sendUpdate

pub fn sendUpdate(
    fuzz: *Fuzz,
    socket: *std.http.Server.WebSocket,
    prev: *Previous,
) !void {
    fuzz.coverage_mutex.lock();
    defer fuzz.coverage_mutex.unlock();

    const coverage_maps = fuzz.coverage_files.values();
    if (coverage_maps.len == 0) return;
    // TODO: handle multiple fuzz steps in the WebSocket packets
    const coverage_map = &coverage_maps[0];
    const cov_header: *const abi.SeenPcsHeader = @ptrCast(coverage_map.mapped_memory[0..@sizeOf(abi.SeenPcsHeader)]);
    // TODO: this isn't sound! We need to do volatile reads of these bits rather than handing the
    // buffer off to the kernel, because we might race with the fuzzer process[es]. This brings the
    // whole mmap strategy into question. Incidentally, I wonder if post-writergate we could pass
    // this data straight to the socket with sendfile...
    const seen_pcs = cov_header.seenBits();
    const n_runs = @atomicLoad(usize, &cov_header.n_runs, .monotonic);
    const unique_runs = @atomicLoad(usize, &cov_header.unique_runs, .monotonic);
    if (prev.unique_runs != unique_runs) {
        // There has been an update.
        if (prev.unique_runs == 0) {
            // We need to send initial context.
            const header: abi.SourceIndexHeader = .{
                .directories_len = @intCast(coverage_map.coverage.directories.entries.len),
                .files_len = @intCast(coverage_map.coverage.files.entries.len),
                .source_locations_len = @intCast(coverage_map.source_locations.len),
                .string_bytes_len = @intCast(coverage_map.coverage.string_bytes.items.len),
                .start_timestamp = coverage_map.start_timestamp,
            };
            var iovecs: [5][]const u8 = .{
                @ptrCast(&header),
                @ptrCast(coverage_map.coverage.directories.keys()),
                @ptrCast(coverage_map.coverage.files.keys()),
                @ptrCast(coverage_map.source_locations),
                coverage_map.coverage.string_bytes.items,
            };
            try socket.writeMessageVec(&iovecs, .binary);
        }

        const header: abi.CoverageUpdateHeader = .{
            .n_runs = n_runs,
            .unique_runs = unique_runs,
        };
        var iovecs: [2][]const u8 = .{
            @ptrCast(&header),
            @ptrCast(seen_pcs),
        };
        try socket.writeMessageVec(&iovecs, .binary);

        prev.unique_runs = unique_runs;
    }

    if (prev.entry_points != coverage_map.entry_points.items.len) {
        const header: abi.EntryPointHeader = .init(@intCast(coverage_map.entry_points.items.len));
        var iovecs: [2][]const u8 = .{
            @ptrCast(&header),
            @ptrCast(coverage_map.entry_points.items),
        };
        try socket.writeMessageVec(&iovecs, .binary);

        prev.entry_points = coverage_map.entry_points.items.len;
    }
}