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docs: warn packagers that a system uchardet detects non-UTF-8 less accurately #67

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wbarnha merged 1 commit into from
Aug 4, 2026
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docs: warn packagers that a system uchardet detects non-UTF-8 less accurately #67

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31 changes: 30 additions & 1 deletion README.md
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Expand Up @@ -56,7 +56,7 @@ Meson [feature option](https://mesonbuild.com/Build-options.html#features):
| `-Dsystem-uchardet=` | Behaviour |
| --- | --- |
| `disabled` (default) | Always build the bundled copy (used by the wheels). |
| `enabled` | Require the system library; the build **fails** if it is missing or too old. Recommended for distro packaging. |
| `enabled` | Require the system library; the build **fails** if it is missing or too old. Intended for distro packaging — read the detection-quality caveat below first. |
| `auto` | Use the system library if it is new enough, otherwise fall back to the bundled copy. |

Pass the option through your build front-end. With `pip` / `build` the
Expand All @@ -72,6 +72,35 @@ pip install . --config-settings=setup-args=-Dsystem-uchardet=enabled
> release provides it yet — so `enabled`/`auto` probe for it and reject an
> older library.

> [!WARNING]
> **A system build currently detects non-UTF-8 input less accurately than the
> bundled build.** The bundled copy compiles a cChardet-specific replacement
> for uchardet's multi-byte group prober, which both restores detection
> throughput and rejects non-UTF-8 byte sequences that upstream reports as
> UTF-8. A system build links upstream's prober, so neither applies.
>
> Measured over 1650 non-UTF-8 documents spanning 7 encodings
> (`benchmarks/make_nonutf8_corpus.py`, median of 3 runs):
>
> | Build | Throughput | Non-UTF-8 reported as UTF-8 |
> | --- | ---: | ---: |
> | bundled (wheel default) | 3.10 MB/s | 0.0% |
> | system `uchardet` | 1.44 MB/s | **16.2%** |
>
> Reporting non-UTF-8 bytes as UTF-8 is the defect that caused v3.0.0 to be
> yanked from PyPI, so this is not a cosmetic difference. It is an upstream
> issue rather than a packaging mistake: `nsUTF8Prober` does not reject
> invalid sequences on its own, and its confidence never falls low enough for
> the candidate to be discarded. Patches have been sent upstream; once they
> land, the build can require a `uchardet` version that includes them and this
> caveat goes away.
>
> If you are packaging cChardet for a distribution that forbids bundled
> libraries, consider carrying the overlay
> (`src/cchardet/uchardet-overlay/nsMBCSGroupProber.cpp`) as a patch against
> your system `uchardet` until then. The corpus generator and benchmark above
> are in-tree, so you can verify the result yourself.

## Supported Languages/Encodings

- International (Unicode)
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