[clangd] Provide a way to disable external index

Users can reset any external index set by previous fragments by
putting a `None` for the external block, e.g:

```
Index:
  External: None
```

Differential Revision: https://reviews.llvm.org/D100106

clang-tools-extra/clangd/Config.h

@@ -70,7 +70,7 @@ struct Config {
   enum class BackgroundPolicy { Build, Skip };
   /// Describes an external index configuration.
   struct ExternalIndexSpec {
-    enum { File, Server } Kind;
+    enum { None, File, Server } Kind;
     /// This is one of:
     /// - Address of a clangd-index-server, in the form of "ip:port".
     /// - Absolute path to an index produced by clangd-indexer.

clang-tools-extra/clangd/ConfigCompile.cpp

@@ -329,10 +329,11 @@ struct FragmentCompiler {
     }
 #endif
     // Make sure exactly one of the Sources is set.
-    unsigned SourceCount =
+    unsigned SourceCount = External.File.hasValue() +
-        External.File.hasValue() + External.Server.hasValue();
+                           External.Server.hasValue() + *External.IsNone;
     if (SourceCount != 1) {
-      diag(Error, "Exactly one of File or Server must be set.", BlockRange);
+      diag(Error, "Exactly one of File, Server or None must be set.",
+           BlockRange);
       return;
     }
     Config::ExternalIndexSpec Spec;
@@ -346,20 +347,29 @@ struct FragmentCompiler {
       if (!AbsPath)
         return;
       Spec.Location = std::move(*AbsPath);
+    } else {
+      assert(*External.IsNone);
+      Spec.Kind = Config::ExternalIndexSpec::None;
     }
-    // Make sure MountPoint is an absolute path with forward slashes.
+    if (Spec.Kind != Config::ExternalIndexSpec::None) {
-    if (!External.MountPoint)
+      // Make sure MountPoint is an absolute path with forward slashes.
-      External.MountPoint.emplace(FragmentDirectory);
+      if (!External.MountPoint)
-    if ((**External.MountPoint).empty()) {
+        External.MountPoint.emplace(FragmentDirectory);
-      diag(Error, "A mountpoint is required.", BlockRange);
+      if ((**External.MountPoint).empty()) {
-      return;
+        diag(Error, "A mountpoint is required.", BlockRange);
+        return;
+      }
+      auto AbsPath = makeAbsolute(std::move(*External.MountPoint), "MountPoint",
+                                  llvm::sys::path::Style::posix);
+      if (!AbsPath)
+        return;
+      Spec.MountPoint = std::move(*AbsPath);
     }
-    auto AbsPath = makeAbsolute(std::move(*External.MountPoint), "MountPoint",
-                                llvm::sys::path::Style::posix);
-    if (!AbsPath)
-      return;
-    Spec.MountPoint = std::move(*AbsPath);
     Out.Apply.push_back([Spec(std::move(Spec))](const Params &P, Config &C) {
+      if (Spec.Kind == Config::ExternalIndexSpec::None) {
+        C.Index.External.reset();
+        return;
+      }
       if (P.Path.empty() || !pathStartsWith(Spec.MountPoint, P.Path,
                                             llvm::sys::path::Style::posix))
         return;

clang-tools-extra/clangd/ConfigFragment.h

@@ -173,6 +173,9 @@ struct Fragment {
     /// usually prepared using clangd-indexer.
     /// Exactly one source (File/Server) should be configured.
     struct ExternalBlock {
+      /// Whether the block is explicitly set to `None`. Can be used to clear
+      /// any external index specified before.
+      Located<bool> IsNone = false;
       /// Path to an index file generated by clangd-indexer. Relative paths may
       /// be used, if config fragment is associated with a directory.
       llvm::Optional<Located<std::string>> File;

clang-tools-extra/clangd/ConfigYAML.cpp

@@ -12,6 +12,7 @@
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/YAMLParser.h"
+#include <string>
 #include <system_error>
 
 namespace clang {
@@ -149,13 +150,34 @@ private:
                 [&](Node &N) { F.Background = scalarValue(N, "Background"); });
     Dict.handle("External", [&](Node &N) {
       Fragment::IndexBlock::ExternalBlock External;
-      parse(External, N);
+      // External block can either be a mapping or a scalar value. Dispatch
+      // accordingly.
+      if (N.getType() == Node::NK_Mapping) {
+        parse(External, N);
+      } else if (N.getType() == Node::NK_Scalar ||
+                 N.getType() == Node::NK_BlockScalar) {
+        parse(External, scalarValue(N, "External").getValue());
+      } else {
+        error("External must be either a scalar or a mapping.", N);
+        return;
+      }
       F.External.emplace(std::move(External));
       F.External->Range = N.getSourceRange();
     });
     Dict.parse(N);
   }
 
+  void parse(Fragment::IndexBlock::ExternalBlock &F,
+             Located<std::string> ExternalVal) {
+    if (!llvm::StringRef(*ExternalVal).equals_lower("none")) {
+      error("Only scalar value supported for External is 'None'",
+            ExternalVal.Range);
+      return;
+    }
+    F.IsNone = true;
+    F.IsNone.Range = ExternalVal.Range;
+  }
+
   void parse(Fragment::IndexBlock::ExternalBlock &F, Node &N) {
     DictParser Dict("External", this);
     Dict.handle("File", [&](Node &N) { F.File = scalarValue(N, "File"); });

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

@@ -327,21 +327,31 @@ TEST_F(ConfigCompileTests, ExternalBlockWarnOnMultipleSource) {
 #ifdef CLANGD_ENABLE_REMOTE
   EXPECT_THAT(
       Diags.Diagnostics,
-      Contains(AllOf(DiagMessage("Exactly one of File or Server must be set."),
+      Contains(
-                     DiagKind(llvm::SourceMgr::DK_Error))));
+          AllOf(DiagMessage("Exactly one of File, Server or None must be set."),
+                DiagKind(llvm::SourceMgr::DK_Error))));
 #else
   ASSERT_TRUE(Conf.Index.External.hasValue());
   EXPECT_EQ(Conf.Index.External->Kind, Config::ExternalIndexSpec::File);
 #endif
 }
 
+TEST_F(ConfigCompileTests, ExternalBlockDisableWithNone) {
+  Fragment::IndexBlock::ExternalBlock External;
+  External.IsNone = true;
+  Frag.Index.External = std::move(External);
+  compileAndApply();
+  EXPECT_FALSE(Conf.Index.External.hasValue());
+}
+
 TEST_F(ConfigCompileTests, ExternalBlockErrOnNoSource) {
   Frag.Index.External.emplace(Fragment::IndexBlock::ExternalBlock{});
   compileAndApply();
   EXPECT_THAT(
       Diags.Diagnostics,
-      Contains(AllOf(DiagMessage("Exactly one of File or Server must be set."),
+      Contains(
-                     DiagKind(llvm::SourceMgr::DK_Error))));
+          AllOf(DiagMessage("Exactly one of File, Server or None must be set."),
+                DiagKind(llvm::SourceMgr::DK_Error))));
 }
 
 TEST_F(ConfigCompileTests, ExternalBlockDisablesBackgroundIndex) {

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

@@ -147,6 +147,23 @@ horrible
   ASSERT_THAT(Results, IsEmpty());
 }
 
+TEST(ParseYAML, ExternalBlockNone) {
+  CapturedDiags Diags;
+  Annotations YAML(R"yaml(
+Index:
+  External: None
+  )yaml");
+  auto Results =
+      Fragment::parseYAML(YAML.code(), "config.yaml", Diags.callback());
+  ASSERT_THAT(Diags.Diagnostics, IsEmpty());
+  ASSERT_EQ(Results.size(), 1u);
+  ASSERT_TRUE(Results[0].Index.External);
+  EXPECT_FALSE(Results[0].Index.External.getValue()->File.hasValue());
+  EXPECT_FALSE(Results[0].Index.External.getValue()->MountPoint.hasValue());
+  EXPECT_FALSE(Results[0].Index.External.getValue()->Server.hasValue());
+  EXPECT_THAT(*Results[0].Index.External.getValue()->IsNone, testing::Eq(true));
+}
+
 TEST(ParseYAML, ExternalBlock) {
   CapturedDiags Diags;
   Annotations YAML(R"yaml(