Re: [PATCH v5 14/16] merge-recursive: offer an option to retain the output in 'obuf'

[Junio C Hamano <gitster@pobox.com>]

Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:

>> I actually now see how this would work well for "reason 2)".  If a
>> caller wants to run the function and wants to pretend as if it did
>> not run anything when it failed, for example, using this to spool
>> all output and error to a strbuf and discard it when the function
>> returns an error, and emit the spooled output to standard output and
>> standard error in the order the lines were collected when the
>> function returns a success, would be a good way to do so.
>
> That is actually the exact opposite of the intended usage: when any `pick`
> in an interactive rebase succeeds, its output is discarded so as not to
> bother the user. We show the complete output only when it fails.

Oh, it makes sense, too, to show the output only when there is an
error.

But in that case, there would be both messages meant for the
standard output and also meant for the standard error, and we need
some way to make sure they go to the right channel.

I however do not think an array of <bool, const char *> is the only
way to achieve that.  We can get away by a single strbuf that
accumulates all output() that we have seen so far, i.e. "we only
accumulate output() and ignore flush() as long as what we see are
only from output()" mode.

Then the err() routine operating under this new mode can show what
has been accumulated to the standard output (because with this tweak
I am outlining here, by definition, the strbuf will only keep the
output() material and not err() things), show the err() message, and
switch back to the normal "we accumulate output() and honor flush()"
mode.  Of course, when we are doing multiple rounds, the mode must
be reset to "accumulate output and ignore flush" mode at the
beginning of each rouhd.

That would give us "silence if there is no error, but if we are
showing error, show them to the standard error, while giving
non-error message to the standard output".