Add documentation for debugging [v]python with VSCode

Change-Id: If8268b5526f9e8c294dd63aa365f650f7e0a0a66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2472018Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Ryan Heise <heiserya@google.com>
Auto-Submit: Ryan Heise <heiserya@google.com>
Cr-Commit-Position: refs/heads/master@{#817274}

docs/vscode.md

@@ -25,6 +25,7 @@ Here's what works well:
     well, even though startup times can be fairly high (~40 seconds with
     gdb on Linux, much lower on Windows). You can step through code, inspect
     variables, view call stacks for multiple threads etc.
+    *   For more information on debugging Python code, see [here](vscode_python.md).
 *   Opening files and searching solution-wide works well now after having
     problems in earlier versions.
 *   Building works well. Build tools are easy to integrate. Warnings and errors

docs/vscode_python.md

+# Debugging Chromium Python With The VSCode Debugger
+
+## Before You Begin
+
+1. Patch in [this CL](https://chromium-review.googlesource.com/c/chromium/src/+/2466896).
+2. Run gclient sync.
+
+## Via SSH
+
+SSH is useful if you’re modifying and debugging code on another device, such as
+the desktop sitting at your office desk. To do so:
+
+1. Set up VSCode to work with normal development by following the instructions
+   in the Remote Visual Studio Code section
+   [here](https://docs.google.com/document/d/1ZlG8VQxudxvDs-EtpQvaPVcAPfSMdYlXr42s_487wLo/edit#bookmark=id.j10hyv6nlkws).
+2. Open the Connection Dialog of Chrome’s SSH plugin: ![open
+   dialog](images/vscode_python_connection_dialog.png)
+3. Create a new connection and set the username, hostname, port, and SSH relay
+   server options as you normally would. Then, set SSH arguments to "-2 -L
+   50371:localhost:50371"
+
+    a. You can replace 50371 with a different value, so long as it's consistent
+    with step 7b.
+
+4. Open a connection, and set this window aside.
+5. In VSCode, open the code you want to set a breakpoint in, and add the
+   following:
+
+```
+import debugpy
+
+# Your code here!
+debugpy.listen(50371)
+print("Wait for attach...")
+debugpy.wait_for_attach()
+debugpy.brerakpoint()
+```
+
+Note: The port passed to debugpy.listen() should match the port configured in (3).
+
+6. Click on the Debug tab
+7. Click Run. A dialog will appear asking you to set up a debug configuration.
+   Do so, and select “Remote Debug”.
+
+    a. Leave the hostname as-is
+
+    b. Set the port to 50371
+
+8. Run your program on the remote machine. It should stop executing at “Wait for
+   attach”.
+9. Start the debugger in VSCode. It should attach!
+
+## Locally
+
+1. On the debug tab, on the drop-down next to the play button, select “Add
+   Config”
+2. Add the following to the configurations array in “launch”:
+
+```
+{
+    "name":"Python: Local",
+    "type":"python",
+    "request":"attach",
+    "processId":"${command:pickProcess}"
+}
+```
+
+3. Add the following to your program:
+
+```
+import debugpy
+
+# Your code here!
+
+print("Wait for attach...")
+debugpy.wait_for_attach()
+debugpy.brerakpoint()
+```
+
+4. Start your program.
+5. Start the debugger. A dialog box will pop up asking you to select your
+   running program.