Fix bootstrap.py script to not depend on tools/shared.py

[juj]

Fix bootstrap.py script to not depend on tools/shared.py so that it can be run during emsdk installation phase when no .emscripten config yet exists. Fixes emscripten-core/emsdk#1545

[sbc100]

I wonder we add test for this in test_bootstrap in test_sanify.py

[juj]

I wonder we add test for this in test_bootstrap in test_sanify.py

Added a sanity test. It is quite convoluted to test since config.py tries its best to locate Emscripten tools via so many different means. But looks like it worked.

[sbc100]

Can you just do @with_env_modify({'EM_CONFIG': "/path/does/not/exist"}) maybe?

[juj]

Sure - folded that to the other env. modify code.

[sbc100]

But can you literally just write this test as:

@with_env_modify({'EM_CONFIG': "/path/does/not/exist"})
 def test_bootstrap_without_em_config(self):
   # Verify that bootstrap works even without a valid config file.
   self.run_process([shared.bat_suffix(shared.path_from_root('bootstrap'))])

[sbc100]

(verified that that test fails as written without this PR).

[juj]

But can you literally just write this test as:

No, that doesn't work. It didn't work for me, but falsely passed the test.

That is because

emscripten/tools/config.py

Line 85 in 52815ef

path = shutil.which(tool_binary)

goes and finds clang.exe in PATH, and then assigns LLVM_ROOT based on that, sidestepping the problem.

Those env. removals are important to make sure to match the clean install scenario.

[juj]

Oh I see, the behavior of from tools import shared is different with

@with_env_modify({'EM_CONFIG': "/path/does/not/exist"})

versus

@with_env_modify({'EM_CONFIG': None})

It is better to test the scenario that first-time emsdk install also exercises, when there are no env. vars yet set. This way the test matches what happens on a new installation, and won't rely on EM_CONFIG pointing to a nonexistent file (emsdk doesn't do that either on first-time install)

[sbc100]

Fair enough. I think its probably overkill but I get that there could be difference.