Index: clangd/CMakeLists.txt
===================================================================
--- clangd/CMakeLists.txt
+++ clangd/CMakeLists.txt
@@ -21,6 +21,7 @@
   ProtocolHandlers.cpp
   SourceCode.cpp
   Trace.cpp
+  URI.cpp
   XRefs.cpp
   index/FileIndex.cpp
   index/Index.cpp
Index: clangd/URI.h
===================================================================
--- /dev/null
+++ clangd/URI.h
@@ -0,0 +1,95 @@
+//===--- URI.h - File URIs with schemas --------------------------*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_PATHURI_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_PATHURI_H
+
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Registry.h"
+
+namespace clang {
+namespace clangd {
+
+/// \brief An URI for a file path in a certain schema. This is invalid if either
+/// schema or path is empty.
+struct FileURI {
+  std::string Schema;
+  // This is interpreted according to the schema.
+  std::string Path;
+
+  FileURI() = default;
+
+  FileURI(std::string Schema, std::string Path)
+      : Schema(std::move(Schema)), Path(std::move(Path)) {}
+
+  bool IsValid() const { return !Schema.empty() && !Path.empty(); }
+
+  /// \brief Returns a URI string "<schema>://<path>".
+  std::string toString() const;
+
+  /// \brief Parse a URI string "<schema>://<path>". If this is an invalid URI,
+  /// this returns an empty URI with empty schema and path.
+  static FileURI fromURI(llvm::StringRef Uri);
+
+  friend bool operator==(const FileURI &LHS, const FileURI &RHS) {
+    return std::tie(LHS.Schema, LHS.Path) == std::tie(RHS.Schema, RHS.Path);
+  }
+};
+
+/// \brief This manages file paths in different schemas. Different
+/// codebases/projects can have different file schemas, and clangd interprets a
+/// file path according to the schema. For example, a file path provided by a
+/// remote symbol index can follow a certain schema (e.g. relative to a project
+/// root directory), and clangd needs to combine the schema path with execution
+/// environment (e.g. working/build directory) in order to get a file path in
+/// the file system.
+///
+/// By default, a "file" schema is supported where URI paths are always absolute
+/// in the file system.
+class URISchema {
+public:
+  virtual ~URISchema() = default;
+
+  /// \brief Returns the absolute path of the file corresponding to \p Uri in
+  /// the file system. \p CurrentFile is the file from which the request is
+  /// issued. This is needed because the same URI in different workspace may
+  /// correspond to different files.
+  ///
+  /// Returns "" if \p Uri is not in the schema.
+  virtual std::string getAbsolutePath(llvm::StringRef CurrentFile,
+                                      const FileURI &Uri) const = 0;
+
+  /// \brief Creates an URI for the file \p AbsolutePath in this schema.
+  virtual FileURI uriFromAbsolutePath(llvm::StringRef AbsolutePath) const = 0;
+};
+
+/// \brief This manages file paths in the file system. All paths in the schema
+/// are absolute (with leading '/').
+class FileSystemSchema : public URISchema {
+public:
+  static const char *Schema;
+
+  std::string getAbsolutePath(llvm::StringRef /*CurrentFile*/,
+                              const FileURI &Uri) const override;
+
+  FileURI uriFromAbsolutePath(llvm::StringRef AbsolutePath) const override;
+};
+
+typedef llvm::Registry<URISchema> URISchemaRegistry;
+
+/// \brief Resolves the absolute path of \p Uri with the first matching schema
+/// registered.
+///
+/// Returns "" if there is no matching schema.
+std::string resolve(llvm::StringRef CurrentFile, const FileURI &Uri);
+
+} // namespace clangd
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANGD_PATHURI_H
Index: clangd/URI.cpp
===================================================================
--- /dev/null
+++ clangd/URI.cpp
@@ -0,0 +1,54 @@
+//===---- URI.h - File URIs with schemas -------------------------*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "URI.h"
+
+LLVM_INSTANTIATE_REGISTRY(clang::clangd::URISchemaRegistry)
+
+namespace clang {
+namespace clangd {
+
+std::string FileURI::toString() const { return Schema + "://" + Path; }
+
+FileURI FileURI::fromURI(llvm::StringRef Uri) {
+  auto Pos = Uri.find("://");
+  if (Pos == llvm::StringRef::npos)
+    return {"", ""};
+  return {Uri.substr(0, Pos), Uri.substr(Pos + 3)};
+}
+
+const char *FileSystemSchema::Schema = "file";
+
+std::string FileSystemSchema::getAbsolutePath(llvm::StringRef /*CurrentFile*/,
+                                              const FileURI &Uri) const {
+  return Schema == Uri.Schema ? Uri.Path : "";
+}
+
+FileURI
+FileSystemSchema::uriFromAbsolutePath(llvm::StringRef AbsolutePath) const {
+  assert(AbsolutePath.startswith("/") && "Expect an absolute path.");
+  return {Schema, AbsolutePath};
+}
+
+static URISchemaRegistry::Add<FileSystemSchema>
+    X(FileSystemSchema::Schema,
+      "URI schema for absolute paths in the file system.");
+
+std::string resolve(llvm::StringRef CurrentFile, const FileURI &Uri) {
+  for (auto I = URISchemaRegistry::begin(), E = URISchemaRegistry::end();
+       I != E; ++I) {
+    if (I->getName() != Uri.Schema)
+      continue;
+    return I->instantiate()->getAbsolutePath(CurrentFile, Uri);
+  }
+  return "";
+}
+
+} // namespace clangd
+} // namespace clang
Index: unittests/clangd/CMakeLists.txt
===================================================================
--- unittests/clangd/CMakeLists.txt
+++ unittests/clangd/CMakeLists.txt
@@ -18,6 +18,7 @@
   FuzzyMatchTests.cpp
   IndexTests.cpp
   JSONExprTests.cpp
+  URITests.cpp
   TestFS.cpp
   TraceTests.cpp
   SourceCodeTests.cpp
Index: unittests/clangd/URITests.cpp
===================================================================
--- /dev/null
+++ unittests/clangd/URITests.cpp
@@ -0,0 +1,75 @@
+//===-- URITests.cpp  ---------------------------------*- C++ -*-----------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "URI.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace clang {
+namespace clangd {
+namespace {
+
+TEST(URITest, URIStrings) {
+  EXPECT_EQ(FileURI::fromURI("abc:///x/y/z").toString(), "abc:///x/y/z");
+  EXPECT_EQ(FileURI::fromURI("abc:///x/y/z"), FileURI("abc", "/x/y/z"));
+  EXPECT_EQ(FileURI::fromURI("abc://x/y/z"), FileURI("abc", "x/y/z"));
+
+  EXPECT_TRUE(FileURI::fromURI("abc:///x/y/z").IsValid());
+  EXPECT_FALSE(FileURI::fromURI("://x/y/z").IsValid());
+  EXPECT_FALSE(FileURI::fromURI("/x/y/z").IsValid());
+}
+
+TEST(URITest, FileSystemSchema) {
+  FileURI Uri{"file", "/a/b/c"};
+  FileSystemSchema S;
+  EXPECT_EQ(S.getAbsolutePath(/*CurrentFile*/ "", Uri), "/a/b/c");
+  EXPECT_EQ(S.uriFromAbsolutePath("/a/b/c"), Uri);
+}
+
+// Assume all files in the schema have a "test-root/" root directory, and the
+// schema path is the relative path to the root directory.
+// So the schema of "/some-dir/test-root/x/y/z" is "test://x/y/z".
+class TestSchema : public URISchema {
+public:
+  static const char *Schema;
+
+  static const char *TestRoot;
+
+  std::string getAbsolutePath(llvm::StringRef CurrentFile,
+                              const FileURI &Uri) const override {
+    auto Pos = CurrentFile.find(TestRoot);
+    assert(Pos != llvm::StringRef::npos);
+    return CurrentFile.substr(0, Pos + llvm::StringRef(TestRoot).size()).str() +
+           Uri.Path;
+  }
+
+  FileURI uriFromAbsolutePath(llvm::StringRef AbsolutePath) const override {
+    auto Pos = AbsolutePath.find(TestRoot);
+    assert(Pos != llvm::StringRef::npos);
+    return {Schema,
+            AbsolutePath.substr(Pos + Pos + llvm::StringRef(TestRoot).size())};
+  }
+};
+
+const char *TestSchema::Schema = "test";
+const char *TestSchema::TestRoot = "/test-root/";
+
+static URISchemaRegistry::Add<TestSchema> X(TestSchema::Schema, "Test schema");
+
+TEST(URITest, ResolveURIs) {
+  EXPECT_EQ(resolve("", FileURI("file", "/a/b/c")), "/a/b/c");
+  EXPECT_EQ(resolve("/dir/test-root/x/y/z", FileURI("test", "a/b/c")),
+            "/dir/test-root/a/b/c");
+  // Unsupported schema.
+  EXPECT_EQ(resolve("/x/y/z", FileURI("invalid", "a/b/c")), "");
+}
+
+} // namespace
+} // namespace clangd
+} // namespace clang