Problem

The current AppHost orphan detection mechanism only uses the ASPIRE_CLI_PID environment variable to check if the launching Aspire CLI process is still alive. This approach is vulnerable to PID reuse on some systems, where the OS may assign the same PID to a different process after the original CLI process dies. This can cause the AppHost to incorrectly continue running when it should have shut down.

Solution

This PR enhances the orphan detection to be robust against PID reuse by:

1. Adding process start time verification: Introduces a new ASPIRE_CLI_STARTED environment variable that stores the CLI process start time in binary format
2. Robust process checking: The CliOrphanDetector now verifies both PID existence and start time match (with ±1 second tolerance) to conclusively determine if the monitored process is the same original CLI process
3. Backwards compatibility: Gracefully falls back to PID-only logic when the start time environment variable is not present or invalid

Changes Made

Core Implementation

• KnownConfigNames.cs: Added CliProcessStarted constant for the new ASPIRE_CLI_STARTED environment variable
• DotNetCliRunner.cs: Enhanced to set both ASPIRE_CLI_PID and ASPIRE_CLI_STARTED environment variables when launching AppHost processes
• CliOrphanDetector.cs: Added robust IsProcessRunningWithStartTime function and updated detection logic to use both PID and start time verification

Testing

• Added comprehensive test coverage for new scenarios including:
  • Robust detection with start time verification
  • Backwards compatibility fallback
  • PID reuse detection
  • Invalid start time handling
  • Process lifecycle monitoring with enhanced detection

Technical Details

• Uses DateTime.ToBinary() for reliable cross-platform start time serialization
• 1-second tolerance accommodates timing differences in process start detection
• Maintains full backwards compatibility with older CLI versions
• Minimal code changes with surgical precision

Example Scenario

Before: If CLI process (PID 1234) dies and OS reuses PID 1234 for a different process, AppHost incorrectly continues running.

After: AppHost verifies both PID 1234 exists AND has the expected start time. If PID 1234 exists but has a different start time (indicating PID reuse), AppHost correctly shuts down.

Testing

All changes have been thoroughly tested with:

• ✅ Successful compilation of all affected projects
• ✅ Manual verification of core logic (process start time verification, binary serialization)
• ✅ Comprehensive unit test coverage for new functionality
• ✅ Backwards compatibility validation

This enhancement significantly improves the reliability of Aspire's orphan detection while maintaining full backwards compatibility.

This pull request was created as a result of the following prompt from Copilot chat.

Enhance orphan detection in Aspire AppHost to be robust against PID reuse. Currently, the AppHost only uses the ASPIRE_CLI_PID environment variable to detect if the launching Aspire CLI process is still alive, by checking for the PID. This is vulnerable to PID reuse on some systems. The solution is to set an additional environment variable, ASPIRE_CLI_STARTED, which stores the CLI process start time. The AppHost orphan detector (CliOrphanDetector) should check both the PID and the process start time (if present) to conclusively determine if the parent process is the same. If ASPIRE_CLI_STARTED is not present, fallback to PID-only logic for backwards compatibility. Also update relevant tests.

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