[Emulator] Fix the emulator hub locator missing bug when emulators:start run concurrently

[fredzqm]

Issue

When emulator is started quickly, there is a race condition that leaves the hub locator file deleted with active emulator process running.

We can easily reproduce this bug by clicking "run local" button repeatedly quickly. Reproduce

1. p1 firebase emulator:start
2. p1 write locator file
3. p2 firebase emulator:start
4. p2 write locator file
5. p2 exits due to port occupied
6. p2 deletes the locator file

....
p1 continues to run without a locator file, so VS Code don't know about it.

Description

Once a process wrote the locator file, subsequent emulator shouldn't override it.
Add pid to the locator file to ensure a process can only delete its own locator.

1. Read existing file
2. Check if another process is active based on locator.pid.
3. Write the file with {version, origins, pid}.
4. Delete the locator file only if the process wrote it.

There is still a possibilities of race condition if between step 1 and 3. The chances are much smaller than prior.

data-connect.entry.tsx‎ is not used

When "run local" is clicked, try to connect to the emulator first before start one.

[gemini-code-assist]

Summary of Changes

Hello @fredzqm, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces several key improvements to the Firebase VS Code extension's emulator management, primarily addressing issues related to the emulator hub file and preventing multiple emulator instances. It refactors the emulator startup sequence to be more resilient, ensures proper cleanup of locator files, and streamlines event handling for starting emulators. Additionally, a specific Data Connect webview entry file has been removed, indicating a potential restructuring of the Data Connect UI.

Highlights

• Prevent Duplicate Emulator Starts: The startEmulators function now includes a check to ensure emulators are not already running before attempting to start them, preventing redundant operations when the 'run local' command is clicked repeatedly.
• Improved Emulator Hub File Management: The emulator hub's locator file now includes the process ID (PID) and is managed more robustly. It checks for existing live emulator processes before writing a new file and automatically cleans up the file upon process termination (SIGINT, SIGTERM, exit).
• Refactored Emulator Start Event Handling: The direct broker.on("runStartEmulators") listener in firebase-vscode/src/data-connect/terminal.ts has been removed, centralizing the emulator start logic within the EmulatorsController.
• Removed Data Connect Webview Entry: The firebase-vscode/webviews/data-connect/data-connect.entry.tsx file has been entirely removed, suggesting a refactor or removal of a Data Connect webview component.

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[gemini-code-assist]

Code Review

This pull request provides a robust solution to prevent issues when the 'run local' command for emulators is triggered multiple times. The changes in firebase-vscode/src/core/emulators.ts correctly add a guard to prevent re-starting the emulators if they are already running. More importantly, the modifications in src/emulator/hub.ts significantly improve the management of the emulator hub's locator file. By adding the process ID (pid) to the locator file and checking if the process is still live, you've effectively prevented conflicts between multiple or stale emulator instances. The move to handle locator file cleanup using process exit signals (SIGINT, SIGTERM, exit) is a much more reliable approach than the previous implementation. Overall, these are excellent, high-quality changes that increase the stability of the emulator startup process.