[build-system] Fold ExternalWorkspace into BuildSystem

Simplifies clients to only need to care about one thing, and makes it
easier for BuildSystems to grow new functionality.

Author: benlangmuir

Sources/SKCore/BuildSettingsProviderList.swift

@@ -10,17 +10,25 @@
 //
 //===----------------------------------------------------------------------===//
 
+import Basic
 import LanguageServerProtocol
 
 /// Provides build settings from a list of providers in priority order.
-public final class BuildSettingsProviderList: BuildSystem {
+public final class BuildSettingsProviderList {
 
   /// The build settings providers to try (in order).
   public var providers: [BuildSystem] = [
     FallbackBuildSettingsProvider()
   ]
 
   public init() {}
+}
+
+extension BuildSettingsProviderList: BuildSystem {
+
+  public var indexStorePath: AbsolutePath? { return providers.first?.indexStorePath }
+
+  public var indexDatabasePath: AbsolutePath? { return providers.first?.indexDatabasePath }
 
   public func settings(for url: URL, _ language: Language) -> FileBuildSettings? {
     for provider in providers {

Sources/SKCore/BuildSystem.swift

@@ -11,10 +11,24 @@
 //===----------------------------------------------------------------------===//
 
 import LanguageServerProtocol
+import Basic
 
-/// Provider of build settings.
+/// Provider of FileBuildSettings and other build-related information.
+///
+/// The primary role of the build system is to answer queries for FileBuildSettings and (TODO) to
+/// notify clients when they change. The BuildSystem is also the source of related informatino,
+/// such as where the index datastore is located.
+///
+/// For example, a SwiftPMWorkspace provides compiler arguments for the files contained in a
+/// SwiftPM package root directory.
 public protocol BuildSystem {
 
+  /// The path to the raw index store data, if any.
+  var indexStorePath: AbsolutePath? { get }
+
+  /// The path to put the index database, if any.
+  var indexDatabasePath: AbsolutePath? { get }
+
   /// Returns the settings for the given url and language mode, if known.
   func settings(for: URL, _ language: Language) -> FileBuildSettings?
 

Sources/SKCore/CompilationDatabaseBuildSystem.swift

@@ -17,7 +17,7 @@ import LanguageServerProtocol
 ///
 /// Provides build settings from a `CompilationDatabase` found by searching a project. For now, only
 /// one compilation database, located at the project root.
-public final class CompilationDatabaseBuildSystem: BuildSystem {
+public final class CompilationDatabaseBuildSystem {
 
   /// The compilation database.
   var compdb: CompilationDatabase? = nil
@@ -30,6 +30,13 @@ public final class CompilationDatabaseBuildSystem: BuildSystem {
       self.compdb = tryLoadCompilationDatabase(directory: path, fileSystem: fileSystem)
     }
   }
+}
+
+extension CompilationDatabaseBuildSystem: BuildSystem {
+
+  // FIXME: derive from the compiler arguments.
+  public var indexStorePath: AbsolutePath? { return nil }
+  public var indexDatabasePath: AbsolutePath? { return nil }
 
   public func settings(for url: URL, _ language: Language) -> FileBuildSettings? {
     guard let db = database(for: url),

Sources/SKCore/ExternalWorkspace.swift

@@ -26,6 +26,10 @@ public final class FallbackBuildSettingsProvider: BuildSystem {
     return nil
   }()
 
+  public var indexStorePath: AbsolutePath? { return nil }
+
+  public var indexDatabasePath: AbsolutePath? { return nil }
+
   public func settings(for url: URL, _ language: Language) -> FileBuildSettings? {
     guard let path = try? AbsolutePath(validating: url.path) else {
       return nil

Sources/SKCore/FallbackBuildSettingsProvider.swift

@@ -162,9 +162,7 @@ public final class SwiftPMWorkspace {
   }
 }
 
-extension SwiftPMWorkspace: ExternalWorkspace, BuildSystem {
-
-  public var buildSystem: BuildSystem { return self }
+extension SwiftPMWorkspace: BuildSystem {
 
   public var buildPath: AbsolutePath {
     return buildParameters.buildPath

Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift

@@ -228,7 +228,6 @@ extension SourceKitServer {
       self.workspace = Workspace(
         rootPath: nil,
         clientCapabilities: req.params.capabilities,
-        external: nil,
         buildSettings: BuildSettingsProviderList(),
         index: nil
       )

Sources/SourceKit/SourceKitServer.swift

@@ -23,23 +23,15 @@ import SKSwiftPMWorkspace
 /// In LSP, this represents the per-workspace state that is typically only available after the
 /// "initialize" request has been made.
 ///
-/// Typically a workspace is contained in a root directory, and may be represented by an
-/// `ExternalWorkspace` if this workspace is part of a workspace in another tool such as a swiftpm
-/// package.
+/// Typically a workspace is contained in a root directory.
 public final class Workspace {
 
   /// The root directory of the workspace.
   public let rootPath: AbsolutePath?
 
   public let clientCapabilities: ClientCapabilities
 
-  /// The external workspace connection, if any.
-  public let external: ExternalWorkspace?
-
   /// The build settings provider to use for documents in this workspace.
-  ///
-  /// If `external` is not `nil`, this will typically include `external.buildSystem`. It may also
-  /// provide settings for files outside the workspace using additional providers.
   public let buildSettings: BuildSystem
 
   /// The source code index, if available.
@@ -54,13 +46,11 @@ public final class Workspace {
   public init(
     rootPath: AbsolutePath?,
     clientCapabilities: ClientCapabilities,
-    external: ExternalWorkspace?,
     buildSettings: BuildSystem,
     index: IndexStoreDB?)
   {
     self.rootPath = rootPath
     self.clientCapabilities = clientCapabilities
-    self.external = external
     self.buildSettings = buildSettings
     self.index = index
   }
@@ -79,21 +69,17 @@ public final class Workspace {
 
     self.rootPath = try AbsolutePath(validating: url.path)
     self.clientCapabilities = clientCapabilities
-    self.external = SwiftPMWorkspace(url: url, toolchainRegistry: toolchainRegistry)
-
     let settings = BuildSettingsProviderList()
     self.buildSettings = settings
 
     settings.providers.insert(CompilationDatabaseBuildSystem(projectRoot: rootPath), at: 0)
 
-    guard let external = self.external else {
-      return
+    if let swiftpm = SwiftPMWorkspace(url: url, toolchainRegistry: toolchainRegistry) {
+      settings.providers.insert(swiftpm, at: 0)
     }
 
-    settings.providers.insert(external.buildSystem, at: 0)
-
-    if let storePath = external.indexStorePath,
-       let dbPath = external.indexDatabasePath,
+    if let storePath = buildSettings.indexStorePath,
+       let dbPath = buildSettings.indexDatabasePath,
        let libPath = toolchainRegistry.default?.libIndexStore
     {
       do {