DefaultCommand signal behavior improvements for plugins

Correct signal semantics for plugins: We exec into plugin binaries. If
the parent CLI keeps SIGINT/SIGTERM handlers installed, it can
intercept/alter signal behavior intended for the plugin (e.g.,
preventing graceful shutdown in foreground workflows).

Author: mazdak

Sources/CLI/DefaultCommand.swift

@@ -18,6 +18,7 @@ import ArgumentParser
 import Foundation
 import ContainerClient
 import ContainerPlugin
+import Darwin
 
 struct DefaultCommand: AsyncParsableCommand {
     static let configuration = CommandConfiguration(
@@ -84,7 +85,17 @@ struct DefaultCommand: AsyncParsableCommand {
                 """
             )
         }
+        // Before execing into the plugin, restore default SIGINT/SIGTERM so the plugin can manage signals.
+        Self.resetSignalsForPluginExec()
         // Exec performs execvp (with no fork).
         try plugin.exec(args: remaining)
     }
 }
+
+extension DefaultCommand {
+    // Exposed for tests to verify signal reset semantics.
+    static func resetSignalsForPluginExec() {
+        signal(SIGINT, SIG_DFL)
+        signal(SIGTERM, SIG_DFL)
+    }
+}