gh-157242: Add _PyBytes_IsMutable() assertion

[vstinner]

Elaborate mutability in _PyBytes_Resize() and PyBytesWriter documentation.

Check _PyBytes_IsMutable() in functions which require a mutable bytes object like PyBytesWriter_Resize().

• Issue: bytearray.resize() must not clear the object on error (memory allocation failure) #157242

[vstinner]

I also wanted to add these assertions:

assert(_PyObject_IsUniquelyReferenced(v));
assert(get_ob_shash((PyBytesObject*)v) == -1);

• Check that a bytes object is not shared between threads while it's being modified.
• Check that a bytes object has not been used a dictionary key (check that its hash value has not been computed yet).

But IMO these checks are too strict and can fail in use cases which work currently.

The PyBytesWriter has this warning:

The API is not thread safe. A PyBytesWriter object must only be used in a single thread, it must not be shared between threads.

But there is no check at runtime, even in debug mode.

[vstinner]

cc @cmaloney

[StanFromIreland]

This just re-iterates the .. soft-deprecated:: note below.

[vstinner]

Ah yes, you're right, it's redundant. I removed the paragraph.

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #34528345 | 📁 Comparing 176a305 against main (575fe39)

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2 files changed

± c-api/bytes.html
± whatsnew/changelog.html