[WSLC] Stop state-aware exec failures from corrupting the output they report on

[Soham Das (SohamDas2021)]

📖 Description

Two ways a state-aware exec reported something untrue about a run it had already finished.

A failed exec corrupted its own output. The error envelope was appended to the stdout the script had already been streaming to, leaving neither valid JSON nor clean program output: PRE_FAILURE_MARKER{"error":{"code":"backend_error",...}}. It now goes to stderr on its own line, so stdout belongs solely to the script and the envelope survives a script whose own stderr ends mid-line. Non-exec phases and exec dry-run stream nothing, so their envelope stays on stdout.

A truncated exec overwrote the reason it ended. Dropping output replaced the terminal outcome: a clean run lost its exit code, and a timed-out or cancelled run lost the truncation notice entirely — callers were told the output was complete when it was not. Truncation now travels alongside the outcome, so a timeout still reports as a timeout and a truncated clean run carries its exit code in details.exitCode.

Also corrects two docs that described the old behaviour: wslc-state-aware.md presented the overwriting as intentional, and the API doc's SDK-discrimination section still sent consumers to stdout for the error envelope.

🔗 References

Addresses defect 3 of #843. The truncation defects were surfaced while analysing it; the second one became reachable on every terminal variant when #1252 added TimedOut and Cancelled.

Of #843's four defects: 1 is closed (#1187, #954), 2's output half is closed (#1252) and its stdin half is blocked on the WSLc SDK surfacing WSLCProcessFlagsStdin (#1258), 4 is closed (#1167 → #1240). This is 3.

🔍 Validation

• cargo fmt --all -- --check and cargo clippy --all-targets --features wslc -- -D warnings clean across wslc_common, wxc_wslc_daemon, wxc, and mxc_engine.
• cargo test --workspace, plus new unit tests at each seam: the stream-selection branch in the executor, the daemon's frame sequence for every terminal variant, and the exit code surviving a truncation failure.
• On a real WSL2 host: run_wslc_state_aware_tests.ps1, 83/83.
• Both stderr-framing guarantees have E2E regression guards, and both were verified to fail against a pre-fix binary rather than passing vacuously: a command that emits output and then fails (82/83 pre-fix), and a command whose last stderr write has no trailing newline (82/83 pre-fix).
• Truncation has no E2E coverage — provoking it means out-racing the daemon's bounded queue, which is inherently racy from PowerShell. It is covered by unit tests against in-memory duplex pipes only, so that behaviour has not been exercised against a real container.

PROTOCOL_VERSION goes 5 → 6 for the new frame. The persistent per-user daemon is refused by a newer client until it restarts, which is the existing exact-equality check working as intended.

✅ 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 docs/pull-requests.md)

📋 Issue Type

• Bug fix
• Feature
• Task

[azure-pipelines]

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[Copilot]

Copilot review overview

🟡 Changes recommended

Error-envelope routing remains inconsistent for parse failures, and raw stderr cannot safely distinguish workload output from typed errors.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Workload stderr can spoof the typed error dispatch marker · New
• Inconsistent error channel for pre-dispatch request failures · New

What changed in this PR

Prevents WSLc state-aware exec failures from corrupting workload output while preserving truncation and terminal outcomes.

Changes:

• Routes live-exec error envelopes to stderr.
• Adds protocol-level truncation notices without replacing terminal outcomes.
• Updates regression tests and lifecycle documentation.

File	Description
tests/​scripts/​run_wslc_state_aware_tests.ps1	Tests stderr framing and output isolation.
tests/​scripts/​run_isolation_session_state_aware_tests.ps1	Updates exec-error envelope parsing.
src/​core/​wxc/​src/​main.rs	Routes non-dry-run exec errors to stderr.
src/​backends/​wslc/​daemon/​src/​control_server.rs	Emits truncation before terminal frames.
src/​backends/​wslc/​common/​src/​state_aware.rs	Preserves outcomes and truncated exit codes.
src/​backends/​wslc/​common/​src/​daemon_protocol.rs	Adds Truncated and bumps protocol version.
src/​backends/​wslc/​common/​src/​daemon_client.rs	Tracks truncation independently of completion.
docs/​wsl/​wslc-state-aware.md	Documents revised WSLc behavior.
docs/​state-aware-lifecycle/​mxc-state-aware-sandbox-api.md	Updates executor stream semantics.
docs/​isolation-session/​state-aware-rust.md	Documents stderr error envelopes.

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[Jeff Whiteside (jsidewhite)]

write_frame

won't this go away when we move away from framed output?

[Jeff Whiteside (jsidewhite)]

🕐

[Jeff Whiteside (jsidewhite)]