[clangd] Enable reading config from files by default.

[hokein]

clang-tools-extra/clangd/tool/ClangdMain.cpp

436

nit: maybe worth mentioning that the user config has higher privilege.

440

nit: I'd keep this flag off in this patch, and flip it in a new patch.

[kadircet]

clang-tools-extra/clangd/tool/ClangdMain.cpp

701

should we first update the background index storage location?

[kadircet]

LG from my side

[sammccall]

clang-tools-extra/clangd/tool/ClangdMain.cpp

436

I think this is the sort of detail best left to the actual documentation.
I've added a... forward declaration of that documentation here, to the URL where it will soon exist.

701

This is done. (There will still be old .clangd directories around, but that will ~always be true)

[hokein]

looks like you didn't upload the latest version.

clang-tools-extra/clangd/tool/ClangdMain.cpp

opt<Path> IndexFile{
     init(""),
     Hidden,
 };
 
 opt<bool> Test{
     "lit-test",
     cat(Misc),
     desc("Abbreviation for -input-style=delimited -pretty -sync "
-         "-enable-test-scheme -log=verbose. "
+         "-enable-test-scheme -enable-config=0 -log=verbose. "
          "Intended to simplify lit tests"),
     init(false),
     Hidden,
 };
 
 enum PCHStorageFlag { Disk, Memory };
 opt<PCHStorageFlag> PCHStorage{
     "pch-storage",
opt<bool> AsyncPreamble{
     "async-preamble",
     cat(Misc),
     desc("Reuse even stale preambles, and rebuild them in the background. This "
          "improves latency at the cost of accuracy."),
     init(ClangdServer::Options().AsyncPreambleBuilds),
     Hidden,
 };
 
+opt<bool> EnableConfig{
+    "enable-config",
+    cat(Misc),
+    desc(
+        "Read user and project configuration from YAML files.\n"
+        "Project config is from a .clangd file in the project directory.\n"
+        "User config is from clangd/config.yaml in the following directories:\n"
+        "\tWindows: %USERPROFILE%\\AppData\\Local\n"
+        "\tMac OS: ~/Library/Preferences/\n"
+        "\tOthers: $XDG_CONFIG_HOME, usually ~/.config\n"
+        "Configuration is documented at https://clangd.llvm.org/config.html"),
+    init(false),
+};
+
 /// Supports a test URI scheme with relaxed constraints for lit tests.
 /// The path in a test URI will be combined with a platform-specific fake
 /// directory to form an absolute path. For example, test:///a.cpp is resolved
 /// C:\clangd-test\a.cpp on Windows and /clangd-test/a.cpp on Unix.
 class TestScheme : public URIScheme {
 public:
   llvm::Expected<std::string>
   getAbsolutePath(llvm::StringRef /*Authority*/, llvm::StringRef Body,
)";
   llvm::cl::HideUnrelatedOptions(ClangdCategories);
   llvm::cl::ParseCommandLineOptions(argc, argv, Overview,
                                     /*Errs=*/nullptr, FlagsEnvVar);
   if (Test) {
     Sync = true;
     InputStyle = JSONStreamStyle::Delimited;
     LogLevel = Logger::Verbose;
     PrettyPrint = true;
+    // Disable config system by default to avoid external reads.
+    if (!EnableConfig.getNumOccurrences())
+      EnableConfig = false;
     // Disable background index on lit tests by default to prevent disk writes.
     if (!EnableBackgroundIndex.getNumOccurrences())
       EnableBackgroundIndex = false;
     // Ensure background index makes progress.
     else if (EnableBackgroundIndex)
       BackgroundQueue::preventThreadStarvationInTests();
   }
   if (Test || EnableTestScheme) {
if (!HeaderInsertionDecorators) {
     CCOpts.IncludeIndicator.NoInsert.clear();
   }
   CCOpts.SpeculativeIndexRequest = Opts.StaticIndex;
   CCOpts.EnableFunctionArgSnippets = EnableFunctionArgSnippets;
   CCOpts.AllScopes = AllScopesCompletion;
   CCOpts.RunParser = CodeCompletionParse;
 
   RealThreadsafeFS TFS;
+  std::unique_ptr<config::Provider> Config;
+  if (EnableConfig) {
+    std::vector<std::unique_ptr<config::Provider>> ProviderStack;
+    ProviderStack.push_back(
+        config::Provider::fromAncestorRelativeYAMLFiles(".clangd", TFS));
+    llvm::SmallString<256> UserConfig;
+    if (llvm::sys::path::user_config_directory(UserConfig)) {
+      llvm::sys::path::append(UserConfig, "clangd", "config.yaml");
+      vlog("User config file is {0}", UserConfig);
+      ProviderStack.push_back(config::Provider::fromYAMLFile(UserConfig, TFS));
+    } else {
+      elog("Couldn't determine user config file, not loading");
+    }
+    Config = config::Provider::combine(std::move(ProviderStack));
+    Opts.ConfigProvider = Config.get();
+  }
+
   // Initialize and run ClangdLSPServer.
   // Change stdin to binary to not lose \r\n on windows.
   llvm::sys::ChangeStdinToBinary();
 
   std::unique_ptr<Transport> TransportLayer;
   if (getenv("CLANGD_AS_XPC_SERVICE")) {
 #if CLANGD_BUILD_XPC
     log("Starting LSP over XPC service");