CFE_ES_RegisterCDSEx: clear block if new allocation is needed but not new block size

[skliper]

Is your feature request related to a problem? Please describe.
May need to clear block if a new allocation is needed without a changed block size

cFE/modules/es/fsw/src/cfe_es_cds.c

Line 383 in e80aae9

if (Status == CFE_SUCCESS && RegRecPtr->BlockOffset != 0 && NewBlockSize != RegRecPtr->BlockSize)

Describe the solution you'd like
Analyze, clear block if needed

Describe alternatives you've considered
If not needed, document why for future reference

Additional context
Code review

Requester Info
Jacob Hageman - NASA/GSFC

[jphickey]

Looked into this code, its one of those areas where the original design appears to have evolved in a certain way, and so the "classic" behavior from old CFE versions was preserved.

I went back to the requirements to see what it says, and only these three:

cES1315	Upon receipt of a Request, the cFE shall reserve the Request specified amount of memory in the Critical Data Store for the cFE Application using the Request specified name.
cES1315.1	If a Critical Data Store exists for the Request specified name but has a different size than what is specified in the Request, the cFE shall remove the existing Critical Data Store and create a new one using the Request specified name and size.
cES1315.2	If a Critical Data Store exists for the Request specified name but the Data Integrity value is invalid, the cFE shall remove the existing Critical Data Store and create a new one using the Request specified name and size.

But none of these requirements say anything about "pre-clearing" the CDS block, and historically the code has not done this in either case (i.e. for new allocations or size changes). I can't say for sure why not, but most likely for these reasons:

• CDS access might be slow (e.g. flash/eeprom memory, possibly indirectly accessed through an of intermediate serial link) so zeroing it out could take a fair bit of time.
• CDS memory might have a limited number of write cycles and zeroing it out might reduce its lifespan
• CDS blocks are protected by a CRC-32. So in the event that a user attempted to read a CDS that had never been written to, it would fail the CRC check and the user would get CFE_ES_CDS_BLOCK_CRC_ERR. Notably this is dependent on the size of the block too, so if the block size were to change, even if it had a previous content with a good CRC, the CRC check would likely fail for the new/different block size.

My recommendation is to leave this as is, for the reasons above. Changing behavior is likely to have unintended side effects.