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`next/kernel/log_server.erl` (behaviour gen_server) wraps the pure Step 3c.a `log` substrate behind a per-actor process so
concurrent writers serialise through `gen_server:call` instead of racing on the disk segment writer.
API mirrors the pure log substrate:
start_link(ActorId, BasePath) -> Pid
start_link(ActorId, BasePath, Opts) -> Pid %% Opts forwarded to log:open_disk/3
append(Pid, Activity) -> {ok, Seq}
tip(Pid) -> Seq
entries(Pid) -> [Activity, ...]
replay(Pid, InitAcc, Fun) -> Acc
segments(Pid) -> [SegLen, ...]
stop(Pid) -> ok
Per the port's gen_server convention, `gen_server:start_link/2` returns a raw Pid (not `{ok, Pid}`); the API takes the Pid
directly so multiple per-actor servers coexist without a registered-name collision.
`init/1` dispatches on the Opts arg to call either `log:open_disk/2` (default 1 GiB threshold = effectively no rotation) or
`log:open_disk/3` (opt-in `{segment_size, N}`). `handle_call/3` translates each public op to the corresponding pure log call
and threads the new state through.
New `next/tests/log_server.sh` (15 cases):
- API smoke: start_link returns a Pid, single append+tip+entries round-trip, replay/3 chronological, segments visible
through the wrapper, rotation through wrapper with opt-in `{segment_size, 16}`, stop returns ok.
- Five concurrent-writer tests, each: spawn N=3 writers, each firing M=2 appends of `{I, J}`, parent waits on N `{done,_}`
messages via a Y-combinator-shaped receive loop. Assertions cover (a) tip = N*M, (b) length(entries) = N*M, (c) every
`{I, J}` pair appears exactly once via `lists:all/2` membership (no losses, no dupes), (d) reopening from disk via
`log:open_disk/2` reproduces a byte-equal entries list, (e) every writer's index appears in the entries list
(interleaving witnessed).
Erlang-port gotchas worked around this iteration:
(a) Named recursive fun `fun WaitFn(0) -> ok; WaitFn(K) -> ... end` errors as "fun-ref syntax not yet supported" — rewritten
as `fun (_, 0) -> ok; (Self, K) -> ... Self(Self, K - 1) end` then called as `Wait(Wait, N)`.
(b) `lists:foreach/2` isn't registered (only `lists:map/2`) — use `lists:map/2` and discard the result list when running
side-effecting closures.
(c) gen_server message round-trip in this interpreter is ~2s per call, so concurrent N*M was tuned to 6 (`N=3, M=2`) to
keep the whole 15-test suite under 60s wall clock; the test's correctness assertions don't depend on N*M magnitude.
Erlang conformance **761/761** unchanged (log_server.erl is in next/, not lib/erlang/). Step 3c (both .a and .b) now
fully ticked in plans/fed-sx-milestone-1.md.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>