The engine's dlopen of libkrunfw and mke2fs's of libarchive use bare
sonames, and dyld reads DYLD_FALLBACK_LIBRARY_PATH once, at process
startup — a setenv afterwards is invisible to it. When the variable
is unset and the brew kegs exist, the CLI sets it and execv's itself:
exec restarts dyld, which reads the variable fresh. Library consumers
set the variable before their process starts instead (SPEC.md).
The cache-seeding tests seeded entry names under amd64 but resolved
the request against the host arch, so on an arm64 host the lookup
missed and the tests walked into the network path. Requests now pin
arch, matching the seeded names on any host. The tool gate uses
find_tool: on macOS the e2fsprogs binaries are keg-only, never on
PATH.
mke2fs built against libarchive populates an ext4 from a tarball in
one pass; Homebrew's bottle is not built that way, so when the found
mke2fs cannot take a tarball the builder unpacks the rootfs with the
library's own tar decoder and populates from the directory — slower,
and the run says so. The mkfs child is exec'd fresh, so it reads
DYLD_FALLBACK_LIBRARY_PATH (libarchive's keg) at startup.
The engine is libkrun.1.dylib, dlopen'd from the Homebrew kegs when
the loader's own search fails (bare sonames never reach the brew
prefix); the probe becomes a per-platform hypervisor probe — KVM on
Linux, sysctl + a real hv_vm_create on macOS, so an entitlement
denial (HV_DENIED) names the re-sign fix instead of surfacing as an
opaque krun_start_enter EINVAL. Tool lookup (mke2fs, debugfs) gains
the same keg fallback, and dyld_fallback_dirs() collects the kegs the
bare-soname dlopens (libkrunfw, libarchive) need in
DYLD_FALLBACK_LIBRARY_PATH. The machine child's parent-death contract
moves to a kqueue NOTE_EXIT watcher: macOS has no PR_SET_PDEATHSIG,
and SIGKILL-on-parent-exit is what keeps a killed caller from
stranding a booted machine.
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