Merge pull request #24394 from ziglang/fixes

buffering fixes

lib/std/Io/Reader.zig

@@ -840,6 +840,9 @@ pub fn peekDelimiterExclusive(r: *Reader, delimiter: u8) DelimiterError![]u8 {
 /// Returns number of bytes streamed, which may be zero, or error.EndOfStream
 /// if the delimiter was not found.
 ///
+/// Asserts buffer capacity of at least one. This function performs better with
+/// larger buffers.
+///
 /// See also:
 /// * `streamDelimiterEnding`
 /// * `streamDelimiterLimit`
@@ -858,6 +861,9 @@ pub fn streamDelimiter(r: *Reader, w: *Writer, delimiter: u8) StreamError!usize
 /// Returns number of bytes streamed, which may be zero. End of stream can be
 /// detected by checking if the next byte in the stream is the delimiter.
 ///
+/// Asserts buffer capacity of at least one. This function performs better with
+/// larger buffers.
+///
 /// See also:
 /// * `streamDelimiter`
 /// * `streamDelimiterLimit`
@@ -884,6 +890,9 @@ pub const StreamDelimiterLimitError = error{
 ///
 /// Returns number of bytes streamed, which may be zero. End of stream can be
 /// detected by checking if the next byte in the stream is the delimiter.
+///
+/// Asserts buffer capacity of at least one. This function performs better with
+/// larger buffers.
 pub fn streamDelimiterLimit(
     r: *Reader,
     w: *Writer,

lib/std/Io/Writer.zig

@@ -572,6 +572,10 @@ pub fn writeAllPreserve(w: *Writer, preserve_length: usize, bytes: []const u8) E
 /// A user type may be a `struct`, `vector`, `union` or `enum` type.
 ///
 /// To print literal curly braces, escape them by writing them twice, e.g. `{{` or `}}`.
+///
+/// Asserts `buffer` capacity of at least 2 if a union is printed. This
+/// requirement could be lifted by adjusting the code, but if you trigger that
+/// assertion it is a clue that you should probably be using a buffer.
 pub fn print(w: *Writer, comptime fmt: []const u8, args: anytype) Error!void {
     const ArgsType = @TypeOf(args);
     const args_type_info = @typeInfo(ArgsType);
@@ -942,6 +946,7 @@ pub fn printAddress(w: *Writer, value: anytype) Error!void {
     @compileError("cannot format non-pointer type " ++ @typeName(T) ++ " with * specifier");
 }
 
+/// Asserts `buffer` capacity of at least 2 if `value` is a union.
 pub fn printValue(
     w: *Writer,
     comptime fmt: []const u8,

lib/std/debug.zig

@@ -219,10 +219,14 @@ pub fn unlockStderrWriter() void {
     std.Progress.unlockStderrWriter();
 }
 
-/// Print to stderr, unbuffered, and silently returning on failure. Intended
-/// for use in "printf debugging". Use `std.log` functions for proper logging.
+/// Print to stderr, silently returning on failure. Intended for use in "printf
+/// debugging". Use `std.log` functions for proper logging.
+///
+/// Uses a 64-byte buffer for formatted printing which is flushed before this
+/// function returns.
 pub fn print(comptime fmt: []const u8, args: anytype) void {
-    const bw = lockStderrWriter(&.{});
+    var buffer: [64]u8 = undefined;
+    const bw = lockStderrWriter(&buffer);
     defer unlockStderrWriter();
     nosuspend bw.print(fmt, args) catch return;
 }

lib/std/log.zig

@@ -137,8 +137,11 @@ pub fn defaultLogEnabled(comptime message_level: Level) bool {
     return comptime logEnabled(message_level, default_log_scope);
 }
 
-/// The default implementation for the log function, custom log functions may
+/// The default implementation for the log function. Custom log functions may
 /// forward log messages to this function.
+///
+/// Uses a 64-byte buffer for formatted printing which is flushed before this
+/// function returns.
 pub fn defaultLog(
     comptime message_level: Level,
     comptime scope: @Type(.enum_literal),
@@ -147,16 +150,10 @@ pub fn defaultLog(
 ) void {
     const level_txt = comptime message_level.asText();
     const prefix2 = if (scope == .default) ": " else "(" ++ @tagName(scope) ++ "): ";
-    const stderr = std.fs.File.stderr().deprecatedWriter();
-    var bw = std.io.bufferedWriter(stderr);
-    const writer = bw.writer();
-
-    std.debug.lockStdErr();
-    defer std.debug.unlockStdErr();
-    nosuspend {
-        writer.print(level_txt ++ prefix2 ++ format ++ "\n", args) catch return;
-        bw.flush() catch return;
-    }
+    var buffer: [64]u8 = undefined;
+    const stderr = std.debug.lockStderrWriter(&buffer);
+    defer std.debug.unlockStderrWriter();
+    nosuspend stderr.print(level_txt ++ prefix2 ++ format ++ "\n", args) catch return;
 }
 
 /// Returns a scoped logging namespace that logs all messages using the scope