sys/sys: Add DEBUG_PANIC() macro

[ccollins476ad]

The DEBUG_PANIC() macro triggers a crash if the DEBUG_PANIC_ENABLED syscfg setting is enabled. If this setting is disabled, the macro expands to nothing.

This feature is discussed briefly here: https://lists.apache.org/thread.html/0ce981b90eb74226fd660f25ba88f4ce6804eea2943edac70bf8731e@%3Cdev.mynewt.apache.org%3E

[sjanc]

quick question, how this differ from assert?

[sjanc]

nevermind, I see that those are just convenient macros around asserts

[ccollins476ad]

The dev list email probably goes into more detail than anyone is interested in :). The short answer is:

• DEBUG_ASSERT() does not evaluate an expression to determine success / failure; it assumes failure. Since these failures are expected, I think it is likely that the application will always want to execute some code in the failure case, regardless of whether the macro is configured to trigger a crash. For example: logging a message to the console when the failure is detected.

• DEBUG_ASSERT() is intended to be disabled in production builds, so it should always be followed by code that recovers from the failure. assert() is meant to ensure that failures that "should never happen" indeed never happen, so there is no need for recovery code. In addition, even though assert() can be disabled via NDEBUG, I am of the opinion that it should never be disabled, as it helps eliminate a lot of possibilities when debugging faults in the field.

[wes3]

Changes look good to me.

[mkiiskila]

LGTM. Only additional thing I could think of is adding a 3rd option for DEBUG_PANIC(), which would call user defined routine (defined in app, most likely) which could e.g. log the error. But we can add that later if there's an actual use case for such a thing.