We currently build ports in a somewhat random order (see 
https://trac.macports.org/ticket/52766). By default, the next build that 
buildbot starts is the one that was scheduled first. But if multiple builds 
happened to get scheduled at the same exact time, the order in which they will 
build is undefined.

We can fix this by adding a custom "order" property to each build, and 
modifying our nextBuild function (mentioned in my previous email) so that it 
looks not only at the scheduled timestamp but also the order property to decide 
which build to pick.

I don't know whether we currently compute the *correct* order (dependencies 
should build first) but if not that can be tackled later. A predictable 
incorrect order is better than a random incorrect order.

I'd also like to be able to assign priorities to builds. Low, Medium, High 
would be enough. We could add a custom "priority" property to each build for 

Use case for High priority: suppose hundreds of builds are pending, and you see 
that a build of a library that lots of ports use is broken, let's say 
libnetpbm. You want to be able to fix libnetpbm and force it to build right 
away, before the pending builds go through, because you know many of those 
pending builds would fail without libnetpbm. Fixing libnetpbm might not even 
require a commit: libnetpbm might have been marked as failed because of a fetch 
failure--due to the recent SourceForge outages, for example--and the server 
issue that caused the fetch failure might have been resolved in the mean time.

Use case for Low priority: suppose we set up a new builder for a new macOS 
version. We want to eventually build all ports, but it's more important that 
ports that have just had updates committed are built first. So we would force a 
build of all ports with Low priority, and while the builder is otherwise idle, 
it works its way through building all ports, but when a commit comes in, it 
switches over to building that.

We may not even need to allow the user to specify a priority; we could possibly 
infer it. For example, if a large number of ports (more than 100, say) are 
scheduled by a single commit or force, we might infer that it is of low 
priority. It might be a mass update commit of maintainer lines or checksum 
types or removing $Id$ lines or adjusting whitespace, and we don't want those 
builds to block actual port updates. And if a build of a few ports is forced, 
we might infer that it is of high priority.

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