[wan9chi]

Motivation

The exec interposers copied the program, arguments and environment into std BStrings and Vecs (RawExec::to_exec), resolved and rewrote them there (PATH search, shebang, preload variables), and copied them back out (RawExec::from_exec), all through libc malloc. Exec may run in the child of fork(), where malloc's lock may be held by a thread that no longer exists. Only the final pointer arrays used the interception's bump.

Change

• Exec, Shebang, ensure_env and append_path_env in fspy_shared_unix are generic over an allocator; strings are allocator_api2::vec::Vec<u8, A>, and exec::to_vec_in copies bytes into one. (<[u8]>::to_vec_in as a method trips unstable_name_collisions on nightly.)
• The preload builds the Exec in the interception's bump, so every string derived from it stays there. The supervisor's get_exec/set_exec use Global.
• Callbacks that existed only to keep memory alive return values instead:
  • which returns the found path in the allocator, building each candidate in one reused buffer instead of an alloca buffer handed to a callback; stackalloc is no longer used.
  • RawExec::from_exec leaks the C strings and pointer arrays into the Exec's allocator and returns the RawExec, instead of nesting closures to keep owned strings alive.
  • Client::handle_exec borrows the caller's allocator and returns the arguments to forward with the pre-exec step, so the exec and posix_spawn interposers call the real function directly.

std::path::absolute still allocates on this path; the PR stacked on this one replaces it.

🤖 Generated with Claude Code

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.62%  [ -2.85% ..  +4.85%]  overhead   +56.04%
dynamic/access             change  +0.42%  [ -2.95% ..  +3.46%]  overhead   +19.75%
dynamic/access-relative    change  -2.71%  [ -5.85% ..  +1.22%]  overhead   +69.91%
dynamic/access-contended   change  +0.23%  [ -8.13% ..  +6.67%]  overhead   +56.30%
static/launch              change  -0.21%  [ -2.72% ..  +2.66%]  overhead  +190.33%
static/access              change  -0.74%  [ -1.59% ..  +0.44%]  overhead +1379.04%
static/access-relative     change  -0.28%  [ -1.18% ..  +0.82%]  overhead +1874.75%
static/access-contended    change  +0.40%  [ -1.47% ..  +1.28%]  overhead +1883.35%

macos

dynamic/launch             change  -0.32%  [ -5.00% ..  +4.57%]  overhead  +221.16%
dynamic/access             change  +0.58%  [-13.21% .. +47.46%]  overhead    +2.40%
dynamic/access-relative    change  +1.86%  [ -1.03% ..  +5.60%]  overhead  +254.14%
dynamic/access-contended   change  +2.32%  [ -2.58% ..  +7.19%]  overhead    +5.58%

windows

dynamic/launch             change  +0.65%  [ -5.12% ..  +5.62%]  overhead   +24.50%
dynamic/access             change  +0.54%  [ -4.60% ..  +4.63%]  overhead    +0.92%
dynamic/access-relative    change  +0.18%  [ -1.24% ..  +2.39%]  overhead    +1.65%
dynamic/access-contended   change  +0.73%  [ -7.68% .. +13.79%]  overhead    +2.27%