JavaKit: check each classPath element exists before starting VM

Partial fix for #136: validate each classPath element by attempting to open it
before starting VM. It would be more efficient to stat() it but SystemPackage
lacks an abstraction for checking if a file exists, and we don't want to import
Foundation.

Author: lhoward

Package.swift

@@ -140,6 +140,7 @@ let package = Package(
     .package(url: "https://github.com/swiftlang/swift-syntax.git", branch: "main"),
     .package(url: "https://github.com/apple/swift-argument-parser", from: "1.5.0"),
     .package(url: "https://github.com/apple/swift-collections.git", .upToNextMinor(from: "1.1.0")),
+    .package(url: "https://github.com/apple/swift-system.git", .upToNextMajor(from: "1.4.0")),
     .package(url: "https://github.com/ordo-one/package-benchmark", .upToNextMajor(from: "1.4.0")),
   ],
   targets: [
@@ -161,7 +162,12 @@ let package = Package(
     ),
     .target(
       name: "JavaKit",
-      dependencies: ["JavaRuntime", "JavaKitMacros", "JavaTypes"],
+      dependencies: [
+        "JavaRuntime",
+        "JavaKitMacros",
+        "JavaTypes",
+        .product(name: "SystemPackage", package: "swift-system")
+      ],
       exclude: ["Java2Swift.config"],
       swiftSettings: [
         .swiftLanguageMode(.v5),

Sources/JavaKit/JavaKitVM/JavaVirtualMachine.swift

@@ -12,6 +12,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+import SystemPackage
+
 typealias JavaVMPointer = UnsafeMutablePointer<JavaVM?>
 
 public final class JavaVirtualMachine: @unchecked Sendable {
@@ -54,6 +56,14 @@ public final class JavaVirtualMachine: @unchecked Sendable {
     // Construct the complete list of VM options.
     var allVMOptions: [String] = []
     if !classPath.isEmpty {
+      for path in classPath {
+        do {
+          let fd = try FileDescriptor.open(path, .readOnly)
+          try? fd.close()
+        } catch {
+          throw(ClassPathError(path: path, error: error))
+        }
+      }
       let colonSeparatedClassPath = classPath.joined(separator: ":")
       allVMOptions.append("-Djava.class.path=\(colonSeparatedClassPath)")
     }
@@ -268,4 +278,9 @@ extension JavaVirtualMachine {
       }
     }
   }
+
+  struct ClassPathError: Error {
+    let path: String
+    let error: Error
+  }
 }