fix(posix): make pxh app map initialization thread-safe

[katzfey]

Solved Problem

Fixes a POSIX PX4 daemon race where multiple client handler threads can concurrently initialize the static Pxh::_apps map on first command execution.

Solution

Use pthread_once() to initialize the PX4 shell builtin app map exactly once before process_line() or tab completion accesses it. This prevents concurrent first-use calls from both entering init_app_map(_apps) and mutating the same static std::map.

Changelog Entry

For release notes:

Bugfix: prevent POSIX PX4 daemon crash from concurrent shell app map initialization

Alternatives

A mutex around the _apps.empty() / init_app_map() block would also serialize initialization, but pthread_once() more directly models the intended one-time initialization behavior and avoids locking after initialization completes. Eagerly initializing _apps during daemon startup would also avoid the race, but keeping initialization local to Pxh preserves the existing lazy initialization behavior.

Test coverage

Built successfully, runs on voxl2

Context

A downstream crash report showed a segfault with a backtrace through Pxh::process_line(), init_app_map(), and std::_Rb_tree internals, consistent with concurrent mutation of the static builtin app map during early daemon/client startup.

[github-actions]

🔎 FLASH Analysis

px4_fmu-v5x [Total VM Diff: 0 byte (0 %)]

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   -85.7%      -6  [ = ]       0    [Unmapped]
    -0.0%      -7  [ = ]       0    [section .debug_line]
  +0.0%     +40  [ = ]       0    TOTAL

px4_fmu-v6x [Total VM Diff: 0 byte (0 %)]

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%     +55  [ = ]       0    .debug_abbrev
  -0.0%      -2  [ = ]       0    .debug_info
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     +40%      +2  [ = ]       0    [Unmapped]
    -0.0%      -7  [ = ]       0    [section .debug_line]
  +0.0%     +48  [ = ]       0    TOTAL

Updated: 2026-06-11T20:34:33