nxdns/src/storage/querylog_schema.zig

//! The `querylog.db` schema and its open-or-recreate policy.
//!
//! `querylog.db` is never migrated (PLAN §3.7). It holds expendable log rows,
//! so a schema change replaces the file instead of upgrading it. The
//! replacement trigger is a fingerprint derived from the DDL text itself, so
//! editing the schema below automatically invalidates every existing file — the
//! policy cannot drift out of sync with the SQL.
//!
//! **Recreating is destructive, so the predicate is a positive whitelist.** Only
//! a missing file, `error.Corrupt`, `error.NotADb`, a failed `PRAGMA
//! quick_check` and a fingerprint mismatch recreate. Every other error
//! propagates and the file on disk is not touched. `error.Busy` / `error.Locked`
//! mean another process holds the write lock — waiting is right, deleting is
//! catastrophic. `error.OutOfMemory` is this process's problem. `error.CantOpen`
//! is usually a permission or missing-directory problem that recreating would
//! mask rather than fix. Same for `error.ReadOnly`, `error.IoErr`, `error.Full`,
//! `error.Perm`, `error.Auth` and `error.Canceled`.

const std = @import("std");

const db = @import("db.zig");

const log = std.log.scoped(.querylog_schema);

/// PLAN §11.3, plus the coverage watermark of milestone 28. Multi-statement
/// text — it goes through `db.Db.exec`, never through `prepare`.
///
/// The INSERT seeds `querylog_meta`, which is part of the schema
/// rather than a later step: a `query_log` with no watermark beside it cannot
/// answer whether an empty result means "no queries" or "no history", and every
/// database this program reads from is created by executing this string.
/// `unixepoch()` is SQLite's own UTC clock, which is the clock every
/// `timestamp` in the file is measured against.
///
/// `available_since` starts one second *after* `created_at` on purpose. A row
/// logged in the same second the file was created is not evidence that the
/// second is completely covered, and the watermark's whole job is to be
/// conservative. From there it only ever advances, in `queries_repo.pruneOlderThan`.
///
/// The four `bucket_*` tables are the Overview projections (milestone 36), on a
/// 30-minute grain that divides every serving width the API offers. They carry
/// no history of their own: they are born with the file and maintained in the
/// same transaction as every insert and every prune, so SQLite's transaction is
/// the only coherence mechanism there is. There is no backfill path — a file
/// whose projections could disagree with its rows cannot exist.
pub const ddl: [:0]const u8 =
    \\CREATE TABLE domains (
    \\    id INTEGER PRIMARY KEY,
    \\    domain TEXT NOT NULL UNIQUE
    \\);
    \\
    \\CREATE TABLE query_log (
    \\    id INTEGER PRIMARY KEY,
    \\    timestamp INTEGER NOT NULL,
    \\    domain_id INTEGER NOT NULL REFERENCES domains(id),
    \\    client_ip TEXT NOT NULL,            -- text, not a FK: log rows are immutable facts
    \\    qtype INTEGER,
    \\    blocked INTEGER NOT NULL,
    \\    response_time_us INTEGER,
    \\    cache_hit INTEGER,
    \\    upstream TEXT,
    \\    qclass INTEGER NOT NULL,
    \\    rcode INTEGER NOT NULL,
    \\    group_id INTEGER,                   -- text/id pairs, not FKs: a renamed
    \\    group_name TEXT,                    -- group must not rewrite history
    \\    policy_action TEXT NOT NULL,
    \\    policy_reason TEXT NOT NULL,
    \\    matched TEXT,
    \\    source_id INTEGER,
    \\    source_name TEXT,
    \\    cname_target TEXT,
    \\    safe_search_target TEXT,
    \\    route_kind TEXT NOT NULL,
    \\    forward_zone TEXT,
    \\    CHECK (rcode BETWEEN 0 AND 4095)    -- twelve bits (RFC 6891 6.1.3)
    \\);
    \\CREATE INDEX idx_query_log_ts ON query_log(timestamp);
    \\CREATE INDEX idx_query_log_client ON query_log(client_ip);
    \\CREATE INDEX idx_query_log_domain ON query_log(domain_id);
    \\
    \\CREATE TABLE querylog_meta (
    \\    id INTEGER PRIMARY KEY CHECK (id = 1),   -- one row, enforced by the schema
    \\    created_at INTEGER NOT NULL,
    \\    available_since INTEGER NOT NULL
    \\);
    \\INSERT INTO querylog_meta (id, created_at, available_since)
    \\VALUES (1, unixepoch(), unixepoch() + 1);
    \\
    \\CREATE TABLE bucket_totals (
    \\    bucket INTEGER PRIMARY KEY,
    \\    queries INTEGER NOT NULL,
    \\    blocked INTEGER NOT NULL,
    \\    cached INTEGER NOT NULL,
    \\    rt_sum INTEGER NOT NULL,     -- sum(response_time_us) over timed rows
    \\    rt_count INTEGER NOT NULL    -- count(response_time_us)
    \\) WITHOUT ROWID;
    \\
    \\CREATE TABLE bucket_clients (
    \\    bucket INTEGER NOT NULL,
    \\    client_ip TEXT NOT NULL,
    \\    queries INTEGER NOT NULL,
    \\    PRIMARY KEY (bucket, client_ip)
    \\) WITHOUT ROWID;
    \\
    \\CREATE TABLE bucket_types (
    \\    bucket INTEGER NOT NULL,
    \\    qtype INTEGER NOT NULL,      -- -1 encodes a NULL qtype, losslessly
    \\    count INTEGER NOT NULL,
    \\    PRIMARY KEY (bucket, qtype)
    \\) WITHOUT ROWID;
    \\
    \\CREATE TABLE bucket_routes (
    \\    bucket INTEGER NOT NULL,
    \\    route_kind TEXT NOT NULL,
    \\    source_present INTEGER NOT NULL,  -- 0: source NULL; 1: source = source_text
    \\    source_text TEXT NOT NULL,        -- '' when source_present = 0
    \\    count INTEGER NOT NULL,
    \\    PRIMARY KEY (bucket, route_kind, source_present, source_text),
    \\    CHECK (source_present IN (0, 1)),
    \\    CHECK (source_present = 1 OR source_text = '')
    \\) WITHOUT ROWID;
;

/// The fingerprint of an arbitrary DDL text. `tools/cut.zig` calls this at
/// runtime on the DDL of the previous release tag, so the release gate and the
/// server compute the same number from the same function rather than from two
/// copies of one expression.
pub fn fingerprintOf(text: []const u8) i32 {
    return @bitCast(std.hash.Crc32.hash(text));
}

/// `PRAGMA user_version` is a signed 32-bit field. Deriving the fingerprint from
/// the DDL means editing the schema automatically invalidates every existing
/// file — which is exactly the policy.
pub const fingerprint: i32 = blk: {
    // Covers the CRC lookup-table generation in std.hash.crc, which evaluates
    // under this scope's quota and overflows the 1000 default (and 100k).
    @setEvalBranchQuota(2_000_000);
    break :blk fingerprintOf(ddl);
};

const set_user_version = std.fmt.comptimePrint("PRAGMA user_version = {d};", .{fingerprint});

/// Long enough for any path this program will be handed, plus the aside suffix.
/// A longer path is `error.NameTooLong`, which is what the filesystem calls
/// would have returned anyway.
const path_buf_len = 4096 + 64;

pub const RecreateReason = enum { missing, corrupt, not_a_database, quick_check_failed, fingerprint_mismatch };

pub const OpenResult = struct {
    database: db.Db,
    /// Non-null feeds a counter and the `/api/health` rollup.
    recreated: ?RecreateReason,
    /// The path the previous file was kept as, by value. It existed only in a
    /// stack buffer inside `open` before the diagnostics event needed it, and a
    /// slice of that buffer would dangle the moment `open` returned.
    ///
    /// Empty when nothing was renamed aside, which `.missing` and a clean open
    /// both are.
    aside_buf: [path_buf_len]u8 = undefined,
    aside_len: u16 = 0,

    pub fn aside(self: *const OpenResult) []const u8 {
        return self.aside_buf[0..self.aside_len];
    }
};

pub const Error = db.Error || error{AsideNameCollision} ||
    std.Io.Dir.RenamePreserveError || std.Io.Dir.DeleteFileError || std.Io.Dir.AccessError;

/// Opens `path`, recreating it if and only if it is genuinely unusable.
///
/// `path` is resolved twice by two different mechanisms: `dir`-relative for the
/// filesystem calls, and process-cwd-relative by SQLite's VFS, which knows
/// nothing about `dir`. The caller must therefore pass either an absolute path
/// with `dir` open on its parent, or `std.Io.Dir.cwd()` with a cwd-relative
/// path.
pub fn open(io: std.Io, dir: std.Io.Dir, path: [:0]const u8) Error!OpenResult {
    var handle: ?db.Db = null;
    errdefer if (handle) |*h| h.close();

    const reason: ?RecreateReason = probe: {
        dir.access(io, path, .{}) catch |e| switch (e) {
            error.FileNotFound => break :probe .missing,
            else => |other| return other,
        };

        handle = db.Db.open(path, .{ .mode = .read_write_existing }) catch |e|
            break :probe recreatable(e) orelse return e;
        const opened = &handle.?;

        db.applyPragmas(opened, .{}) catch |e|
            break :probe recreatable(e) orelse return e;

        const healthy = quickCheck(opened) catch |e|
            break :probe recreatable(e) orelse return e;
        if (!healthy) break :probe .quick_check_failed;

        const stamped = opened.queryInt("PRAGMA user_version") catch |e|
            break :probe recreatable(e) orelse return e;
        if (stamped != fingerprint) break :probe .fingerprint_mismatch;

        break :probe null;
    };

    const cause = reason orelse return .{ .database = handle.?, .recreated = null };

    // Close first, so SQLite checkpoints and drops `-wal`/`-shm` where it can.
    if (handle) |*h| h.close();
    handle = null;

    var aside_buf: [path_buf_len]u8 = undefined;
    const aside: ?[]const u8 = if (cause == .missing)
        null
    else
        try renameAside(io, dir, path, cause, &aside_buf);

    // Not optional: a stale WAL left beside the renamed database would be
    // replayed into the freshly created file and corrupt it immediately. Any
    // failure other than "already gone" propagates rather than building the new
    // database on a half-cleaned state.
    try deleteSidecars(io, dir, path);

    const fresh = try createFresh(path);
    if (cause == .missing) {
        log.info("created querylog database '{s}'", .{path});
    } else {
        log.warn("recreated querylog database '{s}': {s}; previous file kept as '{s}'", .{
            path,
            @tagName(cause),
            aside.?,
        });
    }
    var result: OpenResult = .{ .database = fresh, .recreated = cause };
    if (aside) |name| {
        result.aside_len = @intCast(name.len);
        @memcpy(result.aside_buf[0..name.len], name);
    }
    return result;
}

/// An additional connection to a `querylog.db` that `open` has already
/// established, with the pragmas every connection to the file needs.
///
/// The canonical opener for every background connection: the log writer, the
/// retention pass and the web task each own one (`retention.zig`'s contract),
/// and a logger generation opens one per writer for that writer's whole life
/// (`logger_controller.zig`) — two writers must never share a handle.
///
/// `dir` and `path` follow `open`'s resolution rule, and `dir` participates in
/// it the same way: the caller passes either an absolute path with `dir` open
/// on its parent, or `std.Io.Dir.cwd()` with a cwd-relative path. Nothing here
/// touches the directory itself — the file already exists by contract — so the
/// handle is present to make the pairing explicit at every call site rather
/// than to be dereferenced.
pub fn reopen(io: std.Io, dir: std.Io.Dir, path: [:0]const u8) db.Error!db.Db {
    _ = io;
    _ = dir;
    var database = try db.Db.open(path, .{ .mode = .read_write_existing });
    errdefer database.close();
    try db.applyPragmas(&database, .{});
    return database;
}

/// The whitelist. `null` means "propagate, do not touch the file".
fn recreatable(e: db.Error) ?RecreateReason {
    return switch (e) {
        error.Corrupt => .corrupt,
        error.NotADb => .not_a_database,
        else => null,
    };
}

/// `PRAGMA quick_check` rather than `integrity_check`: it skips the expensive
/// index-vs-table cross-check while still catching structural damage, and a
/// damaged index on an expendable log is not worth a multi-second startup scan.
fn quickCheck(database: *db.Db) db.Error!bool {
    var stmt = try database.prepare("PRAGMA quick_check");
    defer stmt.deinit();
    if (!try stmt.step()) return false;
    return std.ascii.eqlIgnoreCase(stmt.columnText(0), "ok");
}

/// What the aside file's name calls the reason it was set aside.
///
/// The name is the only account of the reason an operator gets: the log line
/// naming it scrolls away, the file stays for months. `fingerprint_mismatch` is
/// a database with nothing wrong with it — this build's DDL moved — so calling
/// its file "corrupt" invites the operator to delete evidence of a healthy file.
fn asideTag(reason: RecreateReason) []const u8 {
    return switch (reason) {
        .missing => unreachable, // there is no file to rename
        .corrupt => "corrupt",
        .not_a_database => "not-a-database",
        .quick_check_failed => "quick-check-failed",
        .fingerprint_mismatch => "schema-changed",
    };
}

/// Renames the unusable file out of the way and returns the name it now has.
///
/// `renamePreserve` is `RENAME_NOREPLACE`: it returns `error.PathAlreadyExists`
/// instead of overwriting. A previously saved file must never be destroyed by
/// the next recreate, and two recreates in the same second are not hypothetical
/// on a boot loop — hence the uniquifying retries.
fn renameAside(io: std.Io, dir: std.Io.Dir, path: []const u8, reason: RecreateReason, buf: []u8) Error![]const u8 {
    const tag = asideTag(reason);
    const seconds = std.Io.Clock.real.now(io).toSeconds();
    var attempt: u32 = 0;
    while (attempt < 100) : (attempt += 1) {
        const aside = if (attempt == 0)
            std.fmt.bufPrint(buf, "{s}.{s}-{d}", .{ path, tag, seconds }) catch return error.NameTooLong
        else
            std.fmt.bufPrint(buf, "{s}.{s}-{d}-{d}", .{ path, tag, seconds, attempt }) catch return error.NameTooLong;

        dir.renamePreserve(path, dir, aside, io) catch |e| switch (e) {
            error.PathAlreadyExists => continue,
            else => |other| return other,
        };
        return aside;
    }
    return error.AsideNameCollision;
}

fn deleteSidecars(io: std.Io, dir: std.Io.Dir, path: []const u8) Error!void {
    var buf: [path_buf_len]u8 = undefined;
    for ([_][]const u8{ "-wal", "-shm" }) |suffix| {
        const sidecar = std.fmt.bufPrint(&buf, "{s}{s}", .{ path, suffix }) catch return error.NameTooLong;
        dir.deleteFile(io, sidecar) catch |e| switch (e) {
            error.FileNotFound => {},
            else => |other| return other,
        };
    }
}

fn createFresh(path: [:0]const u8) db.Error!db.Db {
    var database = try db.Db.open(path, .{ .mode = .read_write_create });
    errdefer database.close();
    try db.applyPragmas(&database, .{});

    var tx = try db.Tx.begin(&database);
    errdefer tx.rollback();
    try database.exec(ddl);
    try database.exec(set_user_version);
    try tx.commit();

    return database;
}

const testing = std.testing;

test "fingerprint matches a fresh hash of the DDL" {
    try testing.expectEqual(fingerprint, @as(i32, @bitCast(std.hash.Crc32.hash(ddl))));
    // The runtime entry point the release gate uses is the same function the
    // comptime constant is built from.
    try testing.expectEqual(fingerprint, fingerprintOf(ddl));
    try testing.expect(fingerprintOf(ddl[0 .. ddl.len - 1]) != fingerprint);
}

test "ddl creates the query-log tables and every index" {
    var database = try db.Db.open(":memory:", .{ .mode = .memory });
    defer database.close();
    try db.applyPragmas(&database, .{});
    try database.exec(ddl);

    try testing.expectEqual(
        @as(i64, 7),
        try database.queryInt("SELECT count(*) FROM sqlite_schema WHERE type='table'"),
    );
    // The three explicit indexes plus `domains.domain`'s autoindex, and
    // nothing else: the four projection tables are WITHOUT ROWID, so each
    // one's PRIMARY KEY *is* its storage rather than a second b-tree to keep
    // in step on every insert.
    try testing.expectEqual(
        @as(i64, 4),
        try database.queryInt("SELECT count(*) FROM sqlite_schema WHERE type='index'"),
    );
    const objects = [_][]const u8{
        "domains",              "query_log",
        "idx_query_log_ts",     "idx_query_log_client",
        "idx_query_log_domain", "querylog_meta",
        "bucket_totals",        "bucket_clients",
        "bucket_types",         "bucket_routes",
    };
    for (objects) |name| {
        var stmt = try database.prepare("SELECT count(*) FROM sqlite_schema WHERE name = ?1");
        defer stmt.deinit();
        try stmt.bindText(1, name);
        try testing.expect(try stmt.step());
        try testing.expectEqual(@as(i64, 1), stmt.columnInt(0));
    }
}

test "the schema refuses an rcode outside twelve bits" {
    var database = try db.Db.open(":memory:", .{ .mode = .memory });
    defer database.close();
    try db.applyPragmas(&database, .{});
    try database.exec(ddl);
    try database.exec("INSERT INTO domains (id, domain) VALUES (1, 'a.example');");

    var stmt = try database.prepare(
        \\INSERT INTO query_log
        \\  (timestamp, domain_id, client_ip, blocked, qclass, rcode,
        \\   policy_action, policy_reason, route_kind)
        \\VALUES (1, 1, '10.0.0.1', 0, 1, ?1, 'not_evaluated', 'no_match', 'upstream')
    );
    defer stmt.deinit();

    // The whole range an EDNS extended RCODE can express, and nothing wider:
    // the producers are `u12`, and this is what stops any other writer — a
    // hand-run UPDATE included — from putting a value in the column that the
    // read path would have to reject.
    for ([_]i64{ 0, 4095 }) |accepted| {
        try stmt.reset();
        try stmt.bindInt(1, accepted);
        try stmt.exec();
    }
    for ([_]i64{ -1, 4096, 65535 }) |refused| {
        // `sqlite3_reset` repeats the error of the statement it is resetting,
        // which for every iteration after the first is the constraint failure
        // this loop just asserted — the same reason `BatchWriter.resetAll`
        // discards it.
        stmt.reset() catch {};
        try stmt.bindInt(1, refused);
        try testing.expectError(error.Constraint, stmt.exec());
    }

    try testing.expectEqual(@as(i64, 2), try database.queryInt("SELECT count(*) FROM query_log"));
}

test "querylog_meta is seeded with one row the schema will not let a second join" {
    var database = try db.Db.open(":memory:", .{ .mode = .memory });
    defer database.close();
    try db.applyPragmas(&database, .{});
    try database.exec(ddl);

    try testing.expectEqual(@as(i64, 1), try database.queryInt("SELECT count(*) FROM querylog_meta"));

    const created = try database.queryInt("SELECT created_at FROM querylog_meta");
    const since = try database.queryInt("SELECT available_since FROM querylog_meta");
    // Conservative by exactly one second: a row logged in the creating second
    // must not let a query claim that second is completely covered.
    try testing.expectEqual(created + 1, since);
    try testing.expect(created > 1_700_000_000);

    // `CHECK (id = 1)` is what makes "the singleton row" a schema fact rather
    // than a convention the read path has to defend against.
    try testing.expectError(error.Constraint, database.exec(
        "INSERT INTO querylog_meta (id, created_at, available_since) VALUES (2, 1, 1);",
    ));
    try testing.expectError(error.Constraint, database.exec(
        "INSERT INTO querylog_meta (id, created_at, available_since) VALUES (1, 1, 1);",
    ));
    try testing.expectEqual(@as(i64, 1), try database.queryInt("SELECT count(*) FROM querylog_meta"));
}

test "the user_version statement stamps the fingerprint" {
    var database = try db.Db.open(":memory:", .{ .mode = .memory });
    defer database.close();
    try database.exec(set_user_version);
    try testing.expectEqual(@as(i64, fingerprint), try database.queryInt("PRAGMA user_version"));
}

// The behaviour these two cases describe — a resource error leaves the file on
// disk alone — is proven against a real file by "S7 case 23" in
// `storage_integration_test.zig`, which locks a healthy `querylog.db` from a
// second connection and asserts `open` returns `error.Busy` with the bytes, the
// file and the absence of an aside all intact. `error.OutOfMemory` has no such
// case: `open` takes no allocator, and SQLite allocates through its own global
// allocator, so there is no seam to inject a failure through. The two tests
// below are what covers it.
test "recreatable is a whitelist and never selects a resource error" {
    try testing.expectEqual(RecreateReason.corrupt, recreatable(error.Corrupt).?);
    try testing.expectEqual(RecreateReason.not_a_database, recreatable(error.NotADb).?);
    const propagating = [_]db.Error{
        error.Busy,       error.Locked, error.OutOfMemory, error.CantOpen,
        error.ReadOnly,   error.IoErr,  error.Full,        error.Perm,
        error.Auth,       error.Misuse, error.Constraint,  error.SqliteError,
        error.Unexpected,
    };
    for (propagating) |e| {
        try testing.expect(recreatable(e) == null);
    }
}

test "the aside name says why, and a healthy file is never called corrupt" {
    try testing.expectEqualStrings("corrupt", asideTag(.corrupt));
    try testing.expectEqualStrings("not-a-database", asideTag(.not_a_database));
    try testing.expectEqualStrings("quick-check-failed", asideTag(.quick_check_failed));
    try testing.expectEqualStrings("schema-changed", asideTag(.fingerprint_mismatch));
}

test "recreatable selects exactly two of db.Error's members" {
    // Exhaustive over the whole set, so a variant added to `db.Error` later
    // defaults to propagate. The list above only proves the named errors are
    // safe today; this proves nothing else can join the whitelist unnoticed.
    var whitelisted: usize = 0;
    inline for (@typeInfo(db.Error).error_set.?) |member| {
        if (recreatable(@field(db.Error, member.name)) != null) whitelisted += 1;
    }
    try testing.expectEqual(@as(usize, 2), whitelisted);
}

test "a recreate returns the aside name by value and a fresh create returns none" {
    var threaded: std.Io.Threaded = .init(testing.allocator, .{});
    defer threaded.deinit();
    const io = threaded.io();

    var tmp = testing.tmpDir(.{ .iterate = true });
    defer tmp.cleanup();

    var path_buf: [path_buf_len]u8 = undefined;
    const path = try std.fmt.bufPrintZ(&path_buf, ".zig-cache/tmp/{s}/querylog.db", .{tmp.sub_path});

    // First open: the file is missing, so nothing is renamed aside. The
    // one-shot event is deliberately not emitted for this case.
    var created = try open(io, std.Io.Dir.cwd(), path);
    created.database.close();
    try testing.expectEqual(RecreateReason.missing, created.recreated.?);
    try testing.expectEqualStrings("", created.aside());

    try tmp.dir.writeFile(io, .{ .sub_path = "querylog.db", .data = "not a database at all" });

    var recreated = try open(io, std.Io.Dir.cwd(), path);
    recreated.database.close();
    try testing.expectEqual(RecreateReason.not_a_database, recreated.recreated.?);
    try testing.expect(recreated.aside().len != 0);

    // The name is a real file, which is the whole reason it travels out.
    const kept = std.fs.path.basename(recreated.aside());
    try tmp.dir.access(io, kept, .{});
}

test "a recreate resets coverage to the new file and keeps the old one aside" {
    var threaded: std.Io.Threaded = .init(testing.allocator, .{});
    defer threaded.deinit();
    const io = threaded.io();

    var tmp = testing.tmpDir(.{ .iterate = true });
    defer tmp.cleanup();

    var path_buf: [path_buf_len]u8 = undefined;
    const path = try std.fmt.bufPrintZ(&path_buf, ".zig-cache/tmp/{s}/querylog.db", .{tmp.sub_path});

    var created = try open(io, std.Io.Dir.cwd(), path);
    const first_coverage = try created.database.queryInt("SELECT available_since FROM querylog_meta");
    // A row in the file the operator is about to lose.
    try created.database.exec("INSERT INTO domains (domain) VALUES ('old.example');");
    created.database.close();

    // A healthy file this build's DDL no longer matches — the case milestone
    // 28's own schema edit produces on every upgrade.
    {
        var stamped = try db.Db.open(path, .{ .mode = .read_write_existing });
        defer stamped.close();
        var sql_buf: [64]u8 = undefined;
        try stamped.exec(try std.fmt.bufPrintZ(&sql_buf, "PRAGMA user_version = {d};", .{fingerprint +% 1}));
    }

    var recreated = try open(io, std.Io.Dir.cwd(), path);
    defer recreated.database.close();

    try testing.expectEqual(RecreateReason.fingerprint_mismatch, recreated.recreated.?);
    // The name says the file was healthy and this build moved, not that it rotted.
    try testing.expect(std.mem.indexOf(u8, recreated.aside(), ".schema-changed-") != null);
    try tmp.dir.access(io, std.fs.path.basename(recreated.aside()), .{});

    // Exactly one meta row, and coverage starts at the recreate rather than
    // carrying the replaced file's promise forward.
    try testing.expectEqual(
        @as(i64, 1),
        try recreated.database.queryInt("SELECT count(*) FROM querylog_meta"),
    );
    const new_coverage = try recreated.database.queryInt("SELECT available_since FROM querylog_meta");
    try testing.expect(new_coverage >= first_coverage);

    // Nothing of the old file came across: the history is genuinely gone, which
    // is what the coverage start has to tell the operator.
    try testing.expectEqual(
        @as(i64, 0),
        try recreated.database.queryInt("SELECT count(*) FROM domains"),
    );
}

test "a clean reopen reports no recreate and no aside" {
    var threaded: std.Io.Threaded = .init(testing.allocator, .{});
    defer threaded.deinit();
    const io = threaded.io();

    var tmp = testing.tmpDir(.{ .iterate = true });
    defer tmp.cleanup();

    var path_buf: [path_buf_len]u8 = undefined;
    const path = try std.fmt.bufPrintZ(&path_buf, ".zig-cache/tmp/{s}/querylog.db", .{tmp.sub_path});

    var first = try open(io, std.Io.Dir.cwd(), path);
    first.database.close();

    var second = try open(io, std.Io.Dir.cwd(), path);
    second.database.close();
    try testing.expectEqual(@as(?RecreateReason, null), second.recreated);
    try testing.expectEqualStrings("", second.aside());
}