logging: log output busy loops if log output function can not process buffer

[b0661]

Describe the bug
According to the documentation the log_output_func shall return the number of bytes processed.

typedef int (*log_output_func_t)(u8_t *buf, size_t size, void *ctx)

    Prototype of the function processing output data.

    Return
        Number of bytes processed. 
    Parameters

            data: Data.
            length: Data length.
            ctx: User context.

If the function returns 0 the log buffer_write function busy loops forever.

static void buffer_write(log_output_func_t outf, u8_t *buf, size_t len,
			 void *ctx)
{
	int processed;

	do {
		processed = outf(buf, len, ctx);
		len -= processed;
		buf += processed;
	} while (len);
}

To Reproduce
Steps to reproduce the behavior:
Change the log_output_func to return 0.

Expected behavior
Ether - The logger shall stop the output to the backend and come back later.
Or - Improve documentation to reflect the needed behaviour (which is still unclear to me)

Impact
Application stalls if log output stalls.

Screenshots or console output
N/A

Environment (please complete the following information):

• Zephyr, Commit SHA 3980e0c

Additional context
N/A

[nordic-krch]

@b0661 processed means dropped as well. It is backend responsibility to drop data. I would prefer to keep log_output module simple. It's just a helper module for backends to avoid code repetition. I will add a note to api documentation, ok?

[b0661]

@nordic-krch

It's just a helper module for backends to avoid code repetition. I will add a note to api documentation, ok?

Can you just make it typedef void (*log_output_func_t)(u8_t *buf, size_t size, void *ctx)?

By this it wouid be clear by interface that the whole buffer has to be handled by the backend. You may get an extra loop in the backend but could get rid of e.g. the buffer_write helper.

Otherwise a note in the api documentation is also fine for me.

[carlescufi]

@nordic-krch can you submit a PR for the note in the doc that you agreed to?