Bugzilla

Well, it just happened again.

This took place just after I realized that this tree didn't rebuild C++ code, so I assumed it was configured for artifact  builds and I attempted to reconfigure it for regular builds. The  `./mach bootstrap`  failed with an error message that sounded like bug 1636797. I attach the error message, although it's probably not related to the current bug.

Here's my bash history:

```sh
./mach bootstrap # Failed.
rm `find obj-x86_64-apple-darwin19.4.0/ -name "*.pyc"` # Workaround for bug 1636797
hg commit -m "WIP: Tracing unnamed threads"
hg pull --rebase
./mach build && ./mach run
./mach boostrap # Fails again, same error message, still probably unrelated to the current bug.
```

I'm almost sure that I realized that `~/.mozbuild` had vanished after the first call to `./mach boostrap`.

Also, I found out what happens with `./mach clobber python`. Apparently, I was missing `purge = ` in my .hgrc. I assume that it should be added to `./mach bootstrap`. Now that I have added it, `./mach clobber python` seems to work.

Well, it just happened again.

This took place just after I realized that this tree didn't rebuild C++ code, so I assumed it was configured for artifact  builds and I attempted to reconfigure it for regular builds. The  `./mach bootstrap`  failed with an error message that sounded like bug 1636797. I attach the error message, although it's probably not related to the current bug.

Here's my bash history:

```sh
./mach bootstrap # Failed.
rm `find obj-x86_64-apple-darwin19.4.0/ -name "*.pyc"` # Workaround for bug 1636797
hg commit -m "WIP: Tracing unnamed threads"
hg pull --rebase
./mach build && ./mach run
./mach boostrap # Fails again, same error message, still probably unrelated to the current bug.
```

I'm almost sure that I realized that `~/.mozbuild` had vanished after the first call to `./mach boostrap`.

Also, I found out what happens with `./mach clobber python`. Apparently, I was missing `purge = ` in my .hgrc. I assume that it should be added to `./mach bootstrap`. Now that I have added it, `./mach clobber python` seems to work.

**edit** The latter is bug 1637709.