    //! Thread-local cryptographically secure pseudo-random number generator.
    //! This file has public declarations that are intended to be used internally
    //! by the standard library; this namespace is not intended to be exposed
    //! directly to standard library users.

    const std = @import("std");
    const builtin = @import("builtin");
    const mem = std.mem;
    const os = std.os;

    /// We use this as a layer of indirection because global const pointers cannot
    /// point to thread-local variables.
    pub const interface = std.rand.Random{
        .ptr = undefined,
        .fillFn = tlsCsprngFill,
    };

    const os_has_fork = switch (builtin.os.tag) {
        .dragonfly,
        .freebsd,
        .ios,
        .kfreebsd,
        .linux,
        .macos,
        .netbsd,
        .openbsd,
        .solaris,
        .tvos,
        .watchos,
        .haiku,
        => true,

        else => false,
    };
    const os_has_arc4random = builtin.link_libc and @hasDecl(std.c, "arc4random_buf");
    const want_fork_safety = os_has_fork and !os_has_arc4random and
        (std.meta.globalOption("crypto_fork_safety", bool) orelse true);
    const maybe_have_wipe_on_fork = builtin.os.isAtLeast(.linux, .{
        .major = 4,
        .minor = 14,
        .patch = 0,
    }) orelse true;
    const is_haiku = builtin.os.tag == .haiku;

    const Rng = std.rand.DefaultCsprng;

    const Context = struct {
        init_state: enum(u8) { uninitialized = 0, initialized, failed },
        rng: Rng,
    };

    var install_atfork_handler = std.once(struct {
        // Install the global handler only once.

        // The same handler is shared among threads and is inherinted by fork()-ed

        // processes.

        fn do() void {
            const r = std.c.pthread_atfork(null, null, childAtForkHandler);
            std.debug.assert(r == 0);
        }
    }.do);

    threadlocal var wipe_mem: []align(mem.page_size) u8 = &[_]u8{};

    fn tlsCsprngFill(_: *anyopaque, buffer: []u8) void {
        if (builtin.link_libc and @hasDecl(std.c, "arc4random_buf")) {
            // arc4random is already a thread-local CSPRNG.

            return std.c.arc4random_buf(buffer.ptr, buffer.len);
        }
        // Allow applications to decide they would prefer to have every call to

        // std.crypto.random always make an OS syscall, rather than rely on an

        // application implementation of a CSPRNG.

        if (std.options.crypto_always_getrandom) {
            return defaultRandomSeed(buffer);
        }

        if (wipe_mem.len == 0) {
            // Not initialized yet.

            if (want_fork_safety and maybe_have_wipe_on_fork or is_haiku) {
                // Allocate a per-process page, madvise operates with page

                // granularity.

                wipe_mem = os.mmap(
                    null,
                    @sizeOf(Context),
                    os.PROT.READ | os.PROT.WRITE,
                    os.MAP.PRIVATE | os.MAP.ANONYMOUS,
                    -1,
                    0,
                ) catch {
                    // Could not allocate memory for the local state, fall back to

                    // the OS syscall.

                    return std.options.cryptoRandomSeed(buffer);
                };
                // The memory is already zero-initialized.

            } else {
                // Use a static thread-local buffer.

                const S = struct {
                    threadlocal var buf: Context align(mem.page_size) = .{
                        .init_state = .uninitialized,
                        .rng = undefined,
                    };
                };
                wipe_mem = mem.asBytes(&S.buf);
            }
        }
        const ctx = @as(*Context, @ptrCast(wipe_mem.ptr));

        switch (ctx.init_state) {
            .uninitialized => {
                if (!want_fork_safety) {
                    return initAndFill(buffer);
                }

                if (maybe_have_wipe_on_fork) wof: {
                    // Qemu user-mode emulation ignores any valid/invalid madvise

                    // hint and returns success. Check if this is the case by

                    // passing bogus parameters, we expect EINVAL as result.

                    if (os.madvise(wipe_mem.ptr, 0, 0xffffffff)) |_| {
                        break :wof;
                    } else |_| {}

                    if (os.madvise(wipe_mem.ptr, wipe_mem.len, os.MADV.WIPEONFORK)) |_| {
                        return initAndFill(buffer);
                    } else |_| {}
                }

                if (std.Thread.use_pthreads) {
                    return setupPthreadAtforkAndFill(buffer);
                }

                // Since we failed to set up fork safety, we fall back to always

                // calling getrandom every time.

                ctx.init_state = .failed;
                return std.options.cryptoRandomSeed(buffer);
            },
            .initialized => {
                return fillWithCsprng(buffer);
            },
            .failed => {
                if (want_fork_safety) {
                    return std.options.cryptoRandomSeed(buffer);
                } else {
                    unreachable;
                }
            },
        }
    }

    fn setupPthreadAtforkAndFill(buffer: []u8) void {
        install_atfork_handler.call();
        return initAndFill(buffer);
    }

    fn childAtForkHandler() callconv(.C) void {
        // The atfork handler is global, this function may be called after

        // fork()-ing threads that never initialized the CSPRNG context.

        if (wipe_mem.len == 0) return;
        std.crypto.utils.secureZero(u8, wipe_mem);
    }

    fn fillWithCsprng(buffer: []u8) void {
        const ctx = @as(*Context, @ptrCast(wipe_mem.ptr));
        return ctx.rng.fill(buffer);
    }

    pub fn defaultRandomSeed(buffer: []u8) void {
        os.getrandom(buffer) catch @panic("getrandom() failed to provide entropy");
    }

    fn initAndFill(buffer: []u8) void {
        var seed: [Rng.secret_seed_length]u8 = undefined;
        // Because we panic on getrandom() failing, we provide the opportunity

        // to override the default seed function. This also makes

        // `std.crypto.random` available on freestanding targets, provided that

        // the `std.options.cryptoRandomSeed` function is provided.

        std.options.cryptoRandomSeed(&seed);

        const ctx = @as(*Context, @ptrCast(wipe_mem.ptr));
        ctx.rng = Rng.init(seed);
        std.crypto.utils.secureZero(u8, &seed);

        // This is at the end so that accidental recursive dependencies result

        // in stack overflows instead of invalid random data.

        ctx.init_state = .initialized;

        return fillWithCsprng(buffer);
    }