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Fix Reader.Limited hangs #25109

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Fix Reader.Limited hangs #25109

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ec636c3
When streaming 0 bytes from Reader, return 0
whatisaphone Sep 1, 2025
c95d2b7
Fix Reader.Limited end of stream conditions
whatisaphone Sep 1, 2025
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12 changes: 12 additions & 0 deletions lib/std/Io/Reader.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -168,6 +168,10 @@ pub fn fixed(buffer: []const u8) Reader {
}

pub fn stream(r: *Reader, w: *Writer, limit: Limit) StreamError!usize {
if (limit == .nothing) {
@branchHint(.unlikely);
return 0;
}
Comment on lines +171 to +174

@andrewrk andrewrk Oct 9, 2025

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This should not be logic inside the interface. To enforce this rule, it must be an assertion. There's already an assertion below for this on line 183 but it will be skipped if an error is returned. I suggest instead of this to do the assertion after calling stream but before returning an error. Also, this rule must be mentioned in the VTable documentation for stream.

const buffer = limit.slice(r.buffer[r.seek..r.end]);
if (buffer.len > 0) {
@branchHint(.likely);
Expand Down Expand Up @@ -1329,6 +1333,14 @@ test fixed {
try testing.expectError(error.EndOfStream, r.takeByte());
}

test "fixed: when streaming 0 bytes, return 0" {
var r: Reader = .fixed("i'm dying");
var trash: Writer.Discarding = .init(&.{});

const n = try r.stream(&trash.writer, .nothing);
try testing.expectEqual(n, 0);
}

test peek {
var r: Reader = .fixed("abc");
try testing.expectEqualStrings("ab", try r.peek(2));
Expand Down
44 changes: 44 additions & 0 deletions lib/std/Io/Reader/Limited.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -27,6 +27,7 @@ pub fn init(reader: *Reader, limit: Limit, buffer: []u8) Limited {

fn stream(r: *Reader, w: *Writer, limit: Limit) Reader.StreamError!usize {
const l: *Limited = @fieldParentPtr("interface", r);
if (l.remaining == .nothing) return error.EndOfStream;

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@whatisaphone whatisaphone Sep 2, 2025

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Right now reading 0 bytes is a no-op, not an error. It's like subtracting 0 from a number. I don't think that should change.

@rohlem rohlem Sep 2, 2025 •
edited
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Ah, my bad, I confused the interface implementation function here (in Reader/Limited.zig) with the user-facing stream function in the interface Reader.zig.
Reader.stream already returns 0 if limit == .nothing was passed, in that case this function isn't called - all good then, sorry for the noise.
(We could document and assert(limit != .nothing); here in the implementation functions, but that's probably deemed unnecessary.)

const combined_limit = limit.min(l.remaining);
const n = try l.unlimited.stream(w, combined_limit);
l.remaining = l.remaining.subtract(n).?;
Expand All @@ -51,8 +52,51 @@ test stream {

fn discard(r: *Reader, limit: Limit) Reader.Error!usize {
const l: *Limited = @fieldParentPtr("interface", r);
if (l.remaining == .nothing) return error.EndOfStream;
const combined_limit = limit.min(l.remaining);
const n = try l.unlimited.discard(combined_limit);
l.remaining = l.remaining.subtract(n).?;
return n;
}

test "end of stream, read, hit limit exactly" {
var f: Reader = .fixed("i'm dying");
var l = f.limited(.limited(4), &.{});
const r = &l.interface;

var buf: [2]u8 = undefined;
try r.readSliceAll(&buf);
try r.readSliceAll(&buf);
try std.testing.expectError(error.EndOfStream, l.interface.readSliceAll(&buf));
}

test "end of stream, read, hit limit after partial read" {
var f: Reader = .fixed("i'm dying");
var l = f.limited(.limited(5), &.{});
const r = &l.interface;

var buf: [2]u8 = undefined;
try r.readSliceAll(&buf);
try r.readSliceAll(&buf);
try std.testing.expectError(error.EndOfStream, l.interface.readSliceAll(&buf));
}

test "end of stream, discard, hit limit exactly" {
var f: Reader = .fixed("i'm dying");
var l = f.limited(.limited(4), &.{});
const r = &l.interface;

try r.discardAll(2);
try r.discardAll(2);
try std.testing.expectError(error.EndOfStream, l.interface.discardAll(2));
}

test "end of stream, discard, hit limit after partial read" {
var f: Reader = .fixed("i'm dying");
var l = f.limited(.limited(5), &.{});
const r = &l.interface;

try r.discardAll(2);
try r.discardAll(2);
try std.testing.expectError(error.EndOfStream, l.interface.discardAll(2));
}