This is from a fresh bootstrap.py run on a new machine. After the failure, running ./mach bootstrap fails with the same error.

I also tried removing the clang toolchain archive so that it would re-download, which it did, and still got the same error. The failure looks like this:

 0:04.25 Setting up artifact clang-dist-toolchain.tar.xz
 0:04.26 Downloading artifact to local cache: c:\Users\dapar\.mozbuild\toolchains\9a6f11f6e96f603e-clang-dist-toolchain.tar.xz
 0:04.46 Downloading... 0.0 %
[...]
 0:14.56 Downloading... 100.0 %
 0:03.87 Could not find a toolchain build named `win64-dump-syms`
Error running mach:

    ['bootstrap']

The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.
You can invoke |./mach busted| to check if this issue is already on file. If it
isn't, please use |./mach busted file bootstrap| to report it. If |./mach busted| is
misbehaving, you can also inspect the dependencies of bug 1543241.

If filing a bug, please include the full output of mach, including this error
message.

The details of the failure are as follows:

subprocess.CalledProcessError: Command '['c:/Users/dapar/.mozbuild\\_virtualenvs\\mach\\Scripts\\python.exe', 'c:\\mozilla-src\\mozilla-unified\\mach', 'artifact', 'toolchain', '--bootstrap', '--from-build', 'win64-dump-syms']' returned non-zero exit status 1.

  File "c:\mozilla-src\mozilla-unified\python/mozboot/mozboot/mach_commands.py", line 55, in bootstrap
    bootstrapper.bootstrap(self.settings)
  File "c:\mozilla-src\mozilla-unified\python/mozboot\mozboot\bootstrap.py", line 389, in bootstrap
    self.maybe_install_private_packages_or_exit(state_dir, checkout_root)
  File "c:\mozilla-src\mozilla-unified\python/mozboot\mozboot\bootstrap.py", line 268, in maybe_install_private_packages_or_exit
    self.instance.ensure_dump_syms_packages(state_dir, checkout_root)
  File "c:\mozilla-src\mozilla-unified\python/mozboot\mozboot\mozillabuild.py", line 235, in ensure_dump_syms_packages
    state_dir, checkout_root, dump_syms.WIN64_DUMP_SYMS
  File "c:\mozilla-src\mozilla-unified\python/mozboot\mozboot\base.py", line 417, in install_toolchain_artifact
    subprocess.check_call(cmd, cwd=state_dir)
  File "c:\mozilla-build\python3\lib\subprocess.py", line 347, in check_call
    raise CalledProcessError(retcode, cmd)

This step comes after installing many other packages, otherwise I'd figure it was maybe a down server + a misleading error message.

FYI, this command

./mach artifact toolchain --from-build win64-dump-syms

has the same result. However, running that command on my Mac, which already has a working repo, is successful.

Bug 1719229 broke the install by accident, but configure takes care of
it by default since bug 1716911, bug 1716912 or bug 1717585 depending on
the platform, so we can just stop installing it from mach bootstrap.

Yup, that fixed my issue. Thanks!

https://hg.mozilla.org/mozilla-central/rev/013ecb3d2bf2

Set release status flags based on info from the regressing bug 1719229