std: WIP: debug-level stderr writing

6 files changed, 181 insertions, 125 deletions

diff --git a/lib/std/Io.zig b/lib/std/Io.zig
index 193998f037..833cc4ec0f 100644
--- a/lib/std/Io.zig
+++ b/lib/std/Io.zig
@@ -560,6 +560,13 @@ pub const net = @import("Io/net.zig");
 userdata: ?*anyopaque,
 vtable: *const VTable,
 
+/// This is the global, process-wide protection to coordinate stderr writes.
+///
+/// The primary motivation for recursive mutex here is so that a panic while
+/// stderr mutex is held still dumps the stack trace and other debug
+/// information.
+pub var stderr_thread_mutex: std.Thread.Mutex.Recursive = .init;
+
 pub const VTable = struct {
     /// If it returns `null` it means `result` has been already populated and
     /// `await` will be a no-op.
@@ -733,6 +740,10 @@ pub const VTable = struct {
     netInterfaceNameResolve: *const fn (?*anyopaque, *const net.Interface.Name) net.Interface.Name.ResolveError!net.Interface,
     netInterfaceName: *const fn (?*anyopaque, net.Interface) net.Interface.NameError!net.Interface.Name,
     netLookup: *const fn (?*anyopaque, net.HostName, *Queue(net.HostName.LookupResult), net.HostName.LookupOptions) net.HostName.LookupError!void,
+
+    lockStderrWriter: *const fn (?*anyopaque, buffer: []u8) Cancelable!*Writer,
+    tryLockStderrWriter: *const fn (?*anyopaque, buffer: []u8) ?*Writer,
+    unlockStderrWriter: *const fn (?*anyopaque) void,
 };
 
 pub const Cancelable = error{
@@ -2167,3 +2178,23 @@ pub fn select(io: Io, s: anytype) Cancelable!SelectUnion(@TypeOf(s)) {
         else => unreachable,
     }
 }
+
+/// For doing application-level writes to the standard error stream.
+/// Coordinates also with debug-level writes that are ignorant of Io interface
+/// and implementations. When this returns, `stderr_thread_mutex` will be
+/// locked.
+///
+/// See also:
+/// * `tryLockStderrWriter`
+pub fn lockStderrWriter(io: Io, buffer: []u8) Cancelable!*Writer {
+    return io.vtable.lockStderrWriter(io.userdata, buffer);
+}
+
+/// Same as `lockStderrWriter` but uncancelable and non-blocking.
+pub fn tryLockStderrWriter(io: Io, buffer: []u8) ?*Writer {
+    return io.vtable.tryLockStderrWriter(io.userdata, buffer);
+}
+
+pub fn unlockStderrWriter(io: Io) void {
+    return io.vtable.unlockStderrWriter(io.userdata);
+}
diff --git a/lib/std/Io/Dir.zig b/lib/std/Io/Dir.zig
index 9c4a1f1df8..a9d28440eb 100644
--- a/lib/std/Io/Dir.zig
+++ b/lib/std/Io/Dir.zig
@@ -342,7 +342,7 @@ pub const Walker = struct {
 
 /// Recursively iterates over a directory.
 ///
-/// `dir` must have been opened with `OpenOptions{.iterate = true}`.
+/// `dir` must have been opened with `OpenOptions.iterate` set to `true`.
 ///
 /// `Walker.deinit` releases allocated memory and directory handles.
 ///
@@ -350,7 +350,8 @@ pub const Walker = struct {
 ///
 /// `dir` will not be closed after walking it.
 ///
-/// See also `walkSelectively`.
+/// See also:
+/// * `walkSelectively`
 pub fn walk(dir: Dir, allocator: Allocator) Allocator.Error!Walker {
     return .{ .inner = try walkSelectively(dir, allocator) };
 }
diff --git a/lib/std/Io/Threaded.zig b/lib/std/Io/Threaded.zig
index 639e5cf6cd..9d6d9f979e 100644
--- a/lib/std/Io/Threaded.zig
+++ b/lib/std/Io/Threaded.zig
@@ -77,6 +77,8 @@ use_sendfile: UseSendfile = .default,
 use_copy_file_range: UseCopyFileRange = .default,
 use_fcopyfile: UseFcopyfile = .default,
 
+stderr_writer: Io.Writer,
+
 pub const RobustCancel = if (std.Thread.use_pthreads or native_os == .linux) enum {
     enabled,
     disabled,
@@ -9514,6 +9516,36 @@ fn netLookupFallible(
     return error.OptionUnsupported;
 }
 
+fn lockStderrWriter(userdata: ?*anyopaque, buffer: []u8) Io.Cancelable!*Io.Writer {
+    const t: *Threaded = @ptrCast(@alignCast(userdata));
+    // Only global mutex since this is Threaded.
+    Io.stderr_thread_mutex.lock();
+    if (is_windows) t.stderr_writer.file = .stderr();
+    std.Progress.clearWrittenWithEscapeCodes(&t.stderr_writer) catch {};
+    t.stderr_writer.flush() catch {};
+    t.stderr_writer.buffer = buffer;
+    return &t.stderr_writer;
+}
+
+fn tryLockStderrWriter(userdata: ?*anyopaque, buffer: []u8) ?*Io.Writer {
+    const t: *Threaded = @ptrCast(@alignCast(userdata));
+    // Only global mutex since this is Threaded.
+    if (!Io.stderr_thread_mutex.tryLock()) return null;
+    std.Progress.clearWrittenWithEscapeCodes(t.io()) catch {};
+    if (is_windows) t.stderr_writer.file = .stderr();
+    t.stderr_writer.flush() catch {};
+    t.stderr_writer.buffer = buffer;
+    return &t.stderr_writer;
+}
+
+fn unlockStderrWriter(userdata: ?*anyopaque) void {
+    const t: *Threaded = @ptrCast(@alignCast(userdata));
+    t.stderr_writer.flush() catch {};
+    t.stderr_writer.end = 0;
+    t.stderr_writer.buffer = &.{};
+    Io.stderr_thread_mutex.unlock();
+}
+
 pub const PosixAddress = extern union {
     any: posix.sockaddr,
     in: posix.sockaddr.in,
diff --git a/lib/std/Progress.zig b/lib/std/Progress.zig
index a839de8c4f..5528591620 100644
--- a/lib/std/Progress.zig
+++ b/lib/std/Progress.zig
@@ -13,16 +13,18 @@ const assert = std.debug.assert;
 const posix = std.posix;
 const Writer = std.Io.Writer;
 
-/// `null` if the current node (and its children) should
-/// not print on update()
+/// Currently this API only supports this value being set to stderr, which
+/// happens automatically inside `start`.
 terminal: Io.File,
 
+io: Io,
+
 terminal_mode: TerminalMode,
 
-update_thread: ?std.Thread,
+update_worker: ?Io.Future(void),
 
 /// Atomically set by SIGWINCH as well as the root done() function.
-redraw_event: std.Thread.ResetEvent,
+redraw_event: Io.ResetEvent,
 /// Indicates a request to shut down and reset global state.
 /// Accessed atomically.
 done: bool,
@@ -95,9 +97,9 @@ pub const Options = struct {
     /// Must be at least 200 bytes.
     draw_buffer: []u8 = &default_draw_buffer,
     /// How many nanoseconds between writing updates to the terminal.
-    refresh_rate_ns: u64 = 80 * std.time.ns_per_ms,
+    refresh_rate_ns: Io.Duration = .fromMilliseconds(80),
     /// How many nanoseconds to keep the output hidden
-    initial_delay_ns: u64 = 200 * std.time.ns_per_ms,
+    initial_delay_ns: Io.Duration = .fromMilliseconds(200),
     /// If provided, causes the progress item to have a denominator.
     /// 0 means unknown.
     estimated_total_items: usize = 0,
@@ -330,7 +332,7 @@ pub const Node = struct {
         } else {
             @atomicStore(bool, &global_progress.done, true, .monotonic);
             global_progress.redraw_event.set();
-            if (global_progress.update_thread) |thread| thread.join();
+            if (global_progress.update_worker) |worker| worker.await(global_progress.io);
         }
     }
 
@@ -391,9 +393,10 @@ pub const Node = struct {
 };
 
 var global_progress: Progress = .{
+    .io = undefined,
     .terminal = undefined,
     .terminal_mode = .off,
-    .update_thread = null,
+    .update_worker = null,
     .redraw_event = .unset,
     .refresh_rate_ns = undefined,
     .initial_delay_ns = undefined,
@@ -403,6 +406,7 @@ var global_progress: Progress = .{
     .done = false,
     .need_clear = false,
     .status = .working,
+    .start_failure = .unstarted,
 
     .node_parents = &node_parents_buffer,
     .node_storage = &node_storage_buffer,
@@ -411,6 +415,13 @@ var global_progress: Progress = .{
     .node_end_index = 0,
 };
 
+pub const StartFailure = union(enum) {
+    unstarted,
+    spawn_ipc_worker: error{ConcurrencyUnavailable},
+    spawn_update_worker: error{ConcurrencyUnavailable},
+    parse_env_var: error{},
+};
+
 const node_storage_buffer_len = 83;
 var node_parents_buffer: [node_storage_buffer_len]Node.Parent = undefined;
 var node_storage_buffer: [node_storage_buffer_len]Node.Storage = undefined;
@@ -437,7 +448,7 @@ const noop_impl = builtin.single_threaded or switch (builtin.os.tag) {
 /// Asserts there is only one global Progress instance.
 ///
 /// Call `Node.end` when done.
-pub fn start(options: Options) Node {
+pub fn start(options: Options, io: Io) Node {
     // Ensure there is only 1 global Progress object.
     if (global_progress.node_end_index != 0) {
         debug_start_trace.dump();
@@ -458,10 +469,10 @@ pub fn start(options: Options) Node {
     if (noop_impl)
         return Node.none;
 
-    const io = static_threaded_io.io();
+    global_progress.io = io;
 
     if (std.process.parseEnvVarInt("ZIG_PROGRESS", u31, 10)) |ipc_fd| {
-        global_progress.update_thread = std.Thread.spawn(.{}, ipcThreadRun, .{
+        global_progress.update_worker = io.concurrent(ipcThreadRun, .{
             io,
             @as(Io.File, .{ .handle = switch (@typeInfo(posix.fd_t)) {
                 .int => ipc_fd,
@@ -469,7 +480,7 @@ pub fn start(options: Options) Node {
                 else => @compileError("unsupported fd_t of " ++ @typeName(posix.fd_t)),
             } }),
         }) catch |err| {
-            std.log.warn("failed to spawn IPC thread for communicating progress to parent: {s}", .{@errorName(err)});
+            global_progress.start_failure = .{ .spawn_ipc_worker = err };
             return Node.none;
         };
     } else |env_err| switch (env_err) {
@@ -502,17 +513,17 @@ pub fn start(options: Options) Node {
 
             if (switch (global_progress.terminal_mode) {
                 .off => unreachable, // handled a few lines above
-                .ansi_escape_codes => std.Thread.spawn(.{}, updateThreadRun, .{io}),
-                .windows_api => if (is_windows) std.Thread.spawn(.{}, windowsApiUpdateThreadRun, .{io}) else unreachable,
-            }) |thread| {
-                global_progress.update_thread = thread;
+                .ansi_escape_codes => io.concurrent(updateThreadRun, .{io}),
+                .windows_api => if (is_windows) io.concurrent(windowsApiUpdateThreadRun, .{io}) else unreachable,
+            }) |future| {
+                global_progress.update_worker = future;
             } else |err| {
-                std.log.warn("unable to spawn thread for printing progress to terminal: {s}", .{@errorName(err)});
+                global_progress.start_failure = .{ .spawn_update_worker = err };
                 return Node.none;
             }
         },
         else => |e| {
-            std.log.warn("invalid ZIG_PROGRESS file descriptor integer: {s}", .{@errorName(e)});
+            global_progress.start_failure = .{ .parse_env_var = e };
             return Node.none;
         },
     }
@@ -545,10 +556,10 @@ fn updateThreadRun(io: Io) void {
         maybeUpdateSize(resize_flag);
 
         const buffer, _ = computeRedraw(&serialized_buffer);
-        if (stderr_mutex.tryLock()) {
-            defer stderr_mutex.unlock();
-            write(io, buffer) catch return;
+        if (io.tryLockStderrWriter(&.{})) |w| {
+            defer io.unlockStderrWriter();
             global_progress.need_clear = true;
+            w.writeAll(buffer) catch return;
         }
     }
 
@@ -556,18 +567,18 @@ fn updateThreadRun(io: Io) void {
         const resize_flag = wait(global_progress.refresh_rate_ns);
 
         if (@atomicLoad(bool, &global_progress.done, .monotonic)) {
-            stderr_mutex.lock();
-            defer stderr_mutex.unlock();
-            return clearWrittenWithEscapeCodes(io) catch {};
+            const w = io.lockStderrWriter(&.{}) catch return;
+            defer io.unlockStderrWriter();
+            return clearWrittenWithEscapeCodes(w) catch {};
         }
 
         maybeUpdateSize(resize_flag);
 
         const buffer, _ = computeRedraw(&serialized_buffer);
-        if (stderr_mutex.tryLock()) {
-            defer stderr_mutex.unlock();
-            write(io, buffer) catch return;
+        if (io.tryLockStderrWriter(&.{})) |w| {
+            defer io.unlockStderrWriter();
             global_progress.need_clear = true;
+            w.writeAll(buffer) catch return;
         }
     }
 }
@@ -589,11 +600,11 @@ fn windowsApiUpdateThreadRun(io: Io) void {
         maybeUpdateSize(resize_flag);
 
         const buffer, const nl_n = computeRedraw(&serialized_buffer);
-        if (stderr_mutex.tryLock()) {
-            defer stderr_mutex.unlock();
+        if (io.tryLockStderrWriter()) |w| {
+            defer io.unlockStderrWriter();
             windowsApiWriteMarker();
-            write(io, buffer) catch return;
             global_progress.need_clear = true;
+            w.writeAll(buffer) catch return;
             windowsApiMoveToMarker(nl_n) catch return;
         }
     }
@@ -602,74 +613,25 @@ fn windowsApiUpdateThreadRun(io: Io) void {
         const resize_flag = wait(global_progress.refresh_rate_ns);
 
         if (@atomicLoad(bool, &global_progress.done, .monotonic)) {
-            stderr_mutex.lock();
-            defer stderr_mutex.unlock();
+            _ = io.lockStderrWriter() catch return;
+            defer io.unlockStderrWriter();
             return clearWrittenWindowsApi() catch {};
         }
 
         maybeUpdateSize(resize_flag);
 
         const buffer, const nl_n = computeRedraw(&serialized_buffer);
-        if (stderr_mutex.tryLock()) {
-            defer stderr_mutex.unlock();
+        if (io.tryLockStderrWriter()) |w| {
+            defer io.unlockStderrWriter();
             clearWrittenWindowsApi() catch return;
             windowsApiWriteMarker();
-            write(io, buffer) catch return;
             global_progress.need_clear = true;
+            w.writeAll(buffer) catch return;
             windowsApiMoveToMarker(nl_n) catch return;
         }
     }
 }
 
-/// Allows the caller to freely write to stderr until `unlockStdErr` is called.
-///
-/// During the lock, any `std.Progress` information is cleared from the terminal.
-///
-/// The lock is recursive; the same thread may hold the lock multiple times.
-pub fn lockStdErr() void {
-    const io = stderr_file_writer.io;
-    stderr_mutex.lock();
-    clearWrittenWithEscapeCodes(io) catch {};
-}
-
-pub fn unlockStdErr() void {
-    stderr_mutex.unlock();
-}
-
-/// Protected by `stderr_mutex`.
-const stderr_writer: *Writer = &stderr_file_writer.interface;
-/// Protected by `stderr_mutex`.
-var stderr_file_writer: Io.File.Writer = .{
-    .io = static_threaded_io.io(),
-    .interface = Io.File.Writer.initInterface(&.{}),
-    .file = if (is_windows) undefined else .stderr(),
-    .mode = .streaming,
-};
-var static_threaded_io: Io.Threaded = .init_single_threaded;
-
-/// Allows the caller to freely write to the returned `Writer`,
-/// initialized with `buffer`, until `unlockStderrWriter` is called.
-///
-/// During the lock, any `std.Progress` information is cleared from the terminal.
-///
-/// The lock is recursive; the same thread may hold the lock multiple times.
-pub fn lockStderrWriter(buffer: []u8) *Io.Writer {
-    const io = stderr_file_writer.io;
-    stderr_mutex.lock();
-    clearWrittenWithEscapeCodes(io) catch {};
-    if (is_windows) stderr_file_writer.file = .stderr();
-    stderr_writer.flush() catch {};
-    stderr_writer.buffer = buffer;
-    return stderr_writer;
-}
-
-pub fn unlockStderrWriter() void {
-    stderr_writer.flush() catch {};
-    stderr_writer.end = 0;
-    stderr_writer.buffer = &.{};
-    stderr_mutex.unlock();
-}
-
 fn ipcThreadRun(io: Io, file: Io.File) anyerror!void {
     // Store this data in the thread so that it does not need to be part of the
     // linker data of the main executable.
@@ -793,11 +755,11 @@ fn appendTreeSymbol(symbol: TreeSymbol, buf: []u8, start_i: usize) usize {
     }
 }
 
-fn clearWrittenWithEscapeCodes(io: Io) anyerror!void {
+fn clearWrittenWithEscapeCodes(w: *Io.Writer) anyerror!void {
     if (noop_impl or !global_progress.need_clear) return;
 
+    try w.writeAll(clear ++ progress_remove);
     global_progress.need_clear = false;
-    try write(io, clear ++ progress_remove);
 }
 
 /// U+25BA or ►
@@ -997,7 +959,7 @@ fn serializeIpc(start_serialized_len: usize, serialized_buffer: *Serialized.Buff
             const n = posix.read(fd, pipe_buf[bytes_read..]) catch |err| switch (err) {
                 error.WouldBlock => break,
                 else => |e| {
-                    std.log.debug("failed to read child progress data: {s}", .{@errorName(e)});
+                    std.log.debug("failed to read child progress data: {t}", .{e});
                     main_storage.completed_count = 0;
                     main_storage.estimated_total_count = 0;
                     continue :main_loop;
@@ -1424,10 +1386,6 @@ fn withinRowLimit(p: *Progress, nl_n: usize) bool {
     return nl_n + 2 < p.rows;
 }
 
-fn write(io: Io, buf: []const u8) anyerror!void {
-    try global_progress.terminal.writeStreamingAll(io, buf);
-}
-
 var remaining_write_trash_bytes: usize = 0;
 
 fn writeIpc(io: Io, file: Io.File, serialized: Serialized) error{BrokenPipe}!void {
@@ -1459,7 +1417,7 @@ fn writeIpc(io: Io, file: Io.File, serialized: Serialized) error{BrokenPipe}!voi
             error.WouldBlock => return,
             error.BrokenPipe => return error.BrokenPipe,
             else => |e| {
-                std.log.debug("failed to send progress to parent process: {s}", .{@errorName(e)});
+                std.log.debug("failed to send progress to parent process: {t}", .{e});
                 return error.BrokenPipe;
             },
         }
@@ -1476,7 +1434,7 @@ fn writeIpc(io: Io, file: Io.File, serialized: Serialized) error{BrokenPipe}!voi
         error.WouldBlock => {},
         error.BrokenPipe => return error.BrokenPipe,
         else => |e| {
-            std.log.debug("failed to send progress to parent process: {s}", .{@errorName(e)});
+            std.log.debug("failed to send progress to parent process: {t}", .{e});
             return error.BrokenPipe;
         },
     }
@@ -1568,11 +1526,6 @@ const have_sigwinch = switch (builtin.os.tag) {
     else => false,
 };
 
-/// The primary motivation for recursive mutex here is so that a panic while
-/// stderr mutex is held still dumps the stack trace and other debug
-/// information.
-var stderr_mutex = std.Thread.Mutex.Recursive.init;
-
 fn copyAtomicStore(dest: []align(@alignOf(usize)) u8, src: []const u8) void {
     assert(dest.len == src.len);
     const chunked_len = dest.len / @sizeOf(usize);
diff --git a/lib/std/debug.zig b/lib/std/debug.zig
index 7b111215f3..7e10c5af32 100644
--- a/lib/std/debug.zig
+++ b/lib/std/debug.zig
@@ -262,17 +262,6 @@ pub const sys_can_stack_trace = switch (builtin.cpu.arch) {
     else => true,
 };
 
-/// Allows the caller to freely write to stderr until `unlockStdErr` is called.
-///
-/// During the lock, any `std.Progress` information is cleared from the terminal.
-pub fn lockStdErr() void {
-    std.Progress.lockStdErr();
-}
-
-pub fn unlockStdErr() void {
-    std.Progress.unlockStdErr();
-}
-
 /// Allows the caller to freely write to stderr until `unlockStderrWriter` is called.
 ///
 /// During the lock, any `std.Progress` information is cleared from the terminal.
@@ -281,17 +270,21 @@ pub fn unlockStdErr() void {
 /// times. The primary motivation is that this allows the panic handler to safely dump the stack
 /// trace and panic message even if the mutex was held at the panic site.
 ///
-/// The returned `Writer` does not need to be manually flushed: flushing is performed automatically
-/// when the matching `unlockStderrWriter` call occurs.
+/// The returned `Writer` does not need to be manually flushed: flushing is
+/// performed automatically when the matching `unlockStderrWriter` call occurs.
+///
+/// This is a low-level debugging primitive that bypasses the `Io` interface,
+/// writing directly to stderr using the most basic syscalls available. This
+/// function does not switch threads, switch stacks, or suspend.
+///
+/// Alternatively, use the higher-level `Io.lockStderrWriter` to integrate with
+/// the application's chosen `Io` implementation.
 pub fn lockStderrWriter(buffer: []u8) struct { *Writer, tty.Config } {
-    const global = struct {
-        var conf: ?tty.Config = null;
-    };
+    Io.stderr_thread_mutex.lock();
     const w = std.Progress.lockStderrWriter(buffer);
-    const file_writer: *File.Writer = @fieldParentPtr("interface", w);
     // The stderr lock also locks access to `global.conf`.
-    if (global.conf == null) {
-        global.conf = .detect(file_writer.io, .stderr());
+    if (StderrWriter.singleton.tty_config == null) {
+        StderrWriter.singleton.tty_config = .detect(io, .stderr());
     }
     return .{ w, global.conf.? };
 }
@@ -300,11 +293,17 @@ pub fn unlockStderrWriter() void {
     std.Progress.unlockStderrWriter();
 }
 
-/// Print to stderr, silently returning on failure. Intended for use in "printf
-/// debugging". Use `std.log` functions for proper logging.
+/// Writes to stderr, ignoring errors.
+///
+/// This is a low-level debugging primitive that bypasses the `Io` interface,
+/// writing directly to stderr using the most basic syscalls available. This
+/// function does not switch threads, switch stacks, or suspend.
 ///
 /// Uses a 64-byte buffer for formatted printing which is flushed before this
 /// function returns.
+///
+/// Alternatively, use the higher-level `std.log` or `Io.lockStderrWriter` to
+/// integrate with the application's chosen `Io` implementation.
 pub fn print(comptime fmt: []const u8, args: anytype) void {
     var buffer: [64]u8 = undefined;
     const bw, _ = lockStderrWriter(&buffer);
@@ -312,6 +311,34 @@ pub fn print(comptime fmt: []const u8, args: anytype) void {
     nosuspend bw.print(fmt, args) catch return;
 }
 
+const StderrWriter = struct {
+    interface: Writer,
+    tty_config: ?tty.Config,
+
+    var singleton: StderrWriter = .{
+        .interface = .{
+            .buffer = &.{},
+            .vtable = &.{ .drain = drain },
+        },
+        .tty_config = null,
+    };
+
+    fn drain(io_w: *Writer, data: []const []const u8, splat: usize) Writer.Error!usize {
+        const w: *Writer = @alignCast(@fieldParentPtr("interface", io_w));
+        var n: usize = 0;
+        const header = w.interface.buffered();
+        if (header.len != 0) n += try std.Io.Threaded.debugWrite(header);
+        for (data[0 .. data.len - 1]) |d| {
+            if (d.len != 0) n += try std.Io.Threaded.debugWrite(d);
+        }
+        const pattern = data[data.len - 1];
+        if (pattern.len != 0) {
+            for (0..splat) |_| n += try std.Io.Threaded.debugWrite(pattern);
+        }
+        return io_w.consume(n);
+    }
+};
+
 /// Marked `inline` to propagate a comptime-known error to callers.
 pub inline fn getSelfDebugInfo() !*SelfInfo {
     if (SelfInfo == void) return error.UnsupportedTarget;
@@ -767,7 +794,7 @@ pub const FormatStackTrace = struct {
     stack_trace: StackTrace,
     tty_config: tty.Config,
 
-    pub fn format(context: @This(), writer: *Io.Writer) Io.Writer.Error!void {
+    pub fn format(context: @This(), writer: *Writer) Writer.Error!void {
         try writer.writeAll("\n");
         try writeStackTrace(&context.stack_trace, writer, context.tty_config);
     }
@@ -1608,7 +1635,7 @@ test "manage resources correctly" {
     const gpa = std.testing.allocator;
     var threaded: Io.Threaded = .init_single_threaded;
     const io = threaded.ioBasic();
-    var discarding: Io.Writer.Discarding = .init(&.{});
+    var discarding: Writer.Discarding = .init(&.{});
     var di: SelfInfo = .init;
     defer di.deinit(gpa);
     try printSourceAtAddress(
diff --git a/lib/std/log.zig b/lib/std/log.zig
index 9568f9ba52..461dfca36e 100644
--- a/lib/std/log.zig
+++ b/lib/std/log.zig
@@ -80,6 +80,8 @@ pub fn logEnabled(comptime level: Level, comptime scope: @EnumLiteral()) bool {
     return @intFromEnum(level) <= @intFromEnum(std.options.log_level);
 }
 
+var static_threaded_io: std.Io.Threaded = .init_single_threaded;
+
 /// The default implementation for the log function. Custom log functions may
 /// forward log messages to this function.
 ///
@@ -91,9 +93,19 @@ pub fn defaultLog(
     comptime format: []const u8,
     args: anytype,
 ) void {
+    return defaultLogIo(level, scope, format, args, static_threaded_io.io());
+}
+
+pub fn defaultLogIo(
+    comptime level: Level,
+    comptime scope: @EnumLiteral(),
+    comptime format: []const u8,
+    args: anytype,
+    io: std.Io,
+) void {
     var buffer: [64]u8 = undefined;
-    const stderr, const ttyconf = std.debug.lockStderrWriter(&buffer);
-    defer std.debug.unlockStderrWriter();
+    const stderr, const ttyconf = io.lockStderrWriter(&buffer);
+    defer io.unlockStderrWriter();
     ttyconf.setColor(stderr, switch (level) {
         .err => .red,
         .warn => .yellow,