[PackageGraph] Add cancellation support in DependencyResolver

<rdar://problem/48341369> Add cancellation support in DependencyResolver Unfortunately, I can't think of a good way to unit test this.

Author: aciidgh

Sources/PackageGraph/DependencyResolver.swift

@@ -28,6 +28,9 @@ public enum DependencyResolverError: Error, Equatable, CustomStringConvertible {
  /// The resolver found missing versions for the given constraints.
  case missingVersions([PackageContainerConstraint])
 
+ /// The resolution was cancelled.
+ case cancelled
+
  public static func == (lhs: DependencyResolverError, rhs: DependencyResolverError) -> Bool {
  switch (lhs, rhs) {
  case (.unsatisfiable, .unsatisfiable):
@@ -47,11 +50,17 @@ public enum DependencyResolverError: Error, Equatable, CustomStringConvertible {
  return lhs == rhs
  case (.missingVersions, _):
  return false
+ case (.cancelled, .cancelled):
+ return true
+ case (.cancelled, _):
+ return false
  }
  }
 
  public var description: String {
  switch self {
+ case .cancelled:
+ return "the dependency resolution was cancelled"
  case .unsatisfiable:
  return "unable to resolve dependencies"
  case .cycle(let package):
@@ -821,10 +830,20 @@ public class DependencyResolver {
  /// Skip updating containers while fetching them.
  private let skipUpdate: Bool
 
+ /// Lock used to get and set the error variable.
+ private let errorLock: Lock = Lock()
+
  // FIXME: @testable private
  //
  /// Contains any error encountered during dependency resolution.
- var error: Swift.Error?
+ var error: Swift.Error? {
+ get {
+ return errorLock.withLock { self.__error }
+ } set {
+ errorLock.withLock { self.__error = newValue }
+ }
+ }
+ var __error: Swift.Error?
 
  /// Key used to cache a resolved subtree.
  private struct ResolveSubtreeCacheKey: Hashable {
@@ -884,6 +903,13 @@ public class DependencyResolver {
  case error(Swift.Error)
  }
 
+ /// Cancel the dependency resolution operation.
+ ///
+ /// This method is thread-safe.
+ public func cancel() {
+ self.error = DependencyResolverError.cancelled
+ }
+
  /// Execute the resolution algorithm to find a valid assignment of versions.
  ///
  /// If a valid assignment is not found, the resolver will go into incomplete

Sources/Workspace/Workspace.swift

@@ -300,6 +300,9 @@ public class Workspace {
  /// Skip updating containers while fetching them.
  fileprivate let skipUpdate: Bool
 
+ /// The active package resolver. This is set during a dependency resolution operation.
+ fileprivate var activeResolver: PackageResolver?
+
  /// Typealias for dependency resolver we use in the workspace.
  fileprivate typealias PackageDependencyResolver = DependencyResolver
  fileprivate typealias PubgrubResolver = PubgrubDependencyResolver
@@ -539,6 +542,17 @@ extension Workspace {
  try? fileSystem.removeFileTree(dataPath)
  }
 
+ /// Cancel the active dependency resolution operation.
+ ///
+ /// Only legacy resolver is supported right now. This method is thread-safe.
+ public func cancelActiveResolverOperation() {
+ switch activeResolver {
+ case .legacy(let resolver)?:
+ resolver.cancel()
+ default: break
+ }
+ }
+
  /// Updates the current dependencies.
  ///
  /// - Parameters:
@@ -579,7 +593,14 @@ extension Workspace {
  let resolutionStartTime = Date()
 
  // Resolve the dependencies.
- let updateResults = resolveDependencies(dependencies: updateConstraints, diagnostics: diagnostics)
+ let resolver = createResolver()
+ activeResolver = resolver
+
+ let updateResults = resolveDependencies(resolver: resolver, dependencies: updateConstraints, diagnostics: diagnostics)
+
+ // Reset the active resolver.
+ activeResolver = nil
+
  guard !diagnostics.hasErrors else { return }
 
  // Emit the time taken to perform dependency resolution.
@@ -1217,8 +1238,11 @@ extension Workspace {
  // Perform dependency resolution.
  let resolverDiagnostics = DiagnosticsEngine()
  var resolver = createResolver()
+ activeResolver = resolver
+
  var result = resolveDependencies(
  resolver: resolver, dependencies: constraints, pins: validPins, diagnostics: resolverDiagnostics)
+ activeResolver = nil
 
  // If we fail, we just try again without any pins because the pins might
  // be completely incompatible.
@@ -1586,13 +1610,11 @@ extension Workspace {
 
  /// Runs the dependency resolver based on constraints provided and returns the results.
  fileprivate func resolveDependencies(
- resolver: PackageResolver? = nil,
+ resolver: PackageResolver,
  dependencies: [RepositoryPackageConstraint],
  pins: [RepositoryPackageConstraint] = [],
  diagnostics: DiagnosticsEngine
  ) -> [(container: PackageReference, binding: BoundVersion)] {
- let resolver = resolver ?? createResolver()
-
  let result = resolver.resolve(dependencies: dependencies, pins: pins)
 
  // Take an action based on the result.