PackageModel: relative paths for cross-compilation destinations

This introduces support for relative paths in v2 scheme of `destination.json` files. The path is then normalized relative to the location of `destination.json` files. As a result, it makes it easier for moving cross-compilation sysroots between different directories on the same filesystem, or even between different machines.

Author: MaxDesiatov

Sources/PackageModel/Destination.swift

@@ -12,6 +12,7 @@
 
 import Basics
 import Foundation
+import SystemPackage
 import TSCBasic
 
 import struct TSCUtility.Triple
@@ -212,27 +213,51 @@ extension Destination {
     public init(fromFile path: AbsolutePath, fileSystem: FileSystem) throws {
         let decoder = JSONDecoder.makeWithDefaults()
         let version = try decoder.decode(path: path, fileSystem: fileSystem, as: VersionInfo.self)
+        
         // Check schema version.
-        guard version.version == 1 else {
+        if version.version == 1 {
+            let destination = try decoder.decode(path: path, fileSystem: fileSystem, as: DestinationInfoV1.self)
+            try self.init(
+                target: destination.target.map{ try Triple($0) },
+                sdk: destination.sdk,
+                binDir: destination.binDir,
+                extraCCFlags: destination.extraCCFlags,
+                extraSwiftCFlags: destination.extraSwiftCFlags,
+                extraCPPFlags: destination.extraCPPFlags
+            )
+        } else if version.version == 2 {
+            let destination = try decoder.decode(path: path, fileSystem: fileSystem, as: DestinationInfoV2.self)
+            let destinationDirectory = path.parentDirectory
+            
+            try self.init(
+                target: destination.target.map{ try Triple($0) },
+                sdk: destination.sdk?.absolutePath(relativeTo: destinationDirectory),
+                binDir: destination.binDir.absolutePath(relativeTo: destinationDirectory),
+                extraCCFlags: destination.extraCCFlags,
+                extraSwiftCFlags: destination.extraSwiftCFlags,
+                extraCPPFlags: destination.extraCPPFlags
+            )
+        } else {
             throw DestinationError.invalidSchemaVersion
         }
-        let destination = try decoder.decode(path: path, fileSystem: fileSystem, as: DestinationInfo.self)
-        try self.init(
-            target: destination.target.map{ try Triple($0) },
-            sdk: destination.sdk,
-            binDir: destination.binDir,
-            extraCCFlags: destination.extraCCFlags,
-            extraSwiftCFlags: destination.extraSwiftCFlags,
-            extraCPPFlags: destination.extraCPPFlags
-        )
+    }
+}
+
+fileprivate extension FilePath {
+    func absolutePath(relativeTo directory: AbsolutePath) -> AbsolutePath {
+        if isRelative {
+            return directory.appending(RelativePath(string))
+        } else {
+            return AbsolutePath(string)
+        }
     }
 }
 
 fileprivate struct VersionInfo: Codable {
     let version: Int
 }
 
-fileprivate struct DestinationInfo: Codable {
+fileprivate struct DestinationInfoV1: Codable {
     let target: String?
     let sdk: AbsolutePath?
     let binDir: AbsolutePath
@@ -249,3 +274,21 @@ fileprivate struct DestinationInfo: Codable {
         case extraCPPFlags = "extra-cpp-flags"
     }
 }
+
+fileprivate struct DestinationInfoV2: Codable {
+    let target: String?
+    let sdk: FilePath?
+    let binDir: FilePath
+    let extraCCFlags: [String]
+    let extraSwiftCFlags: [String]
+    let extraCPPFlags: [String]
+
+    enum CodingKeys: String, CodingKey {
+        case target
+        case sdk
+        case binDir = "toolchain-bin-dir"
+        case extraCCFlags = "extra-cc-flags"
+        case extraSwiftCFlags = "extra-swiftc-flags"
+        case extraCPPFlags = "extra-cpp-flags"
+    }
+}