How hg signals error and aux messages

[Jan Vrany]

Hi guys,

I'm working on an Mercurial abstraction layer for Smalltalk.
I hope I'll soon announce it.

I execute commands and parse their output right from the pipe as
I would like to reduce memory and have some feedback while command
is executing. Works fine unless there is an error.

Is there any consistent system in how errors are reported? Is an
error alwyas indicated by a line to stderr (or error channel) starting
with 'abort: '? Is an error always indicated on stderr (or error 
channel) before command output?

Where are auxiliary messages written - just now I've got a line like

invalidating branch cache (tip differs)

when issuing a hg log command. Normal output follows. I've never seen
that before. Is there any documentation that describes output of 
commands in depth, something like a "grammar" for the output so one
can build a parser? (now I use try-and-see technique, but I constantly
run into problems while using it).

I would be happy for any info about this.

Best, Jan