trace: show error message when fork fails in TraceScreen

[dengzhongyuan365-dev]

Summary

• Show a user-visible error message (e.g. "Out of memory", "Resource temporarily unavailable") in the TraceScreen when fork() fails, instead of silently doing nothing
• The error message is the OS error string from strerror(errno)

Changes

File	Change
TraceScreen.c	Add TraceScreen_scan() to display the system error when fork fails; register it in the InfoScreenClass struct
Action.c	Remove conditional if (ok) — InfoScreen_run() is always called so the TraceScreen appears even when fork fails

Test Plan

• Press s on a process when fork is likely to fail (e.g. PID quota exceeded, or use ulimit -u to restrict processes)
• Observe that the TraceScreen appears with a one-line error message instead of nothing

Fixes #1991

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: ASSERTIVE

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Run ID: ba876577-1004-4260-9376-f1ce6315de0a

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Reviewing files that changed from the base of the PR and between 39ff873 and 76e7f5d.

📒 Files selected for processing (1)

• TraceScreen.c

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📝 Walkthrough

Walkthrough

Changes

actionStrace now always calls InfoScreen_run after TraceScreen_forkTracer. TraceScreen.c adds a scan callback that emits a strerror(errno) line when tracing is not alive, registers that callback on TraceScreen_class, and preserves errno during fork error cleanup.

Assessment against linked issues

Objective	Addressed	Explanation
Show the TraceScreen after a trace fork failure and display an error line instead of a blank screen [#1991]	✅
Use an errno-derived message such as strerror(errno) for the failure feedback [#1991]	✅

Assessment against linked issues: Out-of-scope changes

Code Change	Explanation
Preserve errno across TraceScreen_forkTracer cleanup (TraceScreen.c lines 138-145)	This is implementation support for error reporting, but it is not stated as a separate objective in the linked issue.

Poem

A trace once failed in quiet gray,
But now a screen will show the way.
If forking stumbles, errno speaks,
And silence leaves the user’s leaks.

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[coderabbitai]

Actionable comments posted: 1

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 1a3d99c7-dfbc-416a-adb3-d19c7295a466

📥 Commits

Reviewing files that changed from the base of the PR and between 1ef4226 and 39ff873.

📒 Files selected for processing (2)

• Action.c
• TraceScreen.c