index/db/commands/server: TTL indexes

createIndex({expireAt: 1}, {expireAfterSeconds: 60}) now deletes a
document once its indexed date is that many seconds old.

index.zig carries the option: Index.ttl (?i64), parsed from
expireAfterSeconds (int32/int64/integral double, within MongoDB's
[0, 2147483647]; 0 means "expire at the stored instant"), emitted by
spec_pairs and so persisted through the log and reported by listIndexes,
and compared by spec_equal — a same-name re-create with a different
expiry stays IndexOptionsConflict, as in MongoDB, since collMod does not
exist here. The bound keeps the value an int32 on the wire and makes the
emission cast unconditional. TTL is single-field only (a compound key is
error.TtlOnCompoundIndex); an absent option parses to null, so every
existing log record reparses unchanged.

db.zig sweeps: Engine.ttl_sweep(now_ms) walks each TTL index's entries
and deletes through the ordinary remove path, so an expiry is logged and
fsynced like any other write and holds across a restart. Expired ids are
duped before removal — remove frees the docs-map key that Entry.id
aliases — then sorted and deduped, because one document can be expired
by several entries (an array of dates expires on its earliest member,
which multikey expansion gives for free) or by several TTL indexes.
Selecting entries is a type test rather than a range lookup: bson
compare order ranks datetime above null, numbers and strings, so a
datetime upper bound would also select every value of a lesser type. A
sweep that deleted something checks the compaction threshold, since a
TTL-only workload never reaches the one in upsert.

commands.zig maps the new spec errors: CannotCreateIndex (67) for a
compound key or an out-of-range expiry, and InvalidIndexSpecificationOption
(197) for an expiry on {_id: 1}, which the idempotent _id no-op would
otherwise swallow.

server.zig runs the monitor as a member of the connection group, so the
existing group.cancel tears it down; it sleeps first, then sweeps under
the write lock, and logs rather than dies on a sweep failure.
--ttl-sweep-secs sets the interval (default 60, 0 leaves it unspawned).

Expiry is coarse by design, as in MongoDB: a document stays visible
until the next sweep, and a non-date value at the indexed path never
expires. Unit tests cover the spec round-trip and every rejection, the
sweep (inclusive cutoff, string/missing/future values untouched, second
sweep a no-op) and its survival of a reopen, and two TTL indexes over
one collection. e2e4.js exercises it through the Node driver.

src/main.zig

@@ -8,6 +8,8 @@ const usage =
     \\  --port <n>    listen port (default 27017)
     \\  --bind <ip>   bind address (default 127.0.0.1)
     \\  --db <path>   database file (default mongo-light.log)
+    \\  --ttl-sweep-secs <n>
+    \\                seconds between TTL index sweeps (default 60, 0 disables)
     \\  --help        show this help
     \\
 ;
@@ -16,6 +18,7 @@ pub fn main(init: std.process.Init) !void {
     var port: u16 = 27017;
     var bind_ip: []const u8 = "127.0.0.1";
     var db_path: []const u8 = "mongo-light.log";
+    var ttl_sweep_secs: i64 = 60;
 
     var it = std.process.Args.Iterator.init(init.minimal.args);
     defer it.deinit();
@@ -31,6 +34,16 @@ pub fn main(init: std.process.Init) !void {
             bind_ip = it.next() orelse return error.MissingValue;
         } else if (std.mem.eql(u8, arg, "--db")) {
             db_path = it.next() orelse return error.MissingValue;
+        } else if (std.mem.eql(u8, arg, "--ttl-sweep-secs")) {
+            const v = it.next() orelse return error.MissingValue;
+            // i64 is the width std.Io.Duration.fromSeconds takes, so the
+            // value reaches the sweeper without a cast; negatives are the
+            // only thing parseInt would otherwise let through.
+            ttl_sweep_secs = std.fmt.parseInt(i64, v, 10) catch -1;
+            if (ttl_sweep_secs < 0) {
+                std.debug.print("mongo-light: invalid ttl sweep interval '{s}'\n", .{v});
+                return error.InvalidTtlSweepSecs;
+            }
         } else if (std.mem.eql(u8, arg, "--help") or std.mem.eql(u8, arg, "-h")) {
             try std.Io.File.writeStreamingAll(.stdout(), init.io, usage);
             return;
@@ -53,6 +66,7 @@ pub fn main(init: std.process.Init) !void {
         .connection_counter = .init(1),
         .engine = &engine,
         .start_time = std.Io.Timestamp.now(init.io, .real),
+        .ttl_sweep_secs = ttl_sweep_secs,
     };
     try server.run();
 }