Make snap-discard-ns fail gracefully

[zyga]

This patch changes snap-discard-ns to gracefully fail (as in, return
successfully) when any of the following three conditions happen:

• the /run/snapd/ns directory doesn't exist yet
• the /run/snapd/ns/$SNAP_NAME.mnt file doesn't exist
• the /run/snapd/ns/$SNAP_NAME.mnt file is not a mount point

This allows snapd to check for abnormal errors from snap-discard-ns.

Signed-off-by: Zygmunt Krynicki zygmunt.krynicki@canonical.com

[jdstrand]

Please adjust sc_open_ns_group() to take an arg instead as per inline comments

[jdstrand]

This function is identical to sc_open_ns_group() except for this ENOENT check. It would be more maintainable to instead modify sc_open_ns_group() to take a flag and decide how to error based on the flag. Eg:

struct sc_ns_group *sc_open_ns_group(const char *group_name, const int graceful_enoent)
{
...
    if (group->dir_fd < 0) {
                if (graceful_enoent == 1 && errno == ENOENT) {
                        free(group);
                        return NULL;
                }
                die("cannot open directory for namespace group %s", group_name);
        }
...

Then sc-main.c uses:

group = sc_open_ns_group(group_name, 0);

and snap-discard-ns.c uses:

group = sc_open_ns_group(group_name, 1);

Obviously feel free to rename graceful_enoent and be sure to document this argument.

[zyga]

Will do, thank you for the review

[zyga]

Spread failure looks related to packaging. I will fix this separately and improve spread setup so that we see what actually failed.

[jdstrand]

+1 provided you add a test for sc_open_ns_group(group_name, SC_NS_FAIL_GRACEFULLY)

[zyga]

Will do, thank you! :)

[zyga]

I'm merging this because the test failures are not really related to the change being made. Analysis indicates that ubuntu-core is being unmounted at some (random) point during the test process and depending on how fast or slow the VM is, tests just randomly fail when that happens. Snap-confine has no code that performs sanity checks on the presence of the core snap.