Reporting of errors detected by handler_read() before it returns

[David Butterfield <dab21774@xxxxxxxxx>]

In tcmu-runner.h some comments in struct tcmur_handler say this:

	/*
	 * Below callbacks are only executed by generic_handle_cmd.
	 * Returns:
	 * - TCMU_STS_OK if the handler has queued the command.
	 * - TCMU_STS_NO_RESOURCE if the handler was not able to allocate
	 *   resources for the command.
	 *
	 * If TCMU_STS_OK is returned from the callout the handler must call
	 * the tcmulib_cmd->done function with TCMU_STS return code.
	 */
	rw_fn_t write;
	rw_fn_t read;
	flush_fn_t flush;
	unmap_fn_t unmap;

It is not clear what the handler should do if it detects a non-resource error, processing
one of those command types, before returning from the handler's callout function.
    
The comment appears to say that rhandler->write() could return TCMU_STS_OK after calling
cmd->done(..., TCMU_STS_error) (so callers should be warned that the completion can occur
before the request call returns).

But given that the caller has to check for TCMU_STS_NO_RESOURCE anyway, perhaps it is also
permitted to simply return other errors directly from the callout function, always omitting
the call to cmd->done() when TCMU_STS_OK is not returned.

However, the code in file_example.c does not do either of those things, and directly
violates the comment in tcmu-runner.h -- this excerpt is from file_read():

	while (remaining) {
		ret = preadv(state->fd, iov, iov_cnt, offset);
		if (ret < 0) {
			tcmu_err("read failed: %m\n");
			ret = TCMU_STS_RD_ERR;
			goto done;
		}
		...
	}
	ret = TCMU_STS_OK;
done:
	return ret;

The file_read() function operates synchronously, and never issues a call to cmd->done()
for successful reads returning TCMU_STS_OK, contradicting the comment in tcmu-runner.h.

Assuming the function is required to call cmd->done() for a successful operation, a
question remains which of these solutions is more correct or preferred.  (I would prefer
the first one be allowed -- to return any TCMU_STS_error directly from the function and
omit the call to cmd->done() whenever returning TCMU_STS_OK.)

/**************************************/
	ret = TCMU_STS_OK;
	cmd->done(dev, cmd, ret);
done:
	return ret;
/**************************************/

or

/**************************************/
	ret = TCMU_STS_OK;
done:
	cmd->done(dev, cmd, ret);
	return TCMU_STS_OK;
/**************************************/