.. _advanced_check_features:

Advanced Check Features
=======================

This page covers additional ways to improve and extend the check you've added to build/clang-plugin.

Adding Tests
------------

No doubt you've seen the tests for existing checks in `build/clang-plugin/tests <https://searchfox.org/mozilla-central/source/build/clang-plugin/tests>`_. Adding tests is straightforward; and your reviewer should insist you do so. Simply copying the existing format of any test and how diagnostics are marked as expected.

One wrinkle - all clang plugin checks are applied to all tests. We try to write tests so that only one check applies to it.  If you write a check that triggers on an existing test, try to fix the existing test slightly so the new check does not trigger on it.

Using Bind To Output More Useful Information
--------------------------------------------

You've probably been wondering what the heck ``.bind()`` is for. You've been seeing it all over the place but never has it actually been explained what it's for and when to use it.

``.bind()`` is used to give a name to part of the AST discovered through your matcher, so you can use it later.  Let's go back to our sample matcher:

::

  AstMatcher->addMatcher(
    traverse(TK_IgnoreUnlessSpelledInSource,
      ifStmt(allOf(
              has(
                   binaryOperator(
                       has(
                           declRefExpr(hasType(enumDecl()))
                       )
                   )
               ),
               hasElse(
                   ifStmt(allOf(
                      unless(hasElse(anything())),
                      has(
                           binaryOperator(
                               has(
                                   declRefExpr(hasType(enumDecl()))
                               )
                           )
                       )
                   ))
              )
           ))
          .bind("node")),
      this);

Now the ``.bind("node")`` makes more sense. We're naming the If statement we matched, so we can refer to it later when we call ``Result.Nodes.getNodeAs<IfStmt>("node")``.

Let's say we want to provide the *type* of the enum in our warning message.  There are two enums we end up seeing in our matcher - the enum in the first if statement, and the enum in the second.  We're going to arbitrarily pick the first and give it the name ``enumType``:

::

  AstMatcher->addMatcher(
    traverse(TK_IgnoreUnlessSpelledInSource,
      ifStmt(allOf(
              has(
                   binaryOperator(
                       has(
                           declRefExpr(hasType(enumDecl().bind("enumType")))
                       )
                   )
               ),
               hasElse(
                   ifStmt(allOf(
                      unless(hasElse(anything())),
                      has(
                           binaryOperator(
                               has(
                                   declRefExpr(hasType(enumDecl()))
                               )
                           )
                       )
                   ))
              )
           ))
          .bind("node")),
      this);

And in our check() function, we can use it like so:

::

  void MissingElseInEnumComparisons::check(
      const MatchFinder::MatchResult &Result) {
    const auto *MatchedDecl = Result.Nodes.getNodeAs<IfStmt>("node");
    const auto *EnumType = Result.Nodes.getNodeAs<EnumDecl>("enumType");

    diag(MatchedDecl->getIfLoc(),
         "Enum comparisons to %0 in an if/else if block without a trailing else.",
         DiagnosticIDs::Warning) << EnumType->getName();
  }

Repeated matcher calls
--------------------------

If you find yourself repeating the same several matchers in several spots, you can turn it into a variable to use.

::

  auto isTemporaryLifetimeBoundCall =
      cxxMemberCallExpr(
          onImplicitObjectArgument(anyOf(has(cxxTemporaryObjectExpr()),
                                         has(materializeTemporaryExpr()))),
          callee(functionDecl(isMozTemporaryLifetimeBound())));

  auto hasTemporaryLifetimeBoundCall =
      anyOf(isTemporaryLifetimeBoundCall,
            conditionalOperator(
                anyOf(hasFalseExpression(isTemporaryLifetimeBoundCall),
                      hasTrueExpression(isTemporaryLifetimeBoundCall))));

The above example is parameter-less, but if you need to supply a parameter that changes, you can turn it into a lambda:

::

  auto hasConstCharPtrParam = [](const unsigned int Position) {
    return hasParameter(
        Position, hasType(hasCanonicalType(pointsTo(asString("const char")))));
  };

  auto hasParamOfType = [](const unsigned int Position, const char *Name) {
    return hasParameter(Position, hasType(asString(Name)));
  };

  auto hasIntegerParam = [](const unsigned int Position) {
    return hasParameter(Position, hasType(isInteger()));
  };

  AstMatcher->addMatcher(
      callExpr(
        hasName("fopen"),
        hasConstCharPtrParam(0))
          .bind("funcCall"),
      this);


Allow-listing existing callsites
--------------------------------

While it's not a great situation, you can set up an allow-list of existing callsites if you need to.  A simple allow-list is demonstrated in `NoGetPrincipalURI <https://hg.mozilla.org/mozilla-central/rev/fb60b22ee6616521b386d90aec07b03b77905f4e>`_. The `NoNewThreadsChecker <https://hg.mozilla.org/mozilla-central/rev/f400f164b3947b4dd54089a36ea31cca2d72805b>`_ is an example of a more sophisticated way of setting up a larger allow-list.


Custom Annotations
------------------
It's possible to create custom annotations that will be a no-op when compiled, but can be used by a static analysis check. These can be used to annotate special types of sources and sinks (for example).  We have some examples of this in-tree presently (such as ``MOZ_CAN_RUN_SCRIPT``) but currently don't have a detailed walkthrough in this documentation of how to set these up and use them. (Patches welcome.)
