Signal handling may be delayed, which leads to dropped samples #5
Description
I've previously mentioned this issue in the comments of PR #1, but now that I understand what the problem is, I think it's worth creating a separate issue, not tied to any particular PR.
djdt-flamegraph uses interval timers that periodically generate signals, and registers a signal handler to sample the current stack frame. Now, the way CPython implements signal handlers is that they set an internal flag, which is checked the next time the CPython interpreter enters the main eval loop.
A consequence of this is that if the main thread is blocked in some C code, signal handling gets delayed for an unbounded time, and you're getting a skewed profile picture because you're missing a significant number of samples. Example: executing an SQL query via psycopg2 may delay stack sampling for hundreds or thousands of milliseconds.