[Tooling] Restore current directory after processing each file.

[alexfh]

Summary

If we actually change directory before processing a file, we need to
restore it afterwards. This was broken in r216620.

Added a comment for the changes in r216620.

[klimek]

lg

lib/Tooling/Tooling.cpp

312–315

I'd rephrase this to say what state we want to be in rather than what the current state is.

[alexfh]

lib/Tooling/Tooling.cpp

312–315

Done. See r221600.

lib/Tooling/Tooling.cpp

int ClangTool::run(ToolAction *Action) {
   // The driver detects the builtin header path based on the path of the
   // executable.
   // FIXME: On linux, GetMainExecutable is independent of the value of the
   // first argument, thus allowing ClangTool and runToolOnCode to just
   // pass in made-up names here. Make sure this works on other platforms.
   std::string MainExecutable =
       llvm::sys::fs::getMainExecutable("clang_tool", &StaticSymbol);
 
+  llvm::SmallString<128> InitialDirectory;
+  if (std::error_code EC = llvm::sys::fs::current_path(InitialDirectory))
+    llvm::report_fatal_error("Cannot detect current path: " +
+                             Twine(EC.message()));
   bool ProcessingFailed = false;
   for (const auto &SourcePath : SourcePaths) {
     std::string File(getAbsolutePath(SourcePath));
 
+    // FIXME: getCompileCommands can change the state of the file system (e.g.
+    // prepare generated headers), so it needs to run right before we invoke the
+    // tool, as the next file may require a different (incompatible) state of
+    // the file system.
     std::vector<CompileCommand> CompileCommandsForFile =
         Compilations.getCompileCommands(File);
     if (CompileCommandsForFile.empty()) {
       // FIXME: There are two use cases here: doing a fuzzy
       // "find . -name '*.cc' |xargs tool" match, where as a user I don't care
       // about the .cc files that were not found, and the use case where I
       // specify all files I want to run over explicitly, where this should
       // be an error. We'll want to add an option for this.
for (CompileCommand &CompileCommand : CompileCommandsForFile) {
       for (const auto &MappedFile : MappedFileContents) {
         Invocation.mapVirtualFile(MappedFile.first, MappedFile.second);
       }
       if (!Invocation.run()) {
         // FIXME: Diagnostics should be used instead.
         llvm::errs() << "Error while processing " << File << ".\n";
         ProcessingFailed = true;
       }
+      // Return to the initial directory to correctly resolve next file by
+      // relative path.
+      if (chdir(InitialDirectory.c_str()))
+        llvm::report_fatal_error("Cannot chdir into \"" +
+                                 Twine(InitialDirectory) + "\n!");
     }
   }
   return ProcessingFailed ? 1 : 0;
 }
 
 namespace {
 
 class ASTBuilderAction : public ToolAction {