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/FixedCompilationDatabaseBuildSystem.swift

@@ -0,0 +1,154 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+func lastIndexStorePathArgument(in compilerArgs: [String]) -> String? {
+  for i in compilerArgs.indices.reversed() {
+    if compilerArgs[i] == "-index-store-path" && i + 1 < compilerArgs.count {
+      return compilerArgs[i + 1]
+    }
+  }
+  return nil
+}
+
+/// A `BuildSystem` that provides compiler arguments from a `compile_flags.txt` file.
+package actor FixedCompilationDatabaseBuildSystem: BuiltInBuildSystem {
+  package static let dbName = "compile_flags.txt"
+
+  private let connectionToSourceKitLSP: any Connection
+
+  package let configPath: URL
+
+  /// The compiler arguments from the fixed compilation database.
+  ///
+  /// Note that this does not contain the path to a compiler.
+  var compilerArgs: [String]
+
+  // Watch for all all changes to `compile_flags.txt` 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_flags.txt` in `mybuild`. In order to pick it  up, they create a
+  // symlink from `<project root>/compile_flags.txt` to `mybuild/compile_flags.txt`.  We want to get notified
+  // about the change to `mybuild/compile_flags.txt` because it effectively changes the contents of
+  // `<project root>/compile_flags.txt`.
+  package let fileWatchers: [FileSystemWatcher] = [
+    FileSystemWatcher(globPattern: "**/compile_flags.txt", kind: [.create, .change, .delete])
+  ]
+
+  package var indexStorePath: URL? {
+    guard let indexStorePath = lastIndexStorePathArgument(in: compilerArgs) else {
+      return nil
+    }
+    return URL(fileURLWithPath: indexStorePath, relativeTo: configPath.deletingLastPathComponent())
+  }
+
+  package var indexDatabasePath: URL? {
+    indexStorePath?.deletingLastPathComponent().appendingPathComponent("IndexDatabase")
+  }
+
+  package nonisolated var supportsPreparation: Bool { false }
+
+  private static func parseCompileFlags(at configPath: URL) throws -> [String] {
+    let fileContents: String = try String(contentsOf: configPath, encoding: .utf8)
+
+    var compilerArgs: [String] = []
+    fileContents.enumerateLines { line, _ in
+      compilerArgs.append(line.trimmingCharacters(in: .whitespacesAndNewlines))
+    }
+    return compilerArgs
+  }
+
+  package init(
+    configPath: URL,
+    connectionToSourceKitLSP: any Connection
+  ) throws {
+    self.connectionToSourceKitLSP = connectionToSourceKitLSP
+    self.configPath = configPath
+    self.compilerArgs = try Self.parseCompileFlags(at: 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: [SourceItem(uri: URI(configPath.deletingLastPathComponent()), kind: .directory, generated: false)]
+      )
+    ])
+  }
+
+  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? {
+    let compilerName: String
+    switch request.language {
+    case .swift: compilerName = "swiftc"
+    case .c, .cpp, .objective_c, .objective_cpp: compilerName = "clang"
+    default: return nil
+    }
+    return TextDocumentSourceKitOptionsResponse(
+      compilerArguments: [compilerName] + compilerArgs + [request.textDocument.uri.pseudoPath],
+      workingDirectory: try? configPath.deletingLastPathComponent().filePath
+    )
+  }
+
+  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 fixed compilation database") {
+      self.compilerArgs = try Self.parseCompileFlags(at: configPath)
+      connectionToSourceKitLSP.send(OnBuildTargetDidChangeNotification(changes: nil))
+    }
+  }
+}