Guarantee of no broken dependencies?

[Peter Polidoro]

For every commit to the Guix master branch, is there a guarantee that there are 
no packages with broken dependencies?

I have not experienced packages with broken dependencies, so I am curious how 
that is checked. In theory, someone could make a commit that accidentally 
breaks other packages correct? Aren’t most inputs just found by name, not name 
and version or hash?

Do packages not get broken, though, because right before every commit the 
updated package is built, along with every package that depends on the updated 
package? Is that enough to make sure all dependencies are fine? There are no 
race conditions with other people making updates at the same time?

How can we do such a consistency check or have such a guarantee when we use 
multiple channels? Does we just rebuild every package in every non-Guix channel 
every time we update Guix to find out what breaks?