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Update docs about hash checking and wheel cache
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sbidoul committed Apr 16, 2023
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Expand Up @@ -59,13 +59,13 @@ It is possible to use multiple hashes for each package. This is important when a

### Interaction with caching

The {ref}`locally-built wheel cache <wheel-caching>` is disabled in hash-checking mode to prevent spurious hash mismatch errors.

These would otherwise occur while installing sdists that had already been automatically built into cached wheels: those wheels would be selected for installation, but their hashes would not match the sdist ones from the requirements file.

A further complication is that locally built wheels are nondeterministic: contemporary modification times make their way into the archive, making hashes unpredictable across machines and cache flushes. Compilation of C code adds further nondeterminism, as many compilers include random-seeded values in their output.
```{versionchanged} 23.1
The {ref}`locally-built wheel cache <wheel-caching>` is used in hash-checking mode too.
```

However, wheels fetched from index servers are required to be the same every time. They land in pip's HTTP cache, not its wheel cache, and are used normally in hash-checking mode. The only downside of having the wheel cache disabled is thus extra build time for sdists, and this can be solved by making sure pre-built wheels are available from the index server.
When installing from the cache of locally built wheel in hash-checking mode, pip verifies
the hashes agains those of the original source distribution that was used to build the wheel.
These original hashes are obtained from a `origin.json` file stored in each cache entry.

### Using hashes from PyPI (or other index servers)

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