Add cache for fullDocumentDiagnosticReport

Author: joehsieh

Sources/SourceKitLSP/Swift/SwiftLanguageServer.swift

@@ -139,6 +139,22 @@ public actor SwiftLanguageServer: ToolchainLanguageServer {
 
   private var stateChangeHandlers: [(_ oldState: LanguageServerState, _ newState: LanguageServerState) -> Void] = []
 
+  /// The cache that stores reports for snapshot ids
+  ///
+  /// Conceptually, this is a dictionary. To prevent excessive memory usage we
+  /// only keep `cacheSize` entries within the array. Older entries are at the
+  /// end of the list, newer entries at the front.
+  private var reportCache:
+    [(
+      snapshotID: DocumentSnapshot.ID,
+      report: RelatedFullDocumentDiagnosticReport
+    )] = []
+
+  /// The number of reports to keep
+  ///
+  /// - Note: This has been chosen without scientific measurements.
+  private let cacheSize = 5
+
   /// Creates a language server for the given client using the sourcekitd dylib specified in `toolchain`.
   /// `reopenDocuments` is a closure that will be called if sourcekitd crashes and the `SwiftLanguageServer` asks its parent server to reopen all of its documents.
   /// Returns `nil` if `sourcektid` couldn't be found.
@@ -970,7 +986,8 @@ extension SwiftLanguageServer {
     let diagnosticReport = try await self.fullDocumentDiagnosticReport(
       DocumentDiagnosticsRequest(
         textDocument: params.textDocument
-      )
+      ),
+      useCache: true
     )
 
     let codeActions = diagnosticReport.items.flatMap { (diag) -> [CodeAction] in
@@ -1065,9 +1082,13 @@ extension SwiftLanguageServer {
   }
 
   private func fullDocumentDiagnosticReport(
-    _ req: DocumentDiagnosticsRequest
+    _ req: DocumentDiagnosticsRequest,
+    useCache: Bool = false
   ) async throws -> RelatedFullDocumentDiagnosticReport {
     let snapshot = try documentManager.latestSnapshot(req.textDocument.uri)
+    if useCache, let report = report(for: snapshot.id) {
+      return report
+    }
     guard let buildSettings = await self.buildSettings(for: req.textDocument.uri), !buildSettings.isFallback else {
       logger.log(
         "Producing syntactic diagnostics from the built-in swift-syntax because we have fallback arguments"
@@ -1076,7 +1097,9 @@ extension SwiftLanguageServer {
       // sourcekitd won't be able to give us accurate semantic diagnostics.
       // Fall back to providing syntactic diagnostics from the built-in
       // swift-syntax. That's the best we can do for now.
-      return try await syntacticDiagnosticFromBuiltInSwiftSyntax(for: snapshot)
+      let report = try await syntacticDiagnosticFromBuiltInSwiftSyntax(for: snapshot)
+      setReport(for: snapshot.id, report: report)
+      return report
     }
 
     try Task.checkCancellation()
@@ -1109,7 +1132,31 @@ extension SwiftLanguageServer {
       return true
     }
 
-    return RelatedFullDocumentDiagnosticReport(items: diagnostics)
+    let report = RelatedFullDocumentDiagnosticReport(items: diagnostics)
+    setReport(for: snapshot.id, report: report)
+    return report
+  }
+
+  /// The report for the given document snapshot.
+  private func report(for snapshotID: DocumentSnapshot.ID) -> RelatedFullDocumentDiagnosticReport? {
+    return reportCache.first(where: { $0.snapshotID == snapshotID })?.report
+  }
+
+  /// Set the report for the given document snapshot.
+  ///
+  /// If we are already storing `cacheSize` many reports, the oldest one
+  /// will get discarded.
+  private func setReport(for snapshotID: DocumentSnapshot.ID, report: RelatedFullDocumentDiagnosticReport) {
+    reportCache.insert((snapshotID, report), at: 0)
+
+    // Remove any reports for old versions of this document.
+    reportCache.removeAll(where: { $0.snapshotID < snapshotID })
+
+    // If we still have more than `cacheSize` reports, delete the ones that
+    // were produced last. We can always re-request them on-demand.
+    while reportCache.count > cacheSize {
+      reportCache.removeLast()
+    }
   }
 
   public func executeCommand(_ req: ExecuteCommandRequest) async throws -> LSPAny? {