//! Whole-database snapshot mode.
//!
//! Produces a single, internally-consistent copy of KoboReader.sqlite and uploads it.
//!
//! Why a snapshot rather than copying the raw files: Nickel keeps the database in WAL mode,
//! and recent commits live in `KoboReader.sqlite-wal` until a checkpoint. Copying the bare
//! `.sqlite` silently loses hours of reading progress (measured: 19h of drift on a live
//! device). Copying all three files instead is non-atomic — a checkpoint landing mid-copy
//! yields a pre-checkpoint main file plus a post-reset WAL, losing data with no error.
//!
//! `VACUUM INTO` runs inside a read transaction, so it sees WAL-resident commits and emits
//! one compacted `journal_mode=delete` file. One file, one consistent read, no reassembly.

const std = @import("std");
const c = @cImport({
    @cInclude("sqlite3.h");
});

pub const default_snapshot_path = "/tmp/kobo_snapshot.sqlite";
pub const default_timeout_secs: u64 = 120;

/// Upper bound on a snapshot we're willing to buffer. The live database is ~2.4 MB; this
/// is a guard against pathological growth, not a real expectation.
const max_snapshot_bytes: usize = 256 * 1024 * 1024;

/// Hard upper bound on the whole run.
///
/// v1 hung for 16 days: the server stopped responding and `std.http.Client` has no timeout,
/// so the process blocked forever *while holding the SQLite handle open*. This guarantees the
/// process dies instead. Snapshotting can stall too (a locked database), so the watchdog
/// covers the entire run, not just the upload.
fn watchdog(secs: u64) void {
    std.Thread.sleep(secs * std.time.ns_per_s);
    std.debug.print("timeout: exceeded {d}s, aborting\n", .{secs});
    std.process.exit(2);
}

/// Arm the watchdog for the current process. Every mode needs this, not just snapshot:
/// legacy highlights mode POSTs to the same kind of endpoint and hangs the same way.
pub fn startWatchdog(secs: u64) !void {
    const guard = try std.Thread.spawn(.{}, watchdog, .{secs});
    guard.detach();
}

fn sha256Hex(data: []const u8, out: *[64]u8) void {
    var digest: [32]u8 = undefined;
    std.crypto.hash.sha2.Sha256.hash(data, &digest, .{});
    const hex = "0123456789abcdef";
    for (digest, 0..) |byte, i| {
        out[i * 2] = hex[byte >> 4];
        out[i * 2 + 1] = hex[byte & 0x0F];
    }
}

/// SQL string literals escape a single quote by doubling it.
fn quoteSqlLiteral(allocator: std.mem.Allocator, s: []const u8) ![]u8 {
    var out: std.ArrayList(u8) = .empty;
    errdefer out.deinit(allocator);
    for (s) |ch| {
        if (ch == '\'') try out.append(allocator, '\'');
        try out.append(allocator, ch);
    }
    return out.toOwnedSlice(allocator);
}

fn deviceName(buf: *[std.posix.HOST_NAME_MAX]u8) []const u8 {
    return std.posix.gethostname(buf) catch "kobo";
}

/// Write a consistent snapshot of `db_path` to `snap_path`, replacing any existing file.
fn writeSnapshot(allocator: std.mem.Allocator, db_path: []const u8, snap_path: []const u8) !void {
    // VACUUM INTO refuses to overwrite. A leftover file from a killed run would otherwise
    // make every subsequent sync fail.
    std.fs.cwd().deleteFile(snap_path) catch |err| switch (err) {
        error.FileNotFound => {},
        else => return err,
    };

    const db_path_z = try allocator.dupeZ(u8, db_path);
    defer allocator.free(db_path_z);

    var db: ?*c.sqlite3 = null;
    if (c.sqlite3_open(db_path_z.ptr, &db) != c.SQLITE_OK) {
        std.debug.print("failed to open database: {s}\n", .{c.sqlite3_errmsg(db)});
        _ = c.sqlite3_close(db);
        return error.DatabaseOpenFailed;
    }
    // Closed explicitly below rather than by defer: the handle must be released before any
    // network I/O so a stalled upload can never pin the database, its -wal and its -shm.
    errdefer _ = c.sqlite3_close(db);

    const escaped = try quoteSqlLiteral(allocator, snap_path);
    defer allocator.free(escaped);
    const sql = try std.fmt.allocPrintSentinel(allocator, "VACUUM INTO '{s}';", .{escaped}, 0);
    defer allocator.free(sql);

    var errmsg: [*c]u8 = null;
    if (c.sqlite3_exec(db, sql.ptr, null, null, &errmsg) != c.SQLITE_OK) {
        std.debug.print("VACUUM INTO failed: {s}\n", .{errmsg});
        c.sqlite3_free(errmsg);
        return error.SnapshotFailed;
    }

    if (c.sqlite3_close(db) != c.SQLITE_OK) return error.DatabaseCloseFailed;
}

/// Caller is expected to have armed `startWatchdog` already.
pub fn run(
    allocator: std.mem.Allocator,
    db_path: []const u8,
    endpoint: ?[]const u8,
    snap_path: []const u8,
) !void {
    try writeSnapshot(allocator, db_path, snap_path);
    // Database handle is released from here on.

    const payload = blk: {
        const file = try std.fs.cwd().openFile(snap_path, .{});
        defer file.close();
        // Deliberately NOT File.stat()/getEndPos(): both issue the `statx` syscall, which
        // does not exist before Linux 4.11. The Kobo Elipsa runs 4.9.77, where it returns
        // ENOSYS and surfaces as a bare `error.Unexpected`. readToEndAlloc with no size
        // hint grows the buffer using plain read() calls instead.
        break :blk try file.readToEndAlloc(allocator, max_snapshot_bytes);
    };
    defer allocator.free(payload);

    var sha: [64]u8 = undefined;
    sha256Hex(payload, &sha);

    var host_buf: [std.posix.HOST_NAME_MAX]u8 = undefined;
    const device = deviceName(&host_buf);

    std.debug.print("snapshot: {d} bytes, sha256={s}\n", .{ payload.len, sha });

    // Without an endpoint this is the inspect/debug path, so the file is deliberately
    // left in place. Cleanup is registered only once we know we're uploading.
    const url = endpoint orelse {
        std.debug.print("no endpoint given; snapshot left at {s}\n", .{snap_path});
        return;
    };
    defer std.fs.cwd().deleteFile(snap_path) catch {};

    std.debug.print("uploading to {s} ...\n", .{url});

    var client: std.http.Client = .{ .allocator = allocator };
    defer client.deinit();

    const result = try client.fetch(.{
        .location = .{ .uri = try std.Uri.parse(url) },
        .method = .POST,
        .payload = payload,
        .headers = .{ .content_type = .{ .override = "application/octet-stream" } },
        .extra_headers = &.{
            .{ .name = "x-kobo-sha256", .value = &sha },
            .{ .name = "x-kobo-device", .value = device },
        },
    });

    const status = @intFromEnum(result.status);
    if (status < 200 or status >= 300) {
        std.debug.print("server returned HTTP {d}\n", .{status});
        return error.HttpRequestFailed;
    }
    std.debug.print("ok: uploaded snapshot (HTTP {d})\n", .{status});
}

test "quoteSqlLiteral doubles single quotes" {
    const a = std.testing.allocator;

    const plain = try quoteSqlLiteral(a, "/tmp/snap.sqlite");
    defer a.free(plain);
    try std.testing.expectEqualStrings("/tmp/snap.sqlite", plain);

    const tricky = try quoteSqlLiteral(a, "/tmp/o'brien.sqlite");
    defer a.free(tricky);
    try std.testing.expectEqualStrings("/tmp/o''brien.sqlite", tricky);
}

test "sha256Hex matches known digest" {
    var out: [64]u8 = undefined;
    sha256Hex("abc", &out);
    try std.testing.expectEqualStrings(
        "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad",
        &out,
    );
}