Containerization: Reduce allocations for image subsystems (#152)

Continue the allocations journey for anything that is in the codepaths
for pulling images. This time there's a couple spots in archive and ext4
we can get rid of some copies.

Sources/ContainerizationArchive/Reader.swift

@@ -101,9 +101,9 @@ extension ArchiveReader: Sequence {
 
     internal func readDataForEntry(_ entry: WriteEntry) -> Data {
         let bufferSize = Int(Swift.min(entry.size ?? 4096, 4096))
-        var data = Data()
+        var entry = Data()
+        var part = Data(count: bufferSize)
         while true {
-            var part = Data(count: bufferSize)
             let c = part.withUnsafeMutableBytes { buffer in
                 guard let baseAddress = buffer.baseAddress else {
                     return 0
@@ -112,9 +112,9 @@ extension ArchiveReader: Sequence {
             }
             guard c > 0 else { break }
             part.count = c
-            data.append(part)
+            entry.append(part)
         }
-        return data
+        return entry
     }
 }
 

Sources/ContainerizationEXT4/EXT4Reader+Export.swift

@@ -130,14 +130,14 @@ extension EXT4.EXT4Reader {
                 entry.fileType = .symbolicLink
                 if size < 60 {
                     let linkBytes = EXT4.tupleToArray(inode.block)
-                    entry.symlinkTarget = String(data: Data(linkBytes), encoding: .utf8) ?? ""
+                    entry.symlinkTarget = String(bytes: linkBytes, encoding: .utf8) ?? ""
                 } else {
                     if let block = item.blocks {
                         try self.seek(block: block.start)
                         guard let linkBytes = try self.handle.read(upToCount: Int(size)) else {
                             throw EXT4.Error.couldNotReadBlock(block.start)
                         }
-                        entry.symlinkTarget = String(data: Data(linkBytes), encoding: .utf8) ?? ""
+                        entry.symlinkTarget = String(bytes: linkBytes, encoding: .utf8) ?? ""
                     }
                 }
                 try writer.writeEntry(entry: entry, data: nil)

Sources/ContainerizationOCI/Client/RegistryClient.swift

@@ -260,12 +260,27 @@ public final class RegistryClient: ContentClient {
         }
     }
 
+    internal func requestBuffer(
+        components: URLComponents,
+        headers: [(String, String)]? = nil
+    ) async throws -> ByteBuffer {
+        try await request(components: components, method: .GET, headers: headers) { response in
+            guard response.status == .ok else {
+                let url = components.url?.absoluteString ?? "unknown"
+                let reason = await ErrorResponse.fromResponseBody(response.body)?.jsonString
+                throw Error.invalidStatus(url: url, response.status, reason: reason)
+            }
+
+            return try await response.body.collect(upTo: self.bufferSize)
+        }
+    }
+
     internal func requestJSON<T: Decodable>(
         components: URLComponents,
         headers: [(String, String)]? = nil
     ) async throws -> T {
-        let data = try await self.requestData(components: components, headers: headers)
-        return try JSONDecoder().decode(T.self, from: data)
+        let buffer = try await self.requestBuffer(components: components, headers: headers)
+        return try JSONDecoder().decode(T.self, from: buffer)
     }
 
     /// A minimal endpoint, mounted at /v2/ will provide version support information based on its response statuses.