Fix maximal supported MTU on Android 13

[marsounjan]

Hello @dariuszseweryn,

since Android 13 the real maximal supported MTU by Android is 515.

Even though in Android source code the GATT_MAX_MTU_SIZE is still set to 517, the data buffer size GAT_MAX_ATTR_LEN is now capped at 512 bytes.

It leads to the situation when Android allows you to negotiate MTU 517, but it drops all the packets with data larger then 512 bytes

Here is the commit where google engineers did this amazing change

[CLAassistant]

All committers have signed the CLA.

[dariuszseweryn]

Hi,
Thats a good finding, thank you! I will sleep with it to think if it would be possible to make a backwards-compatible change.

[weliem]

looks like this change was already done 3 years ago...

https://android.googlesource.com/platform/packages/modules/Bluetooth/+/ab55e0f81c2a09db844cf8c16aef35722ce6ec41%5E%21/

[weliem]

oops...i meant 1 year ago....so might indeed be new to Android 13

[marsounjan]

Hi,
Thats a good finding, thank you! I will sleep with it to think if it would be possible to make a backwards-compatible change.

@dariuszseweryn what do you mean by backwards compatible? this change won't make it incompatible with previous Android versions.

If you have that 2 byte difference on you mind and you'd like to keep the value GATT_MTU_MAXIMUM set to 517 prior Android 13 and use 515 only since then, then we can easily introduce GATT_MTU_MAXIMUM_PRE_API_33 = 517or something like that.

Sadly Java interface don't know computed properties like Kotlin does so you we can't use it for inlining this IF statement :(

[dariuszseweryn]

I'm thinking of deprecating the static value and introduce a function instead.
I'm off for holidays now and I didn't manage to make the change before. I should be back in a week.
@weliem Happy Birthday!

[marsounjan]

@dariuszseweryn great, thanks!

[dariuszseweryn]

Hello, I revisited your PR and added some small suggestions to Javadoc and after that I will happily merge your change :)

[marsounjan]

@dariuszseweryn great, thanks!

[dariuszseweryn]

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