dl-d2-segmented-resume: completed & merged (2026-10-09)

Auto-pilot continuation after a context compaction mid-task. The compaction

note said download.c "ends at seg_filled()" — in fact the previous session had

gotten much further (1124-line engine, 1007-line test file) but 7 of 74 tests

were failing. The last mile was diagnosis, not construction.

The one real gotcha: retry_wait_sec=0 means "use default"

Three tests (T2 kill/resume, T4 etag, T5 retry) hung in SEG_RETRYING forever.

Trace showed retrying until X (now X) — the deadline was set but the tests

kept polling. Root cause: the frozen contract says `retry_wait_sec: backoff

base (default 2s, doubles), so opts_apply_defaults()` turns 0 into 2 —

"0 = unset" in C convention. Tests that wanted no wall-clock backoff had to

set the engine's documented SXDL_NO_BACKOFF=1 env knob instead. One setenv

in main() and 74/74.

Lesson: when a struct field has a documented default, 0 almost never means

"none" — it means "not specified". Check which before writing tests.

the segs array out, never freed the temp → 480 bytes per download.

-Wformat-truncation. Replaced with bounded memcpy (also faster).

stack-use-after-scope. Make fixture buffers static.

The engine "supported" Range but T1's recorded requests showed none:

sxdlh_request_build set header_count = 0 with a comment "unused by io".

The io vtable does read headers. Added an indexable array view to sxdlh_hdrs

(rebuilt on add/build, pointers into the arena). That one fix is why multi-

connection downloads were silently single-stream.

src/backend/fs_posix.c (real fs backend with rename → atomic .sxdl saves) and

a full cmd/dl CLI verb. scorpiox-dl dl --url … --continue now works end to

end once D6 lands the curl transport.

Fast-forward-able, merged --no-ff to main ad797f5, re-verified on a fresh

clone: test rc=0, build rc=0, asan rc=0 with zero diagnostics.

Known wart documented for D6: make asan builds yyjson.o without -fPIC, so a

later non-asan .so link fails until build/vendor/yyjson.o is removed.