Add more resolution safety and debugging tools when a double-resolve …

…happens

Sources/Pwomise/Pwomise.swift

@@ -39,6 +39,10 @@ public class Promise<Output>: CustomDebugStringConvertible {
  case .success(let output):
  outputs.append(output)
  case .failure(let error):
+ guard superPromise.pending else {
+ return
+ }
+
  superPromise.result = .resolved(.failure(error))
  }
  }
@@ -49,6 +53,8 @@ public class Promise<Output>: CustomDebugStringConvertible {
 
  public var resolveQueue: _Scheduler = DispatchQueue.main
 
+ private var resolutionStackTrace: [String] = []
+
  /// The underlying status of the promise
  internal var result: Pending = .pending {
  didSet {
@@ -58,9 +64,10 @@ public class Promise<Output>: CustomDebugStringConvertible {
  willSet {
  guard result == .pending, newValue != .pending else {
  /// Result can only be set once – its a promise of a result, not a publisher
- print(Thread.callStackSymbols.joined(separator: "\n"))
+ print(resolutionStackTrace.joined(separator: "\n"))
  preconditionFailure("result is omnidirectional, from pending to resolved.")
  }
+ resolutionStackTrace = Thread.callStackSymbols
  }
  }
 
@@ -77,14 +84,26 @@ public class Promise<Output>: CustomDebugStringConvertible {
 
  public init(_ cb: (@escaping Resolve, @escaping Reject) -> ()) {
  cb({ output in
+ guard self.pending else {
+ preconditionFailure("cannot overwrite promise state")
+ }
+
  self.result = .resolved(.success(output))
  }, { error in
+ guard self.pending else {
+ preconditionFailure("cannot overwrite promise state")
+ }
+
  self.result = .resolved(.failure(error))
  })
  }
 
  public init(_ cb: (@escaping Resolve) -> ()) {
  cb({ output in
+ guard self.pending else {
+ preconditionFailure("cannot overwrite promise state")
+ }
+
  self.result = .resolved(.success(output))
  })
  }
@@ -136,7 +155,7 @@ public class Promise<Output>: CustomDebugStringConvertible {
  let newPromise = Promise()
  newPromise.resolveQueue = queue
 
- always { result in
+ listeners.append { result in
  newPromise.result = .resolved(result)
  }
 
@@ -147,6 +166,7 @@ public class Promise<Output>: CustomDebugStringConvertible {
  @discardableResult
  public func always<R>(_ cb: @escaping (Completion) throws -> R) -> Promise<R> {
  let promise = Promise<R>()
+
  listeners.append { result in
  do {
  promise.result = .resolved(.success(try cb(result)))
@@ -162,6 +182,7 @@ public class Promise<Output>: CustomDebugStringConvertible {
  @discardableResult
  public func always<R: PromiseConvertible>(_ cb: @escaping (Completion) throws -> R) -> Promise<R.Output> {
  let promise = Promise<R.Output>()
+
  listeners.append { result in
  do {
  try cb(result).asPromise.always { result in