Revert "drop unnecessary realpath usage in the FindBadConstructs plugin"

This reverts commit 07b2231e.

Reason for revert:

We need to be more conservative with plugin behaviour changes.
TODO: document the process, then attempt to re-land this change
via that process.

Original change's description:
> drop unnecessary realpath usage in the FindBadConstructs plugin
>
> All we need to do is ensure that there's a leading '/' in the path, we
> don't expect the path to exist necessarily, and we don't need to resolve
> symlinks.  We just want to be able to search for directories by doing a
> string search of the form "/foo/".  And since we don't use realpath
> anymore, this should work with distributed compilation systems like icecc.
>
> This will also avoid mis-classification of files in chromium checkouts
> that are inside a directory with the same name as one of the so-called
> "banned" directories, eg a checkout in /home/gen/chromium/.
>
> BUG=583454

Change-Id: Ic840387bb2244d967e66fd078684c56435fe940a
Reviewed-on: https://chromium-review.googlesource.com/986435Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Mostyn Bramley-Moore <mostynb@vewd.com>
Cr-Commit-Position: refs/heads/master@{#546928}

tools/clang/plugins/ChromeClassTester.cpp

@@ -95,17 +95,47 @@ ChromeClassTester::LocationType ChromeClassTester::ClassifyLocation(
   if (filename == "<scratch space>")
     return LocationType::kThirdParty;
 
+#if defined(LLVM_ON_UNIX)
+  // Resolve the symlinktastic relative path and make it absolute.
+  char resolvedPath[MAXPATHLEN];
+  if (options_.no_realpath) {
+    // Same reason as windows below.
+    filename.insert(filename.begin(), '/');
+  } else if (realpath(filename.c_str(), resolvedPath)) {
+    filename = resolvedPath;
+  }
+#endif
+
+#if defined(LLVM_ON_WIN32)
+  // Make path absolute.
+  if (options_.no_realpath) {
+    // This turns e.g. "gen/dir/file.cc" to "/gen/dir/file.cc" which lets the
+    // "/gen/" banned_dir work.
+    filename.insert(filename.begin(), '/');
+  } else {
+    // The Windows dance: Convert to UTF-16, call GetFullPathNameW, convert back
+    DWORD size_needed =
+        MultiByteToWideChar(CP_UTF8, 0, filename.data(), -1, nullptr, 0);
+    std::wstring utf16(size_needed, L'\0');
+    MultiByteToWideChar(CP_UTF8, 0, filename.data(), -1,
+                        &utf16[0], size_needed);
+
+    size_needed = GetFullPathNameW(utf16.data(), 0, nullptr, nullptr);
+    std::wstring full_utf16(size_needed, L'\0');
+    GetFullPathNameW(utf16.data(), full_utf16.size(), &full_utf16[0], nullptr);
+
+    size_needed = WideCharToMultiByte(CP_UTF8, 0, full_utf16.data(), -1,
+                                      nullptr, 0, nullptr, nullptr);
+    filename.resize(size_needed);
+    WideCharToMultiByte(CP_UTF8, 0, full_utf16.data(), -1, &filename[0],
+                        size_needed, nullptr, nullptr);
+  }
+#endif
+
   // When using distributed cross compilation build tools, file paths can have
   // separators which differ from ones at this platform. Make them consistent.
   std::replace(filename.begin(), filename.end(), '\\', '/');
 
-  // Ensure that we can search for patterns of the form "/foo/" even
-  // if we have a relative path like "foo/bar.cc".  We don't expect
-  // this transformed path to exist necessarily.
-  if (filename.front() != '/') {
-    filename.insert(0, 1, '/');
-  }
-
   // Don't check autogenerated files. ninja puts them in $OUT_DIR/gen.
   if (filename.find("/gen/") != std::string::npos)
     return LocationType::kThirdParty;

tools/clang/plugins/FindBadConstructsAction.cpp

@@ -53,6 +53,8 @@ bool FindBadConstructsAction::ParseArgs(const CompilerInstance& instance,
       // TODO(tsepez): Enable this by default once http://crbug.com/356815
       // and http://crbug.com/356816 are fixed.
       options_.check_enum_last_value = true;
+    } else if (args[i] == "no-realpath") {
+      options_.no_realpath = true;
     } else if (args[i] == "check-ipc") {
       options_.check_ipc = true;
     } else {

tools/clang/plugins/Options.h

@@ -11,6 +11,9 @@ struct Options {
   bool check_base_classes = false;
   bool enforce_in_thirdparty_webkit = false;  // Use in Blink code itself
   bool check_enum_last_value = false;
+  // This is needed for some distributed build-sytems to respect banned
+  // paths. See https://crbug.com/583454 for details.
+  bool no_realpath = false;
   bool check_ipc = false;
 };