Buffer custom logs until a service connection is established

[jmatsu]

ref: #48

It may be better to buffer all instructions, but we have decided to start addressing this issue from the custom logging.

Approach

• Use Handler to control instructions for logging
• Buffering
  • Buffer logs if a service connection is unavailable
  • Start sending logs when a service connection is established
  • Default backpressure is Drop By Oldest (similar to the current behavior)
• Performance-aspect
  • Any processing doesn't work if the buffer is empty
  • Do not poll the buffer
  • Do not get locks as much as possible
• Fix a bug in the current master
  • 091a284#diff-cc72d904aba4511de76932694c6d4d6cdba24e872a38863bda0589525fb0ecb5L968

Do in another PR

• Measure wasted-time in the buffer

Overview of custom log buffering

sequenceDiagram
    participant Android
    participant UserApp
    participant SDK
    participant ClientApp
opt DirectBoot
    Android->>UserApp: Direct boot event
    UserApp->>SDK: Log a message
    SDK-->SDK: Store the log
end
opt NormalBoot
    Android->>UserApp: Boot completed event
    UserApp->>SDK: Log a message
    SDK-->SDK: Store the log
end

Android->>ClientApp: Boot completed
ClientApp->>SDK: Service connection is established

par Process buffered logs
    SDK->>ClientApp: Send buffered logs
and Process a new fresh log
    UserApp->>SDK: Log a message
    SDK-->ClientApp: Send it
end

loop Process a new log
    UserApp->>SDK: Log a message
    SDK-->ClientApp: Send it
end

Loading

Rejected idea

Polling

• ✅ Easy to keep sending buffered logs
• ❌ Polling costs would be high

Concurrent Deque or else

• ✅ Better performance
• ❌ need to maintain the thread-safe implementation

[tnj]

Overall looks good to me, with minor confirmations and corrections commented

[jmatsu]

@tnj could you please review this again?

[tnj]

LGTM!

[jmatsu]

Thank you so much! 🎉