Implement pdeathSignal handling in Runc (#431)

Add support for parent death signal (pdeathSignal) to ensure child
processes receive a signal when the parent process dies. This addresses
the FIXME comment in Runc.execute().

## Changes
- Add pdeathSignal field to exec_command_attrs C struct
- Implement prctl(PR_SET_PDEATHSIG) in child process handler (Linux
only)
- Expose pdeathSignal through Command.Attrs Swift API
- Wire up pdeathSignal in Runc.execute() to remove FIXME

## Implementation Details
The implementation uses Linux-specific prctl() to set the parent death
signal, ensuring proper cleanup when parent processes terminate. The
feature is conditionally compiled for Linux only, maintaining
compatibility with other platforms.

Sources/CShim/exec_command.c

@@ -32,6 +32,9 @@
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>
+#if defined(__linux__)
+#include <sys/prctl.h>
+#endif
 
 #include "exec_command.h"
 
@@ -98,6 +101,7 @@ void exec_command_attrs_init(struct exec_command_attrs *attrs) {
   attrs->mask = 0;
   attrs->uid = -1;
   attrs->gid = -1;
+  attrs->pdeathSignal = 0;
 }
 
 static void child_handler(const int sync_pipes[2], const char *executable,
@@ -253,6 +257,15 @@ static void child_handler(const int sync_pipes[2], const char *executable,
     }
   }
 
+#if defined(__linux__)
+  // Set parent death signal if specified
+  if (attrs.pdeathSignal != 0) {
+    if (prctl(PR_SET_PDEATHSIG, attrs.pdeathSignal) != 0) {
+      goto fail;
+    }
+  }
+#endif
+
   // close exec everything outside of our child's fd_table.
   if (cloexec_from(file_handle_count) != 0) {
     goto fail;

Sources/CShim/include/exec_command.h

@@ -38,6 +38,8 @@ struct exec_command_attrs {
   gid_t gid;
   /// signal mask for the child process
   int mask;
+  /// parent death signal (Linux only, 0 to disable)
+  int pdeathSignal;
 };
 
 void exec_command_attrs_init(struct exec_command_attrs *attrs);

Sources/ContainerizationOS/Command.swift

@@ -68,6 +68,8 @@ public struct Command: Sendable {
         public var uid: UInt32?
         /// Set the process group ID.
         public var gid: UInt32?
+        /// Signal to send when parent process dies (Linux only).
+        public var pdeathSignal: Int32?
 
         public init(
             setPGroup: Bool = false,
@@ -77,7 +79,8 @@ public struct Command: Sendable {
             setsid: Bool = false,
             setctty: Bool = false,
             uid: UInt32? = nil,
-            gid: UInt32? = nil
+            gid: UInt32? = nil,
+            pdeathSignal: Int32? = nil
         ) {
             self.setPGroup = setPGroup
             self.resetIDs = resetIDs
@@ -87,6 +90,7 @@ public struct Command: Sendable {
             self.setctty = setctty
             self.uid = uid
             self.gid = gid
+            self.pdeathSignal = pdeathSignal
         }
     }
 
@@ -207,6 +211,10 @@ extension Command {
             attrs.gid = gid
         }
 
+        if let pdeathSignal = self.attrs.pdeathSignal {
+            attrs.pdeathSignal = pdeathSignal
+        }
+
         var pid: pid_t = 0
         var argv = ([executable] + arguments).map { strdup($0) } + [nil]
         defer {

vminitd/Sources/vminitd/Runc/Runc.swift

@@ -339,7 +339,9 @@ extension Runc {
         cmd.stdout = stdout ?? outPipe.fileHandleForWriting
         cmd.stderr = stderr ?? outPipe.fileHandleForWriting
 
-        // FIXME: pdeathSignal handling if Command supported it.
+        if let pdeathSignal = pdeathSignal {
+            cmd.attrs.pdeathSignal = pdeathSignal
+        }
 
         if setpgid {
             cmd.attrs.setPGroup = true