fix(procfs): drop stale /proc/<pid> entries after reap

[jiuyue486]

概述

修复进程退出并被回收后，/proc/<pid> 目录项仍可能残留的问题。

问题描述

当前 procfs 会缓存 /proc 下的 pid 目录项。
当进程已经退出并且被 waitpid() 回收后，缓存中的 /proc/<pid> 目录项可能没有及时失效，导致 /proc 视图与实际进程状态不同步。

修复内容

本次修改主要包括：

• 在 /proc 根目录的 populate_children() 中先清理已经失效的 pid 目录缓存，再填充当前仍存在的进程目录
• 在 lookup_child() 中重新检查目标 pid 是否仍然存在；若进程已不存在，则删除缓存项并返回 ENOENT
• 在 /proc/[pid] 目录实现中增加 validate_child()，使进程消失后对应的 pid 子树条目能够正确失效

修复效果

修复后，当进程被回收后，对应的 /proc/<pid> 目录会正确消失，避免 procfs 中出现陈旧的 pid 目录项，使 procfs 行为与实际进程生命周期保持一致。

说明

本次提交仅聚焦于修复 procfs 的 pid 目录同步问题，不包含额外测试或其他功能改动。

[fslongjin]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Validate cached /proc/ entries before returning them

ProcDir::find returns a cached child immediately unless validate_child rejects it (see template/dir.rs), but RootDirOps still uses the default validator, so cached /proc/<pid> entries bypass this new existence check entirely. In practice, if a PID directory was cached before reap, later lookups can still resolve that stale inode without entering this lookup_child path, so /proc/<pid> may remain visible until a separate readdir triggers cache pruning.

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[chatgpt-codex-connector]

Avoid quadratic PID cache pruning in /proc readdir

This cache cleanup performs pid_list.contains(&pid) inside retain, making populate_children() O(number_of_cached_entries × number_of_pids). On systems with many tasks, each /proc directory listing can become disproportionately expensive while holding the cache write lock, causing avoidable latency spikes; converting pid_list to a set before retain would keep pruning linear.

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