fix: write runtime PID file when provided

Ensure the PID is written to the runtime-provided --pid-file after
container creation. This allows supervisors to track and
reap the container process correctly, preventing early cleanup or EOF
failures. This change also ensures that Podman can reliably start urunc
containers.

PR: #439
Signed-off-by: sidneychang <2190206983@qq.com>
Reviewed-by: Charalampos Mainas <cmainas@nubificus.co.uk>
Approved-by: Charalampos Mainas <cmainas@nubificus.co.uk>

Author: sidneychang

cmd/urunc/create.go

@@ -252,9 +252,10 @@ func createUnikontainer(cmd *cli.Command, uruncCfg *unikontainers.UruncConfig) (
 
 	// Retrieve reexec cmd's pid and write to file and state
 	containerPid := reexecPid
+	pidFilePath := cmd.String("pid-file")
 	metrics.Capture(m.TS06)
 
-	err = unikontainer.Create(containerPid)
+	err = unikontainer.Create(containerPid, pidFilePath)
 	if err != nil {
 		return err
 	}

pkg/unikontainers/unikontainers.go

@@ -147,9 +147,14 @@ func (u *Unikontainer) InitialSetup() error {
 }
 
 // Create sets the Unikernel status as created,
-// and saves the given PID in init.pid
-func (u *Unikontainer) Create(pid int) error {
-	err := writePidFile(filepath.Join(u.State.Bundle, initPidFilename), pid)
+// and saves the given PID in the provided pid file path.
+// If pidFilePath is empty, it falls back to the default init.pid path.
+func (u *Unikontainer) Create(pid int, pidFilePath string) error {
+	path := filepath.Join(u.State.Bundle, initPidFilename)
+	if pidFilePath != "" {
+		path = pidFilePath
+	}
+	err := writePidFile(path, pid)
 	if err != nil {
 		return err
 	}