Add nullability support for server parameter delegation

[nomisRev]

Subsystem
Server, core

Motivation
Currently it is not possible to use nullable values with parameter delegation.

get("/nullable") {
    val nameOrNull: String? by call.queryParameters
    val ageOrNull: Int? by parameters
}

Solution

This PR add support by removing the : Any constraint and relying on typeInfo.kotlinType?.isMarkedNullable to check if null is allowed or not.

[nomisRev]

I'm a bit unsure how reliable typeInfo.kotlinType?.isMarkedNullable is across platforms?

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 2973653 and 65a6d88.

📒 Files selected for processing (1)

• ktor-server/ktor-server-core/api/ktor-server-core.klib.api

🚧 Files skipped from review as they are similar to previous changes (1)

• ktor-server/ktor-server-core/api/ktor-server-core.klib.api

---

📝 Walkthrough

Walkthrough

Generic type constraints for Parameters delegated accessors were relaxed to allow nullable result types; implementation now returns null for absent parameters when the target type is nullable and preserves existing exceptions for non-nullable types and conversion failures.

Changes

Parameters nullable handling

Layer / File(s)	Summary
Core implementation change ktor-server/ktor-server-core/common/src/io/ktor/server/util/Parameters.kt	KDoc and operator getValue changed to reified R (no : Any bound); getOrFail and internal getOrFailImpl generalized similarly and now return null as R when R is nullable and the parameter is absent; otherwise previous exception behavior remains.
Exported API signatures ktor-server/ktor-server-core/api/ktor-server-core.klib.api	Exported signatures updated: getOrFailImpl and getValue bounds relaxed from A: Any to A: Any? to reflect nullable-capable generics.
Tests ktor-server/ktor-server-core/common/test/io/ktor/server/http/ParametersTest.kt	Added tests for nullable delegated parameters resolving to value or null, conversion failure for nullable target, and missing non-null delegated value throwing MissingRequestParameterException.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested reviewers

• marychatte

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 11.11% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: adding nullability support for server parameter delegation, which is the primary objective of this PR.
Description check	✅ Passed	The description includes all required sections: Subsystem, Motivation, and Solution. It clearly explains the problem and the approach taken.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch nomisrev/nullable-parameter-delegation

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[bjhham]

Looks like an easy win 👍

I'm pretty sure isMarkedNullable works cross-platform on reified KTypes.

I don't know why we didn't just implement it that way in the first place...

[bjhham]

FYI logged an issue to handle property delegation in the compiler plugin:
KTOR-9422 OpenAPI code inference misses property delegation

[e5l]

lgtm

[nomisRev]

@osipxd I updated the PR to move the nullability check in the actual implementation. This way we now also support nullable types for getOrFail.

TypeInfo is always being created because it's used by the underlying conversions mechanism. Perhaps the conversions mechanism can be optimised to avoid TypeInfo where possible but I left that for outside of this scope since it seems unrelated to my changes.

Could you review again? Thank you for the careful review and pointing it out! 🙏 We're still getting a better outcome thanks to it!

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@ktor-server/ktor-server-core/common/src/io/ktor/server/util/Parameters.kt`:
- Around line 53-54: Update the KDoc for getOrFail<R> to reflect its new
nullable-missing behavior: state that when R is non-nullable a missing parameter
throws MissingRequestParameterException, but when R is a nullable type the
function returns null for missing values; also keep the existing note that
ParameterConversionException is thrown when conversion from String to R fails.
Modify the documentation on the getOrFail<R> declaration to mention the
conditional behavior based on R's nullability and include both exception and
null-return cases for clarity.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: bb869399-a278-451d-b6c4-7876eda331cb

📥 Commits

Reviewing files that changed from the base of the PR and between 1056631 and 2973653.

📒 Files selected for processing (3)

• ktor-server/ktor-server-core/api/ktor-server-core.klib.api
• ktor-server/ktor-server-core/common/src/io/ktor/server/util/Parameters.kt
• ktor-server/ktor-server-core/common/test/io/ktor/server/http/ParametersTest.kt

🚧 Files skipped from review as they are similar to previous changes (2)

• ktor-server/ktor-server-core/common/test/io/ktor/server/http/ParametersTest.kt
• ktor-server/ktor-server-core/api/ktor-server-core.klib.api

[coderabbitai]

⚠️ Potential issue | 🟡 Minor | ⚡ Quick win

Update generic getOrFail KDoc to include nullable-missing behavior.

getOrFail<R> now returns null for missing values when R is nullable, but this KDoc still says missing always throws. Please align docs with the new contract.

As per coding guidelines: "Keep KDoc documentation correct when behavior/signatures change".

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@ktor-server/ktor-server-core/common/src/io/ktor/server/util/Parameters.kt`
around lines 53 - 54, Update the KDoc for getOrFail<R> to reflect its new
nullable-missing behavior: state that when R is non-nullable a missing parameter
throws MissingRequestParameterException, but when R is a nullable type the
function returns null for missing values; also keep the existing note that
ParameterConversionException is thrown when conversion from String to R fails.
Modify the documentation on the getOrFail<R> declaration to mention the
conditional behavior based on R's nullability and include both exception and
null-return cases for clarity.