container: Allow 'container-excursion' to the same namespaces.

Before that, 'container-excursion' would call 'setns' even when the
target namespace is the one the caller is already in, which would fail.

* gnu/build/linux-container.scm (container-excursion): Introduce
'source' and 'target'.  Compare the result of 'readlink' on these
instead of comparing file descriptors to decide whether to call
'setns'.
* tests/containers.scm ("container-excursion, same namespace"): New test.

1 files changed, 9 insertions, 0 deletions

diff --git a/tests/containers.scm b/tests/containers.scm
index bbcff3f51f6..698bef3e47c 100644
--- a/tests/containers.scm
+++ b/tests/containers.scm
@@ -162,4 +162,13 @@
           (waitpid pid)
           (zero? result)))))))
 
+(skip-if-unsupported)
+(test-equal "container-excursion, same namespaces"
+  42
+  ;; The parent and child are in the same namespaces.  'container-excursion'
+  ;; should notice that and avoid calling 'setns' since that would fail.
+  (container-excursion (getpid)
+    (lambda ()
+      (primitive-exit 42))))
+
 (test-end)