add closable protocol (#160)

motivation: marker protocol for entitites that manage resources and need to be closed to to cleanly free such resources

changes:
* define Closable protocol
* conform WritableByteStrea to Closable
* conform PersistenceCache to Closable

Author: tomerd

Sources/TSCBasic/CMakeLists.txt

@@ -11,6 +11,7 @@ add_library(TSCBasic
   ByteString.swift
   CStringArray.swift
   CacheableSequence.swift
+  Closable.swift
   CodableResult.swift
   CollectionAlgorithms.swift
   CollectionExtensions.swift

Sources/TSCBasic/Closable.swift

@@ -0,0 +1,15 @@
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright (c) 2020 Apple Inc. and the Swift project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See http://swift.org/LICENSE.txt for license information
+ See http://swift.org/CONTRIBUTORS.txt for Swift project authors
+*/
+
+/// Closable entity is one that manages underlying resources and needs to be closed for cleanup
+/// The intent of this method is for the sole owner of the refernece/handle of the resource to close it completely, comapred to releasing a shared resource.
+public protocol Closable {
+    func close() throws
+}

Sources/TSCBasic/WritableByteStream.swift

@@ -43,7 +43,7 @@ public protocol ByteStreamable {
 ///
 /// would write each item in the list to the stream, separating them with a
 /// space.
-public protocol WritableByteStream: class, TextOutputStream {
+public protocol WritableByteStream: class, TextOutputStream, Closable {
     /// The current offset within the output stream.
     var position: Int { get }
 
@@ -55,9 +55,6 @@ public protocol WritableByteStream: class, TextOutputStream {
 
     /// Flush the stream's buffer.
     func flush()
-
-    /// Signal that the byte stream will not be used further and should be closed.
-    func close() throws
 }
 
 // Default noop implementation of close to avoid source-breaking downstream dependents with the addition of the close

Sources/TSCUtility/PersistenceCache.swift

@@ -19,7 +19,7 @@ public protocol PersistentCacheProtocol {
 }
 
 /// SQLite backed persistent cache.
-public final class SQLiteBackedPersistentCache: PersistentCacheProtocol {
+public final class SQLiteBackedPersistentCache: PersistentCacheProtocol, Closable {
     let db: SQLite
 
     init(db: SQLite) throws {
@@ -37,7 +37,11 @@ public final class SQLiteBackedPersistentCache: PersistentCacheProtocol {
     }
 
     deinit {
-        try? db.close()
+        try? self.close()
+    }
+    
+    public func close() throws {
+        try db.close()
     }
 
     public convenience init(cacheFilePath: AbsolutePath) throws {