Don't throw on empty responses (#28)

Motivation:

The AsyncDNSResolver currently throws an error when there are no records
associated with a name. However, most query responses are arrays or
contain arrays. As such it's unclear to callers whether they should be
catching an error of a particular type or checking for the presence of
values some combination of the two.

Modifications:

- Modify the `DNSResolver` API to return `nil` where only a single value
  is expected (e.g. CNAME).
- Update documentation on `DNSResolver` to clarify the expected return
  values when no values are resolved.
- Update the c-ares backend to check for no data when parsing and return
  empty responses
- Update the DNSSD backend to pass in `nil` data to the parser when the
  status is timeout
- Remove the `noData` error code as it shouldn't be reachable anymore

Result:

Queries with no response either return empty or nil

Author: glbrntt

Sources/AsyncDNSResolver/AsyncDNSResolver.swift

@@ -64,12 +64,12 @@ public struct AsyncDNSResolver {
     }
 
     /// See ``DNSResolver/queryCNAME(name:)``.
-    public func queryCNAME(name: String) async throws -> String {
+    public func queryCNAME(name: String) async throws -> String? {
         try await self.underlying.queryCNAME(name: name)
     }
 
     /// See ``DNSResolver/querySOA(name:)``.
-    public func querySOA(name: String) async throws -> SOARecord {
+    public func querySOA(name: String) async throws -> SOARecord? {
         try await self.underlying.querySOA(name: name)
     }
 
@@ -102,15 +102,15 @@ public protocol DNSResolver {
     /// - Parameters:
     ///   - name: The name to resolve.
     ///
-    /// - Returns: ``ARecord``s for the given name.
+    /// - Returns: ``ARecord``s for the given name, empty if no records were found.
     func queryA(name: String) async throws -> [ARecord]
 
     /// Lookup AAAA records associated with `name`.
     ///
     /// - Parameters:
     ///   - name: The name to resolve.
     ///
-    /// - Returns: ``AAAARecord``s for the given name.
+    /// - Returns: ``AAAARecord``s for the given name, empty if no records were found.
     func queryAAAA(name: String) async throws -> [AAAARecord]
 
     /// Lookup NS record associated with `name`.
@@ -126,16 +126,16 @@ public protocol DNSResolver {
     /// - Parameters:
     ///   - name: The name to resolve.
     ///
-    /// - Returns: CNAME for the given name.
-    func queryCNAME(name: String) async throws -> String
+    /// - Returns: CNAME for the given name, `nil` if no record was found.
+    func queryCNAME(name: String) async throws -> String?
 
     /// Lookup SOA record associated with `name`.
     ///
     /// - Parameters:
     ///   - name: The name to resolve.
     ///
-    /// - Returns: ``SOARecord`` for the given name.
-    func querySOA(name: String) async throws -> SOARecord
+    /// - Returns: ``SOARecord`` for the given name, `nil` if no record was found.
+    func querySOA(name: String) async throws -> SOARecord?
 
     /// Lookup PTR record associated with `name`.
     ///
@@ -150,23 +150,23 @@ public protocol DNSResolver {
     /// - Parameters:
     ///   - name: The name to resolve.
     ///
-    /// - Returns: ``MXRecord``s for the given name.
+    /// - Returns: ``MXRecord``s for the given name, empty if no records were found.
     func queryMX(name: String) async throws -> [MXRecord]
 
     /// Lookup TXT records associated with `name`.
     ///
     /// - Parameters:
     ///   - name: The name to resolve.
     ///
-    /// - Returns: ``TXTRecord``s for the given name.
+    /// - Returns: ``TXTRecord``s for the given name, empty if no records were found.
     func queryTXT(name: String) async throws -> [TXTRecord]
 
     /// Lookup SRV records associated with `name`.
     ///
     /// - Parameters:
     ///   - name: The name to resolve.
     ///
-    /// - Returns: ``SRVRecord``s for the given name.
+    /// - Returns: ``SRVRecord``s for the given name, empty if no records were found.
     func querySRV(name: String) async throws -> [SRVRecord]
 }
 

Sources/AsyncDNSResolver/Errors.swift

@@ -18,7 +18,6 @@ extension AsyncDNSResolver {
     public struct Error: Swift.Error, Hashable, CustomStringConvertible {
         public struct Code: Hashable, Sendable {
             fileprivate enum Value: Hashable, Sendable {
-                case noData
                 case invalidQuery
                 case serverFailure
                 case notFound
@@ -50,8 +49,6 @@ extension AsyncDNSResolver {
                 self.value = value
             }
 
-            public static var noData: Self { Self(.noData) }
-
             public static var invalidQuery: Self { Self(.invalidQuery) }
 
             public static var serverFailure: Self { Self(.serverFailure) }
@@ -113,8 +110,6 @@ extension AsyncDNSResolver {
 
         public var description: String {
             switch self.code.value {
-            case .noData:
-                return "no data: \(self.message)"
             case .invalidQuery:
                 return "invalid query: \(self.message)"
             case .serverFailure: