From b1189ab038ad1885f961562bf188f0bd3da12153 Mon Sep 17 00:00:00 2001
From: Josh GM Walker <56300765+Josh-Walker-GM@users.noreply.github.com>
Date: Thu, 4 Sep 2025 05:43:47 +0100
Subject: [PATCH] fix: std.fs.File.Writer.seekTo does not flush (#25135)

* add failing test case
* perform flush and allow error
* dont over constrain flush error
* reset seek error during conversion
---
 lib/std/fs/File.zig |  9 ++++++---
 lib/std/fs/test.zig | 25 +++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/lib/std/fs/File.zig b/lib/std/fs/File.zig
index 36e90371ab..c3ee06958b 100644
--- a/lib/std/fs/File.zig
+++ b/lib/std/fs/File.zig
@@ -1505,7 +1505,7 @@ pub const Writer = struct {
     sendfile_err: ?SendfileError = null,
     copy_file_range_err: ?CopyFileRangeError = null,
     fcopyfile_err: ?FcopyfileError = null,
-    seek_err: ?SeekError = null,
+    seek_err: ?Writer.SeekError = null,
     interface: std.Io.Writer,
 
     pub const Mode = Reader.Mode;
@@ -1527,6 +1527,8 @@ pub const Writer = struct {
         Unexpected,
     };
 
+    pub const SeekError = File.SeekError || std.Io.Writer.Error;
+
     /// Number of slices to store on the stack, when trying to send as many byte
     /// vectors through the underlying write calls as possible.
     const max_buffers_len = 16;
@@ -1570,7 +1572,7 @@ pub const Writer = struct {
             .mode = w.mode,
             .pos = w.pos,
             .interface = Reader.initInterface(w.interface.buffer),
-            .seek_err = w.seek_err,
+            .seek_err = null,
         };
     }
 
@@ -2000,7 +2002,8 @@ pub const Writer = struct {
         return n;
     }
 
-    pub fn seekTo(w: *Writer, offset: u64) SeekError!void {
+    pub fn seekTo(w: *Writer, offset: u64) Writer.SeekError!void {
+        try w.interface.flush();
         switch (w.mode) {
             .positional, .positional_reading => {
                 w.pos = offset;
diff --git a/lib/std/fs/test.zig b/lib/std/fs/test.zig
index 7afe322686..65e86e4c2e 100644
--- a/lib/std/fs/test.zig
+++ b/lib/std/fs/test.zig
@@ -2155,3 +2155,28 @@ test "seekBy" {
     try testing.expectEqual(15, n);
     try testing.expectEqualStrings("t's test seekBy", buffer[0..15]);
 }
+
+test "seekTo flushes buffered data" {
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+
+    const contents = "data";
+
+    const file = try tmp.dir.createFile("seek.bin", .{ .read = true });
+    defer file.close();
+    {
+        var buf: [16]u8 = undefined;
+        var file_writer = std.fs.File.writer(file, &buf);
+
+        try file_writer.interface.writeAll(contents);
+        try file_writer.seekTo(8);
+        try file_writer.interface.flush();
+    }
+
+    var read_buffer: [16]u8 = undefined;
+    var file_reader: std.fs.File.Reader = .init(file, &read_buffer);
+
+    var buf: [4]u8 = undefined;
+    try file_reader.interface.readSliceAll(&buf);
+    try std.testing.expectEqualStrings(contents, &buf);
+}