fix: snapshot data in Collector.flush_data to avoid threading race

[alexmv]

Collector.data is a single dict shared across all per-thread tracers, and tracers update its inner sets without taking a lock (one acquire per traced line would be far too expensive). When a switch_context boundary fires on one thread -- which is routine with dynamic_context = test_function -- flush_data runs on that thread while tracers in other threads keep adding lines to the same sets. mapped_file_dict only guarded the outer dict iteration, so add_arcs / add_lines later iterated the live sets and could fail with:

RuntimeError: Set changed size during iteration

...raised from nums_to_numbits in the lines path, or from the arc tuple comprehension in add_arcs. list(packeds) and list(packed_data.items()) in the packed-arcs branch were equally exposed: both go through the Python iterator protocol, which detects concurrent mutation and raises.

Snapshot both the outer dict and each per-file set with .copy(), which is atomic in CPython -- the GIL is held for the duration of the C-level copy, so no other thread can mutate the source mid-copy. Free-threaded builds do them in per-container critical sections, so the same guarantee applies. add_arcs / add_lines now iterate stable copies even while other threads continue recording trace data.

[nedbat]

I can see that a lock for each line would be a lot, though I'm not sure it would be too expensive. Copying the data is also expensive, though it happens infrequently. I'm more concerned about the GIL reasoning, and will it hold up in free-threaded builds?

[alexmv]

.copy() is implemented in C -- and as such, it's free from this style of race.

In free-threaded builds, each object has its own mutex, which is held while modifying the object. These are suspended if it pop back to Python (e.g. using list(thing) will check iterator protocols), but .copy() is explicitly listed as being atomic to other threads, as it is entirely implemented in C.

[nedbat]

This is now released as part of coverage 7.14.0.

[alexmv]

Thanks for the release!