libxo: Need to support fflush and error reporting from fflush

[marcel]

Summary

libxo needs to allow for explicit flushing of output streams when doing
time series output. example: 'netstat --libxo json 1'

Without the flush, then the output is buffered until the stdio buffer
fills.

This patch adds a non-standard (but yet standard in glibc and
solaris) function to peek into stdio buffers to determine if they are
line buffered so that libxo can properly buffer based on lines.

[ngie]

1. xo_emit(3) needs to be updated for the API changes.
2. The library version probably needs to be bumped (I would do that because you're changing a lot of return types and arguments to functions). marcel and other Juniper folks should be consulted on that though.
3. I think this is a good change to make by default, but this could make things more painful on the I/O side, depending on how frequently the code is called. It might be better to move the flushing outside loops to use I/O buffering a bit more intelligently instead of putting it explicitly at the end of xo_flush_h, or having a tunable way to avoid auto-flushing the I/O out to stderr/stdout/etc.

[alfred]

• Using getbufmode() for a second...
• use __flbf() instead of getbufmode
• Need __flbf()

[marcel]

netstat has been converted without this change. The best approach to the problem this change failed to address is that netstat et al is to call the utility every so often and not have the utility produce the equivalent of a line of output every so often. Parsers for structural languages do not yield incomplete objects in the in the interim. Note also that the utility needs a SIGINT to stop, which also aborts the output, leaving with invalid JSON, XML or HTML.

[wblock]

lib/libc/stdio/setbuf.3

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Remember to update this just before commit.

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No paragraph break needed here.

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s/steam/stream/

"returns if" is kind of ambiguous. Maybe

function tests a stream to determine if it is line buffered.