '/static/js/' in path:
             relative_path = os.path.relpath(path, ROOT_DIR)
             line_coverage = coverage_json[path]['s']
             line_mapping = coverage_json[path]['statementMap']
-            if check_line_coverage(line_coverage, line_mapping, log=False) \
+            if check_line_coverage(relative_path, line_coverage, line_mapping, log=False) \
                     and not (relative_path in enforce_fully_covered):
-                ok = False
+                coverage_not_enforced = True
                 print("ERROR: %s has complete node test coverage and is not enforced." % (relative_path,))
-    if not ok:
+
+    if coverage_not_enforced:
         print()
         print("There are one or more fully covered files that are not enforced.")
         print("Add the file(s) to enforce_fully_covered in `tools/test-js-with-node`.")
+
+    problems_encountered = (coverage_lost or coverage_not_enforced)
+    return problems_encountered
+
+if options.coverage and ret == 0:
+    if not individual_files:
+        coverage_json = read_coverage()
+        problems_encountered = enforce_proper_coverage(coverage_json)
+        if problems_encountered:
             ret = 1
 
 print()