debug: fix sporadic failures of memory sampling tests

Memory sampling tests fail sporadically for spike targets. A typical
failure looks as follows (ROI from test log):

```
---------------------------------[ Message ]----------------------------------
139670831 not less than 124104544
--------------------------------[ Traceback ]---------------------------------
    ... SECTION IS SKIPPED FOR READABILITY ...
    raise TestFailed(f"{a!r} not less than {b!r}", comment)
testlib.TestFailed
```

Few observations:

- 139670831 is 0x0853352f in hex, while 124104544 is 0x0765af60
- Now, the assert which is failing corresponds to the following
  expression:

```
  assertLess(value, previous_value + tolerance)
```

- tolerance is `0x500000`. (124104544 - 0x500000) is 0x0715af60

If we look at the sampling output for such failing test, we'll see:

```
...
0x1212340c5c: 0x0715af60
timestamp after: 878087500
timestamp before: 878088133
0x1212340c5c: 0x0853352f
...
```

The log above demonstrates the reason for the failure. Since memory
sampling occures every poll (which by default happens approximately
every 100ms) a value of the counter may exceed the threshold if the time
between subsequent polls is increased (for whatever reason).

In my opinion the failing assert can be safely removed, since the checks
it perform are quite brittle and cannot be generalized. The assert
violation is affected by CPU performance and sporadic delays between
polls.

For now, instead of assert removal we just avoid checks in-between
memory sample bursts. This way we still can be certain that memory
samples are frequent enough and hopefully this will avoid sporadic
failures.

1 files changed, 1 insertions, 0 deletions

diff --git a/debug/gdbserver.py b/debug/gdbserver.py
index f00dcc9..2cc6c8e 100755
--- a/debug/gdbserver.py
+++ b/debug/gdbserver.py
@@ -833,6 +833,7 @@ class MemorySampleTest(DebugTest):
                     first_timestamp = timestamp
                 else:
                     end = (timestamp, total_samples)
+                    previous_value = None
             else:
                 assertRegex(line, r"^0x[0-f]+: 0x[0-f]+$")
                 address, value = line.split(': ')