Guard against racy access to NSError.setUserInfoValueProvider. (#4280)

This API is documented in its headers to only allow being called once
for a particular domain, so we have to make sure our check for an
existing provider is synchronized with the setting.

rdar://problem/27541751

Author: jrose-apple

stdlib/public/SDK/Foundation/NSError.swift

@@ -136,6 +136,9 @@ public extension Error {
   }
 }
 
+internal let _errorDomainUserInfoProviderQueue = DispatchQueue(
+  label: "SwiftFoundation._errorDomainUserInfoProviderQueue")
+
 /// Retrieve the default userInfo dictionary for a given error.
 @_silgen_name("swift_Foundation_getErrorDefaultUserInfo")
 public func _swift_Foundation_getErrorDefaultUserInfo(_ error: Error)
@@ -149,7 +152,8 @@ public func _swift_Foundation_getErrorDefaultUserInfo(_ error: Error)
     // user-info value providers.
     let domain = error._domain
     if domain != NSCocoaErrorDomain {
-      if NSError.userInfoValueProvider(forDomain: domain) == nil {
+      _errorDomainUserInfoProviderQueue.sync {
+        if NSError.userInfoValueProvider(forDomain: domain) != nil { return }
         NSError.setUserInfoValueProvider(forDomain: domain) { (nsError, key) in
           let error = nsError as Error
 

validation-test/stdlib/ErrorProtocol.swift

@@ -1,6 +1,5 @@
 // RUN: %target-run-simple-swift
 // REQUIRES: executable_test
-// REQUIRES: rdar27541751
 // REQUIRES: objc_interop
 
 import SwiftPrivate