selftests/ftrace: Add a basic testcase for snapshot

This testcase is constrived to reproduce a problem that the cpu buffers
become unavailable which is due to 'record_disabled' of array_buffer and
max_buffer being messed up.

Local test result after bugfix:
# ./ftracetest test.d/00basic/snapshot1.tc
=== Ftrace unit tests ===
[1] Snapshot and tracing_cpumask [PASS]
[2] (instance) Snapshot and tracing_cpumask [PASS]

# of passed: 2
# of failed: 0
# of unresolved: 0
# of untested: 0
# of unsupported: 0
# of xfailed: 0
# of undefined(test bug): 0

Link: https://lkml.kernel.org/r/[e-mail]

Cc: <[e-mail]>
Cc: <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Zheng Yejian <[e-mail]>
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

tools/testing/selftests/ftrace/test.d/00basic/snapshot1.tc

···
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+# description: Snapshot and tracing_cpumask
+# requires: trace_marker tracing_cpumask snapshot
+# flags: instance
+
+# This testcase is constrived to reproduce a problem that the cpu buffers
+# become unavailable which is due to 'record_disabled' of array_buffer and
+# max_buffer being messed up.
+
+# Store origin cpumask
+ORIG_CPUMASK=`cat tracing_cpumask`
+
+# Stop tracing all cpu
+echo 0 > tracing_cpumask
+
+# Take a snapshot of the main buffer
+echo 1 > snapshot
+
+# Restore origin cpumask, note that there should be some cpus being traced
+echo ${ORIG_CPUMASK} > tracing_cpumask
+
+# Set tracing on
+echo 1 > tracing_on
+
+# Write a log into buffer
+echo "test input 1" > trace_marker
+
+# Ensure the log writed so that cpu buffers are still available
+grep -q "test input 1" trace
+exit 0