From f27bd87ade2e43dce66963f43ce678e361ddbfc2 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Thu, 18 Dec 2025 23:05:51 -0800
Subject: [PATCH] std.Io.Threaded: allow length-0 file writes

At first I thought about keeping this as an assertion but I can see this
being useful if you already know how many bytes to read and you are
filling the end of the buffer.

This also more closely mirrors POSIX APIs.
---
 lib/std/Io/File.zig     | 4 +---
 lib/std/Io/Threaded.zig | 4 ++++
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/lib/std/Io/File.zig b/lib/std/Io/File.zig
index e06f2ba40d..601a882605 100644
--- a/lib/std/Io/File.zig
+++ b/lib/std/Io/File.zig
@@ -497,7 +497,7 @@ pub fn setTimestampsNow(file: File, io: Io) SetTimestampsError!void {
 
 pub const ReadPositionalError = Reader.Error || error{Unseekable};
 
-/// Returns 0 on end of stream.
+/// Returns 0 on stream end or if `buffer` has no space available for data.
 ///
 /// See also:
 /// * `reader`
@@ -507,8 +507,6 @@ pub fn readPositional(file: File, io: Io, buffer: []const []u8, offset: u64) Rea
 
 pub const WritePositionalError = Writer.Error || error{Unseekable};
 
-/// Returns 0 on end of stream.
-///
 /// See also:
 /// * `writer`
 pub fn writePositional(file: File, io: Io, buffer: []const []const u8, offset: u64) WritePositionalError!usize {
diff --git a/lib/std/Io/Threaded.zig b/lib/std/Io/Threaded.zig
index ce49049f02..ffa292a040 100644
--- a/lib/std/Io/Threaded.zig
+++ b/lib/std/Io/Threaded.zig
@@ -6400,6 +6400,7 @@ fn fileReadStreamingPosix(userdata: ?*anyopaque, file: File, data: []const []u8)
             i += 1;
         }
     }
+    if (i == 0) return 0;
     const dest = iovecs_buffer[0..i];
     assert(dest[0].len > 0);
 
@@ -6482,6 +6483,7 @@ fn fileReadStreamingWindows(userdata: ?*anyopaque, file: File, data: []const []u
     const DWORD = windows.DWORD;
     var index: usize = 0;
     while (data[index].len == 0) index += 1;
+    if (index == 0) return 0;
     const buffer = data[index];
     const want_read_count: DWORD = @min(std.math.maxInt(DWORD), buffer.len);
 
@@ -6519,6 +6521,7 @@ fn fileReadPositionalPosix(userdata: ?*anyopaque, file: File, data: []const []u8
             i += 1;
         }
     }
+    if (i == 0) return 0;
     const dest = iovecs_buffer[0..i];
     assert(dest[0].len > 0);
 
@@ -6614,6 +6617,7 @@ fn fileReadPositionalWindows(userdata: ?*anyopaque, file: File, data: []const []
 
     var index: usize = 0;
     while (data[index].len == 0) index += 1;
+    if (index == 0) return 0;
     const buffer = data[index];
     const want_read_count: DWORD = @min(std.math.maxInt(DWORD), buffer.len);