linux-container: "run-container" scripts shows the container's PID.

* gnu/build/linux-container.scm (call-with-container): Add
 #:process-spawned-hook and honor it.
* gnu/system/linux-container.scm (container-script)[script]:
Define 'explain' and pass it as #:process-spawned-hook'.

1 files changed, 5 insertions, 2 deletions

diff --git a/gnu/build/linux-container.scm b/gnu/build/linux-container.scm
index 6ccb924861..87695c98fd 100644
--- a/gnu/build/linux-container.scm
+++ b/gnu/build/linux-container.scm
@@ -299,8 +299,10 @@ delete it when leaving the dynamic extent of this call."
         (false-if-exception (delete-file-recursively tmp-dir))))))
 
 (define* (call-with-container mounts thunk #:key (namespaces %namespaces)
-                              (host-uids 1) (guest-uid 0) (guest-gid 0))
-  "Run THUNK in a new container process and return its exit status.
+                              (host-uids 1) (guest-uid 0) (guest-gid 0)
+                              (process-spawned-hook (const #t)))
+  "Run THUNK in a new container process and return its exit status; call
+PROCESS-SPAWNED-HOOK with the PID of the new process that has been spawned.
 MOUNTS is a list of <file-system> objects that specify file systems to mount
 inside the container.  NAMESPACES is a list of symbols corresponding to
 the identifiers for Linux namespaces: mnt, ipc, uts, pid, user, and net.  By
@@ -329,6 +331,7 @@ load path must be adjusted as needed."
            (false-if-exception
             (kill pid SIGKILL))))
 
+       (process-spawned-hook pid)
        (match (waitpid pid)
          ((_ . status) status))))))