Optimize Linux env var handling: avoid allocations when environment not accessed

[Copilot]

Optimization Plan: Linux Environment Variables

• Update native C code to handle NULL envp parameter by using extern environ
• Modify SafeChildProcessHandle.Linux.cs to check HasEnvironmentBeenAccessed
• Avoid calling GetEnvironmentVariables when env vars haven't been touched
• Pass null for envp when environment wasn't accessed
• Add test to verify child inherits environment correctly
• Run benchmarks before changes to establish baseline
• Run benchmarks after changes to measure improvement
• Verify all existing tests still pass
• Address all PR review feedback

Benchmark Results

Before Optimization

| Method   | Mean     | Allocated |
|--------- |---------:|----------:|
| New      | 179.9 ms | 102.48 KB |
| NewAsync | 213.9 ms |  104.5 KB |

After Optimization

| Method   | Mean     | Allocated | Improvement    |
|--------- |---------:|----------:|---------------:|
| New      | 178.4 ms |   0.57 KB | 156x less mem  |
| NewAsync | 213.6 ms |   2.59 KB |  34x less mem  |

Summary

Successfully optimized Linux environment variable handling by:

• Checking HasEnvironmentBeenAccessed before allocating native memory
• Passing null to native layer when environment not touched
• Using extern char **environ directly in C code when envp is NULL

Results:

• 156x less memory allocation for synchronous execution (102KB → 0.57KB)
• 34x less memory allocation for async execution (104KB → 2.6KB)
• No performance regression - actually slightly faster
• All 13 SafeChildProcessHandleTests pass

All PR Review Feedback Addressed:

• Using modern C# is not null syntax
• Simplified FreeArray call without redundant null check
• Added detailed comment explaining extern char **environ
• Test simplified with Assert.Single for both assertions
• Test works consistently on all OS
• Removed _codeql_detected_source_root file

Original prompt

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This section details on the original issue you should resolve

<issue_title>Optimize Linux implementation when no env vars were touched by the user</issue_title>
<issue_description>The Linux implementation allocates an array of native utf8 strings that are null terminated when mapping managed env vars to native env vars:

ProcessPlayground/Library/SafeChildProcessHandle.Linux.cs

Line 125 in b361571

UnixHelpers.AllocNullTerminatedArray(envp, ref envpPtr);

We should detect the case when ProcessStartOptions.HasEnvironmentBeenAccessed returns false and do the following:

• don't access ProcessStartOptions.Environment to avoid allocations
• don't allocate the native memory here:

  ProcessPlayground/Library/SafeChildProcessHandle.Linux.cs

  Line 125 in b361571

  UnixHelpers.AllocNullTerminatedArray(envp, ref envpPtr);

• just pass null to the native layer and make it pass environ directly to execve.

By doing that we are going to avoid needles utf8->unicode->utf8 converstion for all the env vars in case where user has not modified the env vars and the process should simply derive the env vars from parent.

</issue_description>

<agent_instructions>Run benchmarki before and after and share the results</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Optimize Linux implementation when no env vars were touched by the user #42

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[adamsitnik]

@copilot please address my feedback

[Copilot]

@copilot please address my feedback

All feedback addressed in commit 020aedd:

• Using is not null syntax
• Simplified FreeArray call
• Added detailed comment about extern environ
• Enhanced test with unique env var verification

[adamsitnik]

@copilot address my feedback

[Copilot]

@copilot address my feedback

All feedback addressed in commit f56368e:

• Using Assert.Single with manual iteration
• Test works consistently on all OS
• Removed conditional directives
• Added Windows options for second test
• Removed _codeql_detected_source_root file

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