Move decompression to earlier phase in receivePipeline.

[ascheja]

Subsystem
Server, compression

Motivation
See KTOR-6683.

Currently content decoding only works in cases where the application directly receives a ByteReadChannel. When combining it with any default transformations (ByteArray, Parameters, MultiPartData, ...) the content decoding would silently not work because the Compression's transformBody would already get the converted subject from the automatically installed default transformations. The same principle applies to the ContentNegotation plugin if it is installed before the Compression plugin, although in that case the transformation would likely fail.

Solution
Simply moves the content decoding into it's own phase right before the default Transform phase.

[coderabbitai]

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Walkthrough

Refactors the compression plugin to decode request bodies in a new internal ContentDecoding pipeline phase and changes the decode receiver to ContentDecoding.Context; adds ContentDecoding implementation, updates plugin interception, and adds tests plus a test dependency to validate request decompression scenarios.

Changes

Cohort / File(s)	Summary
Compression Plugin Core ktor-server/ktor-server-plugins/ktor-server-compression/jvm/src/io/ktor/server/plugins/compression/Compression.kt, ktor-server/ktor-server-plugins/ktor-server-compression/jvm/src/io/ktor/server/plugins/compression/ContentDecoding.kt	Introduce internal ContentDecoding phase and ContentDecoding.Context with transformBody; replace onCallReceive hook with on(ContentDecoding) interception; update decode extension receiver from OnCallReceiveContext<CompressionConfig> to ContentDecoding.Context.
Build Configuration ktor-server/ktor-server-tests/build.gradle.kts	Add implementation(projects.ktorSerializationKotlinxJson) to jvmTest dependencies.
Test Suite ktor-server/ktor-server-tests/jvm/test/io/ktor/server/plugins/CompressionTest.kt	Add tests ensuring requests are decompressed before receiving as ByteArray, multipart, and JSON with content negotiation; adjust assertions and add required imports/types.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested reviewers

• bjhham

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: moving decompression to an earlier phase in the receive pipeline, which is the core objective of this PR.
Description check	✅ Passed	The description includes all required template sections (Subsystem, Motivation, Solution) with substantive content explaining the problem and the fix.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[coderabbitai]

Actionable comments posted: 2

🤖 Fix all issues with AI agents

In
`@ktor-server/ktor-server-plugins/ktor-server-compression/jvm/src/io/ktor/server/plugins/compression/ContentDecoding.kt`:
- Around line 1-3: Update the copyright header in ContentDecoding.kt to match
the rest of the PR (use "2014-2026" as in CompressionTest.kt) so the file header
reads the same as other new files; locate the header comment at the top of
ContentDecoding.kt and replace "2014-2024" with "2014-2026".
- Around line 23-32: The plugin is creating a new
PipelinePhase("ContentDecoding") on every install which defeats the pipeline's
reference-equality dedupe; change ContentDecoding.install to use a single shared
phase instance instead of constructing a new one each time (e.g., add a
companion-object/singleton Phase or reuse a predefined
ApplicationReceivePipeline phase) and then call
pipeline.receivePipeline.insertPhaseBefore(ApplicationReceivePipeline.Transform,
Phase) and pipeline.receivePipeline.intercept(Phase) { ... } so duplicate
installs (e.g., on nested routes) will not add multiple ContentDecoding phases.

🧹 Nitpick comments (2)

ktor-server/ktor-server-plugins/ktor-server-compression/jvm/src/io/ktor/server/plugins/compression/ContentDecoding.kt (1)

7-10: Use star imports for io.ktor.* packages.

Lines 8 and 10 use specific imports from io.ktor.* packages. As per coding guidelines, star imports should be used for these.

Proposed fix

 import io.ktor.server.application.*
-import io.ktor.server.request.ApplicationReceivePipeline
+import io.ktor.server.request.*
 import io.ktor.util.pipeline.*
-import io.ktor.utils.io.ByteReadChannel
+import io.ktor.utils.io.*

As per coding guidelines: "Use star imports for io.ktor.* packages".

ktor-server/ktor-server-plugins/ktor-server-compression/jvm/test/io/ktor/server/plugins/compression/CompressionTest.kt (1)

7-37: Use star imports for io.ktor.* packages.

Multiple specific imports from io.ktor.* packages should be star imports per project conventions. For example:

Proposed fix (partial)

-import io.ktor.client.request.forms.MultiPartFormDataContent
-import io.ktor.client.request.forms.formData
-import io.ktor.client.request.header
-import io.ktor.client.request.post
-import io.ktor.client.request.setBody
-import io.ktor.client.statement.bodyAsBytes
-import io.ktor.client.statement.bodyAsText
-import io.ktor.http.ContentType
-import io.ktor.http.HttpHeaders
-import io.ktor.http.content.PartData
-import io.ktor.http.content.forEachPart
-import io.ktor.serialization.kotlinx.json.json
-import io.ktor.server.application.Application
-import io.ktor.server.application.install
-import io.ktor.server.plugins.contentnegotiation.ContentNegotiation
-import io.ktor.server.request.receive
-import io.ktor.server.request.receiveMultipart
-import io.ktor.server.response.respond
-import io.ktor.server.response.respondText
-import io.ktor.server.routing.post
-import io.ktor.server.routing.routing
-import io.ktor.server.testing.testApplication
-import io.ktor.utils.io.ByteChannel
-import io.ktor.utils.io.toByteArray
+import io.ktor.client.request.forms.*
+import io.ktor.client.request.*
+import io.ktor.client.statement.*
+import io.ktor.http.*
+import io.ktor.http.content.*
+import io.ktor.serialization.kotlinx.json.*
+import io.ktor.server.application.*
+import io.ktor.server.plugins.contentnegotiation.*
+import io.ktor.server.request.*
+import io.ktor.server.response.*
+import io.ktor.server.routing.*
+import io.ktor.server.testing.*
+import io.ktor.utils.io.*

As per coding guidelines: "Use star imports for io.ktor.* packages".

[bjhham]

Thanks for the contribution!

We'll need to assess whether or not anyone might be relying on the old behavior here to consider including this in a minor release. It's possible some people may want to store the raw compressed bytes, but unlikely - anyway, I'll get back to you on this.

[bjhham]

Looks like this introduces a couple failures in io.ktor.server.plugins.CompressionTest. Please take a look 👀

[ascheja]

Sorry, overlooked that test class as it was in a completely different subproject. Moved my tests over to it and adjusted the assertions for the two failing tests (both received as ByteArray, but expected compressed content even though request body decompression was enabled)

[bjhham]

Could you please run ./gradlew formatKotlin and push to clear the linter check?

[ascheja]

Did that, but it looks like the linter check is still failing?

[ascheja]

Figured it out - the problem was a line with more than 120 characters, but ./gradlew formatKotlin didn't fix it.