[clangd] Avoid a code completion crash

[hokein]

Summary

This is a workaround (adding a newline to the eof) in clangd to avoid the code
completion crash, see https://github.com/clangd/clangd/issues/332.

In principle, this is a clang bug, we should fix it in clang, but it is not
trivial.

[sammccall]

This isn't an ideal fix but it's simple, we know the real fix is invasive, and 14 is upon us.

clang-tools-extra/clangd/ClangdServer.cpp

auto Task = [Pos, CodeCompleteOpts, File = File.str(), CB = std::move(CB),
     } else if (CodeCompleteOpts.Index) {
       SpecFuzzyFind.emplace();
       {
         std::lock_guard<std::mutex> Lock(CachedCompletionFuzzyFindRequestMutex);
         SpecFuzzyFind->CachedReq = CachedCompletionFuzzyFindRequestByFile[File];
       }
     }
     ParseInputs ParseInput{IP->Command, &TFS, IP->Contents.str()};
+    // FIXME: Add traling new line if there is none at eof, workaround a crash,
+    // see https://github.com/clangd/clangd/issues/332
+    if (!IP->Contents.endswith("\n"))
+      ParseInput.Contents.append("\n");
     ParseInput.Index = Index;
 
     CodeCompleteOpts.MainFileSignals = IP->Signals;
     CodeCompleteOpts.AllScopes = Config::current().Completion.AllScopes;
     // FIXME(ibiryukov): even if Preamble is non-null, we may want to check
     // both the old and the new version in case only one of them matches.
     CodeCompleteResult Result = clangd::codeComplete(
         File, Pos, IP->Preamble, ParseInput, CodeCompleteOpts,
auto Action = [Pos, File = File.str(), CB = std::move(CB),
     if (!IP)
       return CB(IP.takeError());
 
     const auto *PreambleData = IP->Preamble;
     if (!PreambleData)
       return CB(error("Failed to parse includes"));
 
     ParseInputs ParseInput{IP->Command, &TFS, IP->Contents.str()};
+    // FIXME: Add traling new line if there is none at eof, workaround a crash,
+    // see https://github.com/clangd/clangd/issues/332
+    if (!IP->Contents.endswith("\n"))
+      ParseInput.Contents.append("\n");
     ParseInput.Index = Index;
     CB(clangd::signatureHelp(File, Pos, *PreambleData, ParseInput,
                              DocumentationFormat));
   };
 
   // Unlike code completion, we wait for a preamble here.
   WorkScheduler->runWithPreamble("SignatureHelp", File, TUScheduler::Stale,
                                  std::move(Action));

clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp

completions(R"cpp(
     };
 
     struct Derived : Base {
       ^
     };
   )cpp");
 }
 
+TEST(CompletionTest, NoCrashOnMissingNewLineAtEOF) {
+  auto FooCpp = testPath("foo.cpp");
+
+  MockCompilationDatabase CDB;
+  MockFS FS;
+  Annotations F("#pragma ^ // no new line");
+  FS.Files[FooCpp] = F.code().str();
+  ClangdServer Server(CDB, FS, ClangdServer::optsForTest());
+  runAddDocument(Server, FooCpp, F.code());
+  // Run completion outside the file range.
+  EXPECT_THAT(cantFail(runCodeComplete(Server, FooCpp, F.point(),
+                                       clangd::CodeCompleteOptions()))
+                  .Completions,
+              IsEmpty());
+  EXPECT_THAT(cantFail(runSignatureHelp(Server, FooCpp, F.point(),
+                                        MarkupKind::PlainText))
+                  .signatures,
+              IsEmpty());
+}
+
 TEST(GuessCompletionPrefix, Filters) {
   for (llvm::StringRef Case : {
            "[[scope::]][[ident]]^",
            "[[]][[]]^",
            "\n[[]][[]]^",
            "[[]][[ab]]^",
            "x.[[]][[ab]]^",
            "x.[[]][[]]^",