Add a precondition to prevent the parsing of a command that has itself as its subcommand (#197)

* Add a precondition to prevent the parsing of a command that has itself as its subcommand.

This avoids the infinite recursion that causes a crash and shows the user a meaningful error message.

Fixes: #192

* Fix the detection of a command that has itself as its subcommand

- The recursion detection now works for both the root command and its subcommands
- Add a test to shows that the fix works

* Fix typo in TreeTests

Author: NicFontana

Sources/ArgumentParser/Parsing/CommandParser.swift

@@ -31,7 +31,13 @@ struct CommandParser {
   }
 
   init(_ rootCommand: ParsableCommand.Type) {
-    self.commandTree = Tree(root: rootCommand)
+    do {
+      self.commandTree = try Tree(root: rootCommand)
+    } catch Tree<ParsableCommand.Type>.InitializationError.recursiveSubcommand(let command) {
+      fatalError("The ParsableCommand \"\(command)\" can't have itself as its own subcommand.")
+    } catch {
+      fatalError("Unexpected error: \(error).")
+    }
     self.currentNode = commandTree
 
     // A command tree that has a depth greater than zero gets a `help`

Sources/ArgumentParser/Utilities/Tree.swift

@@ -88,10 +88,17 @@ extension Tree where Element == ParsableCommand.Type {
     children.first(where: { $0.element._commandName == name })
   }
 
-  convenience init(root command: ParsableCommand.Type) {
+  convenience init(root command: ParsableCommand.Type) throws {
     self.init(command)
     for subcommand in command.configuration.subcommands {
-      addChild(Tree(root: subcommand))
+      if subcommand == command {
+        throw InitializationError.recursiveSubcommand(subcommand)
+      }
+      try addChild(Tree(root: subcommand))
     }
   }
+    
+  enum InitializationError: Error {
+    case recursiveSubcommand(ParsableCommand.Type)
+  }
 }

Tests/ArgumentParserUnitTests/TreeTests.swift

@@ -53,3 +53,20 @@ extension TreeTests {
     XCTAssertTrue(tree.path(toFirstWhere: { $0 < 0 }).isEmpty)
   }
 }
+
+extension TreeTests {
+  struct A: ParsableCommand {
+    static let configuration = CommandConfiguration(subcommands: [A.self])
+  }
+  struct Root: ParsableCommand {
+    static let configuration = CommandConfiguration(subcommands: [Sub.self])
+  }
+  struct Sub: ParsableCommand {
+    static let configuration = CommandConfiguration(subcommands: [Sub.self])
+  }
+    
+  func testInitializationWithRecursiveSubcommand() {
+    XCTAssertThrowsError(try Tree(root: A.asCommand))
+    XCTAssertThrowsError(try Tree(root: Root.asCommand))
+  }
+}