Split build systems for JSON compilation database and fixed compilation database

I feel like the implementations are actually simpler if we split them. This will also allow us to add more advanced logic to the JSON compilation database build system in the future, such as inferring the toolchain from the compile command.

Sources/BuildSystemIntegration/JSONCompilationDatabaseBuildSystem.swift

@@ -0,0 +1,140 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import SKLogging
+import SwiftExtensions
+
+#if compiler(>=6)
+package import BuildServerProtocol
+package import Foundation
+package import LanguageServerProtocol
+#else
+import BuildServerProtocol
+import Foundation
+import LanguageServerProtocol
+#endif
+
+/// A `BuildSystem` that provides compiler arguments from a `compile_commands.json` file.
+package actor JSONCompilationDatabaseBuildSystem: BuiltInBuildSystem {
+  package static let dbName: String = "compile_commands.json"
+
+  /// The compilation database.
+  var compdb: JSONCompilationDatabase {
+    didSet {
+      // Build settings have changed and thus the index store path might have changed.
+      // Recompute it on demand.
+      _indexStorePath.reset()
+    }
+  }
+
+  private let connectionToSourceKitLSP: any Connection
+
+  package let configPath: URL
+
+  // Watch for all all changes to `compile_commands.json` and `compile_flags.txt` instead of just the one at
+  // `configPath` so that we cover the following semi-common scenario:
+  // The user has a build that stores `compile_commands.json` in `mybuild`. In order to pick it  up, they create a
+  // symlink from `<project root>/compile_commands.json` to `mybuild/compile_commands.json`.  We want to get notified
+  // about the change to `mybuild/compile_commands.json` because it effectively changes the contents of
+  // `<project root>/compile_commands.json`.
+  package let fileWatchers: [FileSystemWatcher] = [
+    FileSystemWatcher(globPattern: "**/compile_commands.json", kind: [.create, .change, .delete])
+  ]
+
+  private var _indexStorePath: LazyValue<URL?> = .uninitialized
+  package var indexStorePath: URL? {
+    _indexStorePath.cachedValueOrCompute {
+      for command in compdb.commands {
+        if let indexStorePath = lastIndexStorePathArgument(in: command.commandLine) {
+          return URL(
+            fileURLWithPath: indexStorePath,
+            relativeTo: URL(fileURLWithPath: command.directory, isDirectory: true)
+          )
+        }
+      }
+      return nil
+    }
+  }
+
+  package var indexDatabasePath: URL? {
+    indexStorePath?.deletingLastPathComponent().appendingPathComponent("IndexDatabase")
+  }
+
+  package nonisolated var supportsPreparation: Bool { false }
+
+  package init(
+    configPath: URL,
+    connectionToSourceKitLSP: any Connection
+  ) throws {
+    self.compdb = try JSONCompilationDatabase(file: configPath)
+    self.connectionToSourceKitLSP = connectionToSourceKitLSP
+
+    self.configPath = configPath
+  }
+
+  package func buildTargets(request: WorkspaceBuildTargetsRequest) async throws -> WorkspaceBuildTargetsResponse {
+    return WorkspaceBuildTargetsResponse(targets: [
+      BuildTarget(
+        id: .dummy,
+        displayName: nil,
+        baseDirectory: nil,
+        tags: [.test],
+        capabilities: BuildTargetCapabilities(),
+        // Be conservative with the languages that might be used in the target. SourceKit-LSP doesn't use this property.
+        languageIds: [.c, .cpp, .objective_c, .objective_cpp, .swift],
+        dependencies: []
+      )
+    ])
+  }
+
+  package func buildTargetSources(request: BuildTargetSourcesRequest) async throws -> BuildTargetSourcesResponse {
+    guard request.targets.contains(.dummy) else {
+      return BuildTargetSourcesResponse(items: [])
+    }
+    return BuildTargetSourcesResponse(items: [SourcesItem(target: .dummy, sources: compdb.sourceItems)])
+  }
+
+  package func didChangeWatchedFiles(notification: OnWatchedFilesDidChangeNotification) {
+    if notification.changes.contains(where: { $0.uri.fileURL?.lastPathComponent == Self.dbName }) {
+      self.reloadCompilationDatabase()
+    }
+  }
+
+  package func prepare(request: BuildTargetPrepareRequest) async throws -> VoidResponse {
+    throw PrepareNotSupportedError()
+  }
+
+  package func sourceKitOptions(
+    request: TextDocumentSourceKitOptionsRequest
+  ) async throws -> TextDocumentSourceKitOptionsResponse? {
+    guard let cmd = compdb[request.textDocument.uri].first else {
+      return nil
+    }
+    return TextDocumentSourceKitOptionsResponse(
+      compilerArguments: Array(cmd.commandLine.dropFirst()),
+      workingDirectory: cmd.directory
+    )
+  }
+
+  package func waitForBuildSystemUpdates(request: WorkspaceWaitForBuildSystemUpdatesRequest) async -> VoidResponse {
+    return VoidResponse()
+  }
+
+  /// The compilation database has been changed on disk.
+  /// Reload it and notify the delegate about build setting changes.
+  private func reloadCompilationDatabase() {
+    orLog("Reloading compilation database") {
+      self.compdb = try JSONCompilationDatabase(file: configPath)
+      connectionToSourceKitLSP.send(OnBuildTargetDidChangeNotification(changes: nil))
+    }
+  }
+}