Modules with two leading slashes can't be mapped properly in VS Code debugger #1061

Hi, this is a follow-up to a discussion: microsoft/vscode-python#19835

EDIT: See microsoft/vscode#161498 for resolution.

Environment data

• debugpy version: 1.6.3
• OS and version: CentOS 7.9, macOS 11.6.2 (Big Sur)
• Python version : Tested on 3.9 (macOS), 3.7 and 3.6 (CentOS)
• Using VS Code or Visual Studio: VS Code

Actual behavior

In VS Code, breakpoint is hit, but path-mapping does not work with an error in the editor pane.

The editor could not be opened because the file was not found.

Creating the file tries to mkdir /vscode-path and fails.

In a more complex example that I couldn't find a MWE for, the Call Stack pane in VS Code says:

[UriError]: If a URI does not contain an authority component, then the path cannot begin with two slash characters ("//")

Expected behavior

Breakpoint is hit and the path-mapping works.

Steps to reproduce:

Create a file like the following:

$ cat /tmp/vscode-path/vscode_blah.py 
import sys
import pprint
pprint.pprint(sys.path)

import debugpy
debugpy.listen(5678)
print("WAIT")
debugpy.wait_for_client()

print("HEY")

Create a standard "remote attach" debug configuration.

{
        "host": "localhost",
        "name": "Remote Attach",
        "port": 5678,
        "request": "attach",
        "type": "python"
}

In the VS Code editor, set a breakpoint on the final print().

Try to import it via a PYTHONPATH with a leading double slash.

$ PYTHONPATH=//tmp/vscode-path python3 -c "import vscode_blah"

Attach via the debugger.