[Buildbot-devel] Triggerable with waitforFinish=True requires build slot

[Bob Hood]

I have a series of "main" build lanes (a term I use to describe a column of
the Waterfall) that construct toolkits.  After these builds succeed, I use
Triggerable builders to start several secondary builds of third-party tools
that depend on the toolkits made in the "main" lane, waiting for them to
complete before triggering the next ("waitForFinish=True").  This
configuration is repeated over and over to construct variants
(Win32/VS2008/release, Win64/VS2008/release, Win32/VS2008/debug,
Win64/VS2008/debug, Win32/VS2010/release, Win64/VS2010/release, etc.).

I had my configuration limiting the build slave, a single i7 machine, to a
single build ("max_builds=1") because each variant takes about 1 hour. 
However, with that configuration, the triggering of the secondary build halts
because the "main" build lane is holding on to that single build slot.  I have
to change the configuration to "max_builds=2" in order to get the secondary
builds to start.

This leaves me a bit uncomfortable, because that opens the door for a second
"main" variant lane to begin building at the same time as another "main" lane
if I let the lanes respond to SCM changes, which is not what I want.  I only
want the "main" lanes to build in sequential order, after all the secondary
builds complete (whether or not they fail).

I've thought about adding a final trigger to launch the next "main" lane after
all secondary builds are complete, but that doesn't seem to take into account
if a build is actually "pending" for the next "main" lane, and I don't want
the next variant round to start unless there's actually a "pending" build
slotted for it.

Any suggestions regarding how I could configure to keep the build slots
confined to a collection of builds?  Or I would also entertain doing some
"hacking" of the Buildbot Master code to see what I can do there (perhaps
temporarily hand off the sole build slot to another lane) if somebody would
point me at the salient module(s) and lines of code.