[Plugin] exec_cmd(): pass stderr along to sos_get_command_output()

The sosreport of a system with this issue:
```
	# ip netns
	Error: Peer netns reference is invalid.
	Error: Peer netns reference is invalid.
	test-ns
```

Shows this difference between the plugins ebpf and networking (callers
of `ip netns`):
```
	# ./bin/sos report -o ebpf,networking --batch

	# tar tf /tmp/sosreport-*.tar.xz | grep ip_netns_exec
	.../sos_commands/ebpf/ip_netns_exec_test-ns_bpftool_net_list
	.../sos_commands/networking/ip_netns_exec_Error_ip6tables-save
	.../sos_commands/networking/ip_netns_exec_Error_ip6tables-save.1
	...
	.../sos_commands/networking/ip_netns_exec_test-ns_ip6tables-save
	...
```

Only the networking plugin called `ip netns exec <ns> <cmd>` on `Error:`
lines. Hmm.

...

The networking plugin calls `collect_cmd_output()`, which has parameter
stderr=True` as default:

```
    def collect_cmd_output(self, cmd, suggest_filename=None,
                           ...
                           stderr=True, ...
```

The ebpf plugin calls `exec_cmd()`, BUT it has parameter `stderr=True`
as default AS WELL:

```
    def exec_cmd(self, cmd, timeout=cmd_timeout, stderr=True, ...
```

So, why the difference?

...

Well, it turns out `exec_cmd()` does NOT pass the `stderr` parameter
along to `sos_get_command_output()`: [but `collect_cmd_output()` does.]

```
      return sos_get_command_output(cmd, timeout=timeout, chroot=root,
                                    chdir=runat, binary=binary, env=env,
                                    foreground=foreground)
```

And `sos_get_command_output()` has `stderr=False` as default:

```
      def sos_get_command_output(command, timeout=300, stderr=False, ...
```

Thus, despite `exec_cmd()` has `stderr=True` as default it is _ignored_,
as its callee `sos_get_command_output()` has `stderr=False` as default.

This explains the output difference between the ebpf/networking plugins.

...

Looking back for details, the origin of the change is in PR#1807 [1],
with commit e8bb94c (refactor command functions/introduce `exec_cmd()`)
and commit e51d3e6 (update caller plugins).

The PR has no mentions/discussion around `stderr` in comments/review,
so it's apparently an oversight during the refactor, I guess.

Because, previously the refactored callers of `sos_get_command_output()`
(`get_command_output()`, `call_ext_prog()`, `[_]get_cmd_output_now()`)
all had `stderr=True` as parameter/default, passing it along to callee.

(The only exception is in `_collect_cmd_output()` which does the first
call in the chroot with `stderr`, and falls back to non-chroot without
`stderr`, but it was already that way.  Thus not changing this case.)

...

So, it seems right and safe and opportune to fix this inconsistency
between default `stderr` in `exec_cmd()` / `sos_get_command_output()`
by just passing it along between them:

Right: because it restores the previous behavior (before refactor)
assumed/accepted by callers that did not specify `stderr=False`.

Safe: because it restores the actual behavior to previous behavior,
and if plugins handled `stderr` lines previously, they should also
handle it now, for the most part (i.e., not any new `stderr` lines,
but they are currently ignoring those, and should get them covered.)

Opportune: because if any plugin breaks _now_ because of this, it is
a new/post-refactor change that decided to use `exec_cmd()` with the
default that is inconsistent with actual behavior, and it seems okay
to take bugs for that, so to identify and fix such cases.

Hopefully those points are reasonable with the project's philosophy.

...

Test-case:

Setup:
```
	$ sudo python3

	from sos import SoS
	from sos.report.plugins import Plugin

	sos = SoS(['report'])
	plugin = Plugin(sos._component.get_commons())
```

Before:
```
	>>> print(plugin.exec_cmd('ip netns')['output'])
	test-ns

	>>> print(plugin.exec_cmd('ip netns', stderr=True)['output'])
	test-ns

	>>> print(plugin.exec_cmd('ip netns', stderr=False)['output'])
	test-ns
```

After:
```
	>>> print(plugin.exec_cmd('ip netns')['output'])
	Error: Peer netns reference is invalid.
	Error: Peer netns reference is invalid.
	test-ns

	>>> print(plugin.exec_cmd('ip netns', stderr=True)['output'])
	Error: Peer netns reference is invalid.
	Error: Peer netns reference is invalid.
	test-ns

	>>> print(plugin.exec_cmd('ip netns', stderr=False)['output'])
	test-ns
```

...

Test-suite:
```
	# ./tests/simple.sh
	<...>
	Summary

	 failures false          time 2                  -l
	 failures false          time 3                  --list-presets
	 failures false          time 3                  --list-profiles
	 failures false          time 17                 --batch --build
		 --no-env-vars
	 failures false          time 3          Size 14.23KiB   --batch
		 --no-report -o hardware
	 failures false          time 20         Size 2.99MiB    --batch
		 --label TEST -a -c never
	 failures false          time 20         Size 2.81MiB    --batch
		 --debug --log-size 0 -c always
	 failures false          time 20         Size 2.84MiB    --batch
		 -z xz --log-size 1
	 failures false          time 19         Size 4.01MiB    --batch
		 -z gzip
	 failures false          time 32         Size 2.76MiB    --batch
		 -t 1 -n hardware
	 failures false          time 19                 --batch --quiet
		 -e opencl -k kernel.with-timer
	 failures false          time 22         Size 4.31MiB    --batch
		 --case-id 10101 --all-logs --since=20201110
	 failures false          time 21         Size 3.06MiB    --batch
		 --verbose --no-postproc
	 failures false          time 39         Size 2.82MiB    --batch
		 --mask
	Everything worked!
```

[1] https://github.com/sosreport/sos/pull/1807

Resolves: #2306

Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Signed-off-by: Jake Hunsaker <jhunsake@redhat.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/sos/report/plugins/__init__.py b/sos/report/plugins/__init__.py
index ed0a07ef..6eeef5c1 100644
--- a/sos/report/plugins/__init__.py
+++ b/sos/report/plugins/__init__.py
@@ -2099,7 +2099,7 @@ class Plugin(object):
 
         return sos_get_command_output(cmd, timeout=timeout, chroot=root,
                                       chdir=runat, binary=binary, env=env,
-                                      foreground=foreground)
+                                      foreground=foreground, stderr=stderr)
 
     def _get_container_runtime(self, runtime=None):
         """Based on policy and request by the plugin, return a usable