tests,cmd/snap-confine: port older snapd-discard-ns tests

[zyga]

This patch removes the (unused) spread test from snap-confine directory
and adds the useful parts to the existing spread test for
snap-discard-ns.

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

[jdstrand]

According to https://github.com/snapcore/spread, "MATCH - Run grep -q -e on stdin. Without match, print error including content." If I run this as is, it fails:

# 17.04
$ stat -f -c %T /run/snapd/ns/hello-world.mnt
nsfs
$ stat -f -c %T /run/snapd/ns/hello-world.mnt | grep -q -e 'proc|nsfs|0x6e736673' || echo FAIL
FAIL
# 14.04
$ stat -f -c %T /run/snapd/ns/hello-world.mnt
UNKNOWN (0x6e736673)
$ stat -f -c %T /run/snapd/ns/hello-world.mnt | grep -q -e 'proc|nsfs|0x6e736673' || echo FAIL
FAIL

You need to backslash the '|'. Eg:

$ stat -f -c %T /run/snapd/ns/hello-world.mnt | grep -q -e 'proc\|nsfs\|0x6e736673' && echo PASS
PASS

[zyga]

That part of documentation is out of date. Spread's MATCH is actually grep -q -E "$@". If I use \| with -E the expression no longer works.

[jdstrand]

Also, I understand why 'nsfs' and '0x6e736673', but why 'proc'? If that is intentional, please comment on why.

[zyga]

proc is there because on some older kernels there is no nsfs yet.

[jdstrand]

What kernels? What does the output look like? Please document what stat outputs for each of the greps.

[zyga]

NOTE: This is not a new test, I'm just re-enabling an existing test.

The affected kernel is, for example, one used on CentOS 7. I tested 3.10.x which doesn't have nsfs yet. The output of stat is similar, except for the filesystem magic number.

[jdstrand]

Note the added tests are different from the files being removed which is why I felt the comments were warranted.