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Reader: fix fill when implementation writes to reader directly #24677

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Reader: fix fill when implementation writes to reader directly #24677

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30 changes: 28 additions & 2 deletions lib/std/Io/Reader.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -1065,10 +1065,13 @@ fn fillUnbuffered(r: *Reader, n: usize) Error!void {
};
while (r.end < r.seek + n) {
writer.end = r.end;
r.end += r.vtable.stream(r, &writer, .limited(r.buffer.len - r.end)) catch |err| switch (err) {
const streamed = r.vtable.stream(r, &writer, .limited(r.buffer.len - r.end)) catch |err| switch (err) {
error.WriteFailed => unreachable,
error.ReadFailed, error.EndOfStream => |e| return e,
};
// Doing `r.end += r.vtable.stream` is not possible since `r.end` may be modified
// by the implementation, differing from the already evaluated value.
r.end += streamed;
}
}

Expand All @@ -1084,10 +1087,13 @@ pub fn fillMore(r: *Reader) Error!void {
.end = r.end,
.vtable = &.{ .drain = Writer.fixedDrain },
};
r.end += r.vtable.stream(r, &writer, .limited(r.buffer.len - r.end)) catch |err| switch (err) {
const streamed = r.vtable.stream(r, &writer, .limited(r.buffer.len - r.end)) catch |err| switch (err) {
error.WriteFailed => unreachable,
else => |e| return e,
};
// Doing `r.end += r.vtable.stream` is not possible since `r.end` may be modified
// by the implementation, differing from the already evaluated value.
r.end += streamed;
}

/// Returns the next byte from the stream or returns `error.EndOfStream`.
Expand Down Expand Up @@ -1519,6 +1525,26 @@ test fill {
try r.fill(3);
}

test "fill writing directly to reader buffer" {
var buf: [2]u8 = undefined;
var indirect_r: Reader = .{
.buffer = &buf,
.seek = 0,
.end = 0,
.vtable = &.{
.stream = struct {
fn stream(r: *Reader, _: *Writer, _: Limit) StreamError!usize {
@memset(r.buffer[r.seek..], 0xff);
r.end = r.buffer.len;
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actually this is not a valid use of the API

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@gooncreeper gooncreeper Aug 10, 2025 •
edited
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@andrewrk Apologies, though what is invalid with it? The vtable doc comment says you can do this and flate.Decompress.streamIndirectInner takes advantage of this.

return 0;
}
}.stream,
},
};
try indirect_r.fill(2);
try std.testing.expectEqualSlices(u8, &.{ 0xff, 0xff }, indirect_r.buffered());
}

test takeByte {
var r: Reader = .fixed("ab");
try testing.expectEqual('a', try r.takeByte());
Expand Down