Add option to exclude content types from gzip compression#134
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Add GZIP_COMPRESSION_EXCEPT_CONTENT_TYPES, a comma-separated list of content types to exclude from gzip compression in addition to gzhttp's built-in exclusions. Matched by prefix, so "image/" excludes all image types while "image/png" excludes only PNG. gzhttp's DefaultContentTypeFilter already skips video/*, audio/*, and JPEG, but compresses image/png, image/webp, image/avif, etc. Those are already compressed, so gzipping them wastes CPU and forces chunked transfer encoding, which drops Content-Length. The only prior workaround was disabling compression entirely, which also turns it off for text. The new filter extends the default rather than replacing it, so leaving the variable unset is byte-identical to current behaviour.
Cover the behaviours introduced by GZIP_COMPRESSION_EXCEPT_CONTENT_TYPES that the happy-path tests missed: matching a non-first prefix, case-insensitive matching of a mixed-case configured value, blank entries being ignored (rather than excluding everything), built-in exclusions still applying alongside configured ones, and the empty-slice no-op that the production default relies on.
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Pull request overview
Adds a configurable way to skip gzip compression for already-compressed response types (e.g., PNG/WebP/AVIF) while preserving gzhttp’s built-in exclusions and keeping default behavior unchanged when the option is unset. This fits cleanly into Thruster’s existing gzip configuration flow by threading a new config field through Service -> HandlerOptions -> CompressionHandler and extending gzhttp’s content-type filter.
Changes:
- Adds
GZIP_COMPRESSION_EXCEPT_CONTENT_TYPES(comma-separated, prefix-matched) to configuration and documentation. - Extends the gzip wrapper setup to apply an additional content-type exclusion filter on top of
gzhttp.DefaultContentTypeFilter. - Adds focused unit tests covering prefix matching, normalization, parameterized content types, and “no-op when unset/empty” behavior.
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Reviewed changes
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Show a summary per file
| File | Description |
|---|---|
| README.md | Documents the new GZIP_COMPRESSION_EXCEPT_CONTENT_TYPES env var and its prefix-matching behavior. |
| internal/service.go | Passes the new config value into HandlerOptions so it reaches the HTTP pipeline. |
| internal/handler.go | Threads the exception list into NewCompressionHandler. |
| internal/config.go | Parses GZIP_COMPRESSION_EXCEPT_CONTENT_TYPES into Config.GzipCompressionExceptContentTypes. |
| internal/config_test.go | Verifies default/override behavior for the new config field. |
| internal/compression_handler.go | Adds a filter that extends gzhttp.DefaultContentTypeFilter with configurable prefix exclusions. |
| internal/compression_handler_test.go | Adds tests validating the new filtering behavior and ensuring defaults remain unchanged. |
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Thanks for bringing this up @mmenanno! I agree we shouldn't be wasting time compressing these image types. In fact that was a test trying to show that we weren't, but it was only targeting JPEG which seems to be the one image type that gzhttp excludes by default. Given Thruster's goal of being as nearly-zero-config as possible, I think we should just exclude these types by default rather than introduce new config. I've opened #137 to do that. If we hit upon a strong argument for needing to override this on a per-app basis we could introduce the config then. But having this in the default behaviour seems like it should be a win for apps in almost all cases. |
Thruster gzips everything except gzhttp's built-in exclusions (
video/*,audio/*, JPEG). That still compressesimage/png,image/webp,image/avifand similar, which are already compressed, so it just burns CPU and forces chunked encoding that dropsContent-Length. Right now the only way to avoid that is turning compression off entirely, which also disables it for text.This adds
GZIP_COMPRESSION_EXCEPT_CONTENT_TYPES, a comma-separated list of content types to skip on top of the built-in exclusions. It's matched by prefix, soimage/skips all images whileimage/pngskips only PNG.The filter extends gzhttp's default rather than replacing it, so leaving the variable unset behaves exactly as before.