If you cancel a build at just the right time, you'll get ninja in a bad state:
[10:47:13]:/ssd/chromium/src $ ninja -C out/Debug chrome
ninja: Entering directory `out/Debug'
[0/1] Regenerating ninja files^C
ninja: error: rebuilding 'build.ninja': interrupted by user

[10:48:07]:/ssd/chromium/src $ GOMA_DISABLED=true ninja -C out/Debug chrome
ninja: Entering directory `out/Debug'
ninja: error: loading 'build.ninja': No such file or directory

In the above steps, I forgot to disable ninja and tried to quickly cancel my build before it spawned a bunch of processes.

To fix this, you need to run gn args again. This is confusing because the rest of the build can be canceled at any time. Can we make the ninja generation step restore the old files if it is cancelled?