Fix stdin relay for pipes in IsoSession

[Dom Giandinoto (daamenik)]

📖 Description

For a given sandbox, relays copy stdin bytes to the sandbox and stdout/stderr bytes back. There are two stdin relay types: a console-oriented one that listens for a stop event, and a pipe-oriented one that runs until EOF or until a cancel switch is fired.

The isolation_session backend chose the stdin relay type from the interactivity of stdout, not stdin. So piped input was handed the stop-aware console relay instead of the pipe-friendly one. Two consequences:

• Delivery failure (user-visible): the console relay's first console read fails on a pipe handle, so piped input was never read at all — the workload blocked waiting for input it never received.
• Latent handle leak: at teardown the console relay can't be cancelled, so its thread stays blocked and never releases the duplicated write handle to the agent's stdin.

This change classifies the stdin handle at runtime and routes pipe/file stdin to the cancellable pipe relay, so piped I/O works and the destination handle is released on teardown.

🔗 References

🔍 Validation

Manual repro (console stdout + piped stdin): before = no output, ~30s hang, non-zero exit; after = input echoed, prompt exit 0

• Config's command line read a line from stdin, echoed it back in the format "SANDBOX_STDIN="
• 'HELLO FROM CALLER' | .\wxc-exec.exe --experimental $config 2>&1 ; "EXIT=$LASTEXITCODE"

✅ Checklist

• Signed the Contributor License Agreement
• Linked to an issue
• Updated documentation (if applicable)
• Updated Copilot instructions (if build, architecture, or conventions changed)
• If this PR changes Cargo.lock, the dependency-feed-check check passes (see pull request builds)

📋 Issue Type

• Bug fix
• Feature
• Task

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[Copilot]

🟡 Changes recommended

Natural stdin EOF is not propagated to workloads that read until EOF, potentially causing an indefinite wait.

1 open finding

• Propagate source EOF to prevent workloads hanging on stdin · New

What changed in this PR

Fixes IsolationSession stdin relay selection for redirected pipe/file input.

Changes:

• Classifies stdin by its actual Windows handle type.
• Uses cancellable relays for pipe/file stdin.
• Adds cancellation and destination-handle release coverage.

File	Description
pipe_relay.rs	Documents redirected-stdin behavior and tests cancellation cleanup.
manager.rs	Selects and tears down stdin relays by handle type.

🧠 Review effort: Balanced

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