| author |  Ryan Liptak <squeek502@hotmail.com> 2025-09-19 22:10:53-07:00 |
| committer |  Andrew Kelley <andrew@ziglang.org> 2025-09-20 18:31:38-07:00 |
| log | 3fbb88c4bd146ca7bd9e7ab5da9c4b05298f3b34 |
| tree | 8a07a0a5999991b99e31062acd6f87da1a1307d3 |
| parent | 0e47bd16dac9dcdfc12168c07104030af8eaaa8b |
Reader.defaultReadVec: Workaround bad `r.end += r.vtable.stream()` behavior
If `r.end` is updated in the `stream` implementation, then it's possible that `r.end += ...` will behave unexpectedly. What seems to happen is that it reverts back to its value before the function call and then the increment happens. Here's a reproduction:
```zig
test "fill when stream modifies `end` and returns 0" {
var buf: [3]u8 = undefined;
var zero_reader = infiniteZeroes(&buf);
_ = try zero_reader.fill(1);
try std.testing.expectEqual(buf.len, zero_reader.end);
}
pub fn infiniteZeroes(buf: []u8) std.Io.Reader {
return .{
.vtable = &.{
.stream = stream,
},
.buffer = buf,
.end = 0,
.seek = 0,
};
}
fn stream(r: *std.Io.Reader, _: *std.Io.Writer, _: std.Io.Limit) std.Io.Reader.StreamError!usize {
@memset(r.buffer[r.seek..], 0);
r.end = r.buffer.len;
return 0;
}
```
When `fill` is called, it will call into `vtable.readVec` which in this case is `defaultReadVec`. In `defaultReadVec`:
- Before the `r.end += r.vtable.stream` line, `r.end` will be 0
- In `r.vtable.stream`, `r.end` is modified to 3 and it returns 0
- After the `r.end += r.vtable.stream` line, `r.end` will be 0 instead of the expected 3
Separating the `r.end += stream();` into two lines fixes the problem (and this separation is done elsewhere in `Reader` so it seems possible that this class of bug has been encountered before).
Potentially related issues:
- https://github.com/ziglang/zig/issues/4021
- https://github.com/ziglang/zig/issues/120641 files changed, 2 insertions(+), 1 deletions(-)
lib- std
lib/std/Io/Reader.zig+2-1
| ... | ... | @@ -432,10 +432,11 @@ pub fn defaultReadVec(r: *Reader, data: [][]u8) Error!usize { |
| 432 | 432 | .vtable = &.{ .drain = Writer.fixedDrain }, |
| 433 | 433 | }; |
| 434 | 434 | const limit: Limit = .limited(writer.buffer.len - writer.end); |
| 435 | r.end += r.vtable.stream(r, &writer, limit) catch |err| switch (err) { | |
| 435 | const n = r.vtable.stream(r, &writer, limit) catch |err| switch (err) { | |
| 436 | 436 | error.WriteFailed => unreachable, |
| 437 | 437 | else => |e| return e, |
| 438 | 438 | }; |
| 439 | r.end += n; | |
| 439 | 440 | return 0; |
| 440 | 441 | } |
| 441 | 442 |