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std.Io.HostName: fix a bug where expand would include the terminator #25845

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std.Io.HostName: fix a bug where expand would include the terminator #25845

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std.Io.HostName: fix a bug where `expand` would include the terminator
notcancername Nov 7, 2025
8119b14
std.Io.HostName: add regression test for #25811
notcancername Nov 7, 2025
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13 changes: 12 additions & 1 deletion lib/std/Io/net/HostName.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -137,7 +137,6 @@ pub fn expand(noalias packet: []const u8, start_i: usize, noalias dest_buffer: [
i += 1 + label_len;
} else {
dest[dest_i] = 0;
dest_i += 1;
return .{
len orelse i - start_i + 1,
try .init(dest[0..dest_i]),
Expand All @@ -147,6 +146,18 @@ pub fn expand(noalias packet: []const u8, start_i: usize, noalias dest_buffer: [
return error.InvalidDnsPacket;
}

// regression test for <https://github.com/ziglang/zig/issues/25811>
test "HostName.expand correctly parses host names in DNS packets" {
const packet = "\x90A\x81\x80\x00\x01\x00\x03\x00\x00\x00\x00\x03www\x15alditalk-kundenportal\x02de\x00\x00\x01\x00\x01\xc0\x0c\x00\x05\x00\x01\x00\x00\x076\x00\x1f\x06static\x08spectrum\x0atelefonica\x02de\x00\xc0:\x00\x01\x00\x01\x00\x00\x00I\x00\x04\x8deZ'\xc0:\x00\x01\x00\x01\x00\x00\x00I\x00\x04\x8deZ&";
const index = 58;

var out: [max_len]u8 = undefined;
const bytes_consumed, const name = try expand(packet, index, &out);

try std.testing.expectEqual(@as(usize, 31), bytes_consumed);
try std.testing.expectEqualSlices(u8, "static.spectrum.telefonica.de", name.bytes);
}

pub const DnsRecord = enum(u8) {
A = 1,
CNAME = 5,
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