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# Chronos: rebuilding OSS-Fuzz harnesses using cached builds
## Pre-built images.
Daily pre-built images are available at:
- `us-central1-docker.pkg.dev/oss-fuzz/oss-fuzz-gen/<PROJECT>-ofg-cached-address`
- `us-central1-docker.pkg.dev/oss-fuzz/oss-fuzz-gen/<PROJECT>-ofg-cached-coverage`
They can be used as drop-in replacements for the usual `gcr.io/oss-fuzz/<PROJECT>` images.
These images are generated in 2 ways:
- (Preferred) [Generate](https://github.com/google/oss-fuzz/blob/master/infra/base-images/base-builder/bash_parser.py)
a replay build script that can be re-run alongside existing build artifacts,
leveraging existing build system mechanisms to avoid rebuilding (e.g. running
`make` twice should not actually rebuild everything). This is error-prone, so
we validate the script works by running it.
- (Fallback, if the replay build script didn't work). We leverage
[ccache](https://ccache.dev/), to provide a compiler cache. This is often not
as fast as the replay build script, because some project builds spend
significant time doing non-compiler tasks (e.g. checking out submodules,
running configure scripts).
Note: this mechanism does not work for every single OSS-Fuzz project today. The
resulting image may either:
- Not provide much performance improvement compared with a normal image, or
- Not exist at all (if neither approach worked).
Stats from a recent run: <https://gist.github.com/oliverchang/abaf3a1106a2b923c0ac3a577410aaaa>
(Feb 3 2025).
## Usage locally
**Example 1: htslib**
From the OSS-Fuzz root
```sh
$ RUN_ALL=1 ./infra/experimental/chronos/build_cache_local.sh htslib c address
...
...
Vanilla compile time:
17
Replay worked
Replay compile time:
2
Ccache compile time:
9
```
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