io: add Err field to LimitedReader #76156

grishy · 2025-11-03T15:53:15Z

Add an Err field to LimitedReader that allows callers to return a
custom error when the read limit is exceeded, instead of always
returning EOF.

When Err is set to a non-nil, and the limit is reached,
LimitedReader.Read probes the underlying reader with a 1-byte
read to distinguish two cases: stream had exactly N bytes
(returns EOF), or stream has more data. The probe result is cached
using negative N values to avoid repeated reads.

When Err is nil, Read returns EOF, maintaining backward compatibility.
Zero-length reads return (0, nil) without side effects when we
reached end of Reader.

Fixes #51115

Add an Err field to LimitedReader that allows callers to return a custom error when the read limit is exceeded, instead of always returning EOF. When Err is set to a non-nil, non-EOF value, and the limit is reached, LimitedReader.Read probes the underlying reader with a 1-byte read to distinguish two cases: stream had exactly N bytes (returns EOF), or stream has more data (returns the custom Err). The probe result is cached using negative N values to avoid repeated reads. When Err is nil or EOF, Read returns EOF, maintaining backward compatibility. Zero-length reads return (0, nil) without side effects. Fixes golang#51115

gopherbot · 2025-11-03T16:06:13Z

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gopherbot · 2025-11-03T16:16:33Z

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gopherbot · 2025-11-03T16:16:34Z

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gopherbot · 2025-11-03T17:09:59Z

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gopherbot · 2025-11-03T23:02:30Z

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gopherbot · 2025-11-04T06:36:40Z

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gopherbot · 2025-11-04T07:39:20Z

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gopherbot · 2025-11-04T08:21:19Z

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gopherbot · 2025-11-04T08:29:44Z

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gopherbot · 2025-11-05T01:57:35Z

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Patch Set 5: Code-Review+1 Commit-Queue+1

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gopherbot · 2025-11-05T02:05:33Z

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gopherbot · 2025-11-05T02:05:33Z

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Patch Set 5: -Commit-Queue

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gopherbot · 2025-11-05T02:05:34Z

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Patch Set 5:

This CL has failed the run. Reason:

Tryjob golang/try/x_tools-gotip-linux-amd64 has failed with summary (view all results):

go on master (change 717340) / tools on master (commit ce95316)

To reproduce, try gomote repro 8699080481333919025.

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gopherbot · 2025-11-05T02:05:35Z

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Patch Set 5: LUCI-TryBot-Result-1

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gopherbot · 2025-11-05T04:32:58Z

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gopherbot · 2025-11-05T04:41:33Z

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gopherbot · 2025-11-05T05:26:21Z

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Patch Set 7: Commit-Queue+1

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gopherbot · 2025-11-05T05:26:22Z

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Patch Set 7:

Dry run: CV is trying the patch.

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gopherbot · 2025-11-05T05:34:41Z

Message from Ian Lance Taylor:

Patch Set 7: -Commit-Queue

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gopherbot · 2025-11-05T05:34:41Z

Message from Go LUCI:

Patch Set 7:

This CL has failed the run. Reason:

Tryjob golang/try/gotip-linux-amd64_avx512 has failed with summary (view all results):

go on master (change 717340)

To reproduce, try gomote repro 8699067611298001873.

Additional links for debugging:

go tool dist test -json output [shard 1]

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gopherbot · 2025-11-05T05:34:42Z

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Patch Set 7: LUCI-TryBot-Result-1

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austinderek mentioned this pull request Nov 3, 2025

io: add Err field to LimitedReader PullRequestInc/go#134

Open

io: add doc/next file and remove errors.Is from example

0796ec8

io: remove unnecessary EOF checks

ebf18d7

io: use consts in LimitedReader and clean up comments

50cc465

io: use raw string literals in tests

bd71dde

io: add Err field to LimitedReader #76156

Are you sure you want to change the base?

io: add Err field to LimitedReader #76156

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grishy commented Nov 3, 2025 •

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