fix hello world

lib/std/fs/File.zig

@@ -1737,7 +1737,8 @@ pub fn writer_writeFile(
         const smaller_len = if (len_int == 0) max_count else @min(len_int, max_count);
         var off: std.os.linux.off_t = undefined;
         const off_ptr: ?*std.os.linux.off_t = if (in_offset.toInt()) |offset| b: {
-            off = std.math.cast(std.os.linux.off_t, offset) orelse return error.Overflow;
+            off = std.math.cast(std.os.linux.off_t, offset) orelse
+                return writer_writeSplat(context, headers_and_trailers, 1);
             break :b &off;
         } else null;
         if (true) @panic("TODO");
@@ -1747,7 +1748,13 @@ pub fn writer_writeFile(
             error.Unexpected => break :sf,
             else => |e| return e,
         };
-        if (in_offset.toInt()) |offset| assert(n == off - offset);
+        if (in_offset.toInt()) |offset| {
+            assert(n == off - offset);
+        } else if (n == 0 and len_int == 0) {
+            // The caller wouldn't be able to tell that the file transfer is
+            // done and would incorrectly repeat the same call.
+            return writer_writeSplat(context, headers_and_trailers, 1);
+        }
         return n;
     }
     var iovecs_buffer: [max_buffers_len]std.posix.iovec_const = undefined;

lib/std/io/BufferedWriter.zig

@@ -69,7 +69,8 @@ pub fn reset(bw: *BufferedWriter) void {
 pub fn flush(bw: *BufferedWriter) anyerror!void {
     const list = &bw.buffer;
     const send_buffer = list.items;
-    try bw.unbuffered_writer.writeAll(send_buffer);
+    var index: usize = 0;
+    while (index < send_buffer.len) index += try bw.unbuffered_writer.writev(&.{send_buffer[index..]});
     list.items.len = 0;
 }
 

lib/std/io/Writer.zig

@@ -149,6 +149,9 @@ fn null_writeFile(
 ) anyerror!usize {
     _ = context;
     var n: usize = 0;
+    if (offset == .none) {
+        @panic("TODO seek the file forwards");
+    }
     if (len == .entire_file) {
         const headers = headers_and_trailers[0..headers_len];
         for (headers) |bytes| n += bytes.len;

lib/std/posix.zig

@@ -6359,7 +6359,7 @@ pub const SendFileError = PReadError || WriteError || SendError;
 pub fn sendfile(
     out_fd: fd_t,
     in_fd: fd_t,
-    in_offset: ?u64,
+    in_offset: u64,
     in_len: u64,
     headers: []const iovec_const,
     trailers: []const iovec_const,
@@ -6390,9 +6390,8 @@ pub fn sendfile(
 
             const sendfile_sym = if (lfs64_abi) system.sendfile64 else system.sendfile;
             while (true) {
-                var offset: off_t = if (in_offset) |o| o else undefined;
+                var offset: off_t = @bitCast(in_offset);
-                const offset_pointer: ?*off_t = if (in_offset) &offset else null;
+                const rc = sendfile_sym(out_fd, in_fd, &offset, adjusted_count);
-                const rc = sendfile_sym(out_fd, in_fd, offset_pointer, adjusted_count);
                 switch (errno(rc)) {
                     .SUCCESS => {
                         const amt: usize = @bitCast(rc);

test/standalone/simple/hello_world/hello.zig

@@ -1,5 +1,5 @@
 const std = @import("std");
 
 pub fn main() !void {
-    try std.io.getStdOut().writeAll("Hello, World!\n");
+    try std.fs.File.stdout().writeAll("Hello, World!\n");
 }