A command that fails keeps what the engine printed

[glslang]

For windbg-mcp FOLLOWUPS.md item 126: when Execute fails, this crate returns the HRESULT alone and drops the output it captured for the call.

What changed

• DbgEngError::CommandFailedWithOutput { source, output }, read through DbgEngError::output(). The five calls that capture output now answer with it where they answered CommandFailed. That happens when Execute refuses the text or the wait after it fails. The five are execute_command, execute_command_bounded, execute_and_wait, settle and run_to_address.
• The message is CommandFailed's word for word. A caller that renders errors as text sees no change until it asks for output().
• CommandFailed stays. It still covers the failures that had no capture: setting the callbacks, a status read, and the pump inside an open. So branches already open against the old variant keep compiling.
• examples/failed_command_output.rs is the measurement record.

Measured before it was written

The item's open question was whether the engine's reason reaches this call's capture at all. cdb printing the line shows the engine emits it, not that it lands in a DEBUG_OUTCTL_THIS_CLIENT capture. It does land there.

The readings were taken against a user-mode dump on ARM64, with dbgeng 10.0.29617.1000 and 10.0.26100.1742. Both builds answered byte for byte alike, and every failure below is 0x80040205:

command	captured
zzzz	Couldn't resolve error at 'zzz'
? ntdll!Nope, u ntdll!Nope, dd ntdll!Nope	Couldn't resolve error at 'ntdll!Nope'
.echo before the failure; zzzz	before the failure, then the line above
g (through execute_and_wait)	^ No runnable debuggees error in 'g'
||1s, after .opendump of a second dump	^ Illegal debuggee error in '||1s'

Some failures never take this road. These print their complaint and return success:

• dt ntdll!_NOPE prints Symbol ntdll!_NOPE not found.
• !nosuchextension prints No export nosuchextension found.
• dd 0 L4 prints ????????.
• x ntdll!Nope and lm m nope print no match.

The variant is for the failures that do return an error.

Tests

• test_a_refused_command_keeps_what_the_engine_printed drives .echo before the failure; ? nowhere!Nope through execute_command, execute_command_bounded and execute_and_wait against a launched process. For each path it asserts two things are in the capture: the earlier segment's output, and the engine's Couldn't resolve line.
  • It passes on the VM's inbox dbgeng 10.0.26100.1.
  • It fails at each of the three paths with that path's change backed out, one at a time.
• The wait-failure half has no test. That covers settle, and the pump in execute_and_wait and run_to_address. It needs a wait that fails with the target still present and no break raised, which is not something a test can arrange.
• Full suite on the ARM64 VM: 455 passed, 13 ignored, 4 doctests passed.

🤖 Generated with Claude Code

https://claude.ai/code/session_01H7Gnmok17PyEYbcc3aHUm5

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Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-14T11:41:16.664793Z	05bc3df	PR opened

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