Offset field in struct ble_gatt_access_ctxt as an indication of long attribute read

[vandy]

To obtain an attribute value when the attribute size is greater than configured MTU long attribute read procedure could be used.
A client sends consecutive ATT_READ_BLOB_REQ requests (passing along an offset) to consume the attribute value. The offset is used internally in the nimble stack but is not passed to a user provided access callback in a GATT characteristic description. This callback is called the same number of times as the number of blob requests, but it's not possible to say whether it's a first request (or maybe just ATT_READ_REQ) or consecutive request.

The provided offset could indicate this situation to the app developer. However, Core Spec 5.4 Vol 3 Part F 3.4.4.5 notes:

Note: The value of a Long Attribute may change between the server receiving one ATT_READ_BLOB_REQ PDU and the next ATT_READ_BLOB_REQ PDU. A higher layer specification should be aware of this and define appropriate behavior.

It's up to a developer to decide what to do, when consecutive (long attribute) reading takes place. This was discussed in #1090.

[vandy]

I've also made some changes to other parts of the code (only code style without change in functionality). <- Moved to other PRs.

And provided some points to consider with REVIEW comments. I'd love to get a response on this topics and if they are approved I'll make changes to the code as described in this comments.

[vandy]

Right now the nimble stack takes all attribute data from an application and truncates it to be the size of MTU (actually MTU - 1). E.g. attribute length is 500 bytes, MTU is 23 bytes. So the application copies 500 bytes to the provided mbuf, and then nimble stack copies only 22 bytes according to an offset.

I think it would be nice to allow the application callback to provide only part of the data (as it already knows the offset and could obtain mtu length information). What do you think? @sjanc

[andrzej-kaczmarek]

I've also made some changes to other parts of the code (only code style without change in functionality).

code style fixes should be at least in separate commit, but preferably in separate PR

[vandy]

@andrzej-kaczmarek, I've split this PR into 3: #1847 and #1848. But left REVIEW comments here until the discussion and decision is settled. I believe it would be better to make another PR for discussed changes.

[andrzej-kaczmarek]

you can make separate PR for other changes so we can discuss them.
just keep adding the offset field to the struct in this PR and it's good to go. please don't reformat other pieces of code though...

[vandy]

Done, left only changes related to the offset.

[sjanc]

Hi,

Code looks OK to me, thanks! But could you amend commit message to follow project style?
This should be summary line (with area prefix, eg "nimble/host:" ) and short description in commit body.
Also note preferred 72 characters line limit

eg

nimble/host: Add offset in GATT server long read context

This allows application to easily determine if client is performing
GATT Long Read operation.

[vandy]

Amended the commit message. Also, moved issues to discuss into #1849.

[vandy]

@sjanc, would you merge the PR?

[vandy]

@andrzej-kaczmarek, can we merge now?