c++: assert (written == len) is not informative error message for write failure

[james-rms]

Description
Currently we assert that the write succeeds in FileWriter, here:

mcap/cpp/mcap/include/mcap/writer.inl

Line 55 in d850fc5

assert(written == size);

This is an error that can occur in production. if the underlying storage medium fills up, and the user needs some kind of trail to debug why.

• Version: v0.6.0
• Platform: Ubuntu Linux

Steps To Reproduce
Create a tmpfs with a limited max size, try writing past its size limit.

Expected Behavior
Some kind of "storage is full" error

Actual Behavior
"Assertion failed (size == written)"

[jhurliman]

Is the storage medium filling up the only possible cause of this error?

[james-rms]

No, this generalizes to all the errors that could result from calling fwrite. On linux, there's a list here: https://man7.org/linux/man-pages/man3/fputc.3p.html

[jhurliman]

Got it. Is the idea that we check for size != written and then call errno() + strerror()?

[james-rms]

Yes, modulo thread-safety concerns. We may need some OS-specific stuff here.

[jhurliman]

errno() on Linux uses thread-local storage. I don't know about other platforms, and it will need some sort of #ifdef for Windows I imagine.