[PackageLoading] Manifest caching using llbuild

<rdar://problem/35596212> Add support for an evaluated manifest cache

This adds support for caching the package manifest files which have
physical path on the disk. This basically means that manifest caching
will work in all operations except during dependency resolution.

Author: aciidgh

Sources/Basic/Result.swift

@@ -101,6 +101,18 @@ public struct AnyError: Swift.Error, CustomStringConvertible {
     }
 }
 
+/// Represents a string error.
+public struct StringError: Equatable, Codable, CustomStringConvertible, Error {
+
+    /// The description of the error.
+    public let description: String
+
+    /// Create an instance of StringError.
+    public init(_ description: String) {
+        self.description = description
+    }
+}
+
 // AnyError specific helpers.
 extension Result where ErrorType == AnyError {
     /// Initialise with something that throws AnyError.
@@ -133,3 +145,55 @@ extension Result where ErrorType == AnyError {
         }
     }
 }
+
+extension Result where ErrorType == StringError {
+    /// Create an instance of Result<Value, StringError>.
+    ///
+    /// Errors will be encoded as StringError using their description.
+    public init(string body: () throws -> Value) {
+        do {
+            self = .success(try body())
+        } catch let error as StringError {
+            self = .failure(error)
+        } catch {
+            self = .failure(StringError(String(describing: error)))
+        }
+    }
+}
+
+extension Result: Equatable where Value: Equatable, ErrorType: Equatable {}
+
+extension Result: Codable where Value: Codable, ErrorType: Codable {
+    private enum CodingKeys: String, CodingKey {
+        case success, failure
+    }
+
+    public func encode(to encoder: Encoder) throws {
+        var container = encoder.container(keyedBy: CodingKeys.self)
+        switch self {
+        case .success(let value):
+            var unkeyedContainer = container.nestedUnkeyedContainer(forKey: .success)
+            try unkeyedContainer.encode(value)
+        case .failure(let error):
+            var unkeyedContainer = container.nestedUnkeyedContainer(forKey: .failure)
+            try unkeyedContainer.encode(error)
+        }
+    }
+
+    public init(from decoder: Decoder) throws {
+        let values = try decoder.container(keyedBy: CodingKeys.self)
+        guard let key = values.allKeys.first(where: values.contains) else {
+            throw DecodingError.dataCorrupted(.init(codingPath: decoder.codingPath, debugDescription: "Did not find a matching key"))
+        }
+        switch key {
+        case .success:
+            var unkeyedValues = try values.nestedUnkeyedContainer(forKey: key)
+            let value = try unkeyedValues.decode(Value.self)
+            self = .success(value)
+        case .failure:
+            var unkeyedValues = try values.nestedUnkeyedContainer(forKey: key)
+            let error = try unkeyedValues.decode(ErrorType.self)
+            self = .failure(error)
+        }
+    }
+}

Sources/Commands/Options.swift

@@ -40,6 +40,9 @@ public class ToolOptions {
     /// Disables sandboxing when executing subprocesses.
     public var shouldDisableSandbox = false
 
+    /// Disables manifest caching.
+    public var shouldDisableManifestCaching = false
+
     /// Path to the compilation destination describing JSON file.
     public var customCompileDestination: AbsolutePath?
 

Sources/Commands/SwiftTool.swift

@@ -331,6 +331,11 @@ public class SwiftTool<Options: ToolOptions> {
             usage: "Disable using the sandbox when executing subprocesses"),
             to: { $0.shouldDisableSandbox = $1 })
 
+        binder.bind(
+            option: parser.add(option: "--disable-package-manifest-caching", kind: Bool.self,
+            usage: "Disable caching Package.swift manifests"),
+            to: { $0.shouldDisableManifestCaching = $1 })
+
         binder.bind(
             option: parser.add(option: "--version", kind: Bool.self),
             to: { $0.shouldPrintVersion = $1 })
@@ -723,7 +728,9 @@ public class SwiftTool<Options: ToolOptions> {
             try ManifestLoader(
                 // Always use the host toolchain's resources for parsing manifest.
                 resources: self._hostToolchain.dematerialize().manifestResources,
-                isManifestSandboxEnabled: !self.options.shouldDisableSandbox
+                isManifestSandboxEnabled: !self.options.shouldDisableSandbox,
+                isManifestCachingEnabled: !self.options.shouldDisableManifestCaching,
+                cacheDir: self.buildPath
             )
         })
     }()