KTOR-9423 CannotTransformContentToTypeException leaks internal class

[zibet27]

Subsystem
Server

Motivation
KTOR-9423 CannotTransformContentToTypeException leaks internal class names in the response body

Solution

• Move CannotTransformContentToTypeException catching, allowing StatusPages to process it
• Don't send the error message in prod mode

[coderabbitai]

📝 Walkthrough

Walkthrough

The PR centralizes content transformation exception handling by removing default receive-side error handling from BaseApplicationEngine and consolidating exception mapping in DefaultEnginePipeline. Both UnsupportedMediaTypeException and CannotTransformContentToTypeException are now mapped to UnsupportedMediaType status, with logging severity adjusted accordingly. A test case validates the new exception handling path.

Changes

Cohort / File(s)	Summary
Core engine error handling ktor-server/ktor-server-core/common/src/io/ktor/server/engine/BaseApplicationEngine.kt	Removed receive-side transformation error interception that caught CannotTransformContentToTypeException and returned 415; error handling now relies solely on post-render send pipeline phase.
Default pipeline exception mapping ktor-server/ktor-server-core/common/src/io/ktor/server/engine/DefaultEnginePipeline.kt	Expanded defaultExceptionStatusCode to map CannotTransformContentToTypeException to UnsupportedMediaType; refined tryRespondError to consider developmentMode when deciding whether to include error message; adjusted logFailure to treat both transformation exceptions as debug-level.
Test coverage ktor-server/ktor-server-plugins/ktor-server-status-pages/common/test/io/ktor/server/plugins/statuspages/StatusPagesTest.kt	Added test case testCannotTransformContentToTypeException verifying custom StatusPages handler for the exception type with correct status and response body.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• KTOR-6794 Send response in routing on failures #4938: Modifies error handling and response logic in DefaultEnginePipeline, directly related to exception mapping and failure response handling changes.

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 14.29% 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 summarizes the main change: fixing a leak of internal class names in the CannotTransformContentToTypeException response.
Description check	✅ Passed	The description follows the required template with all sections completed: Subsystem (Server), Motivation (references KTOR-9423 issue), and Solution (specific changes outlined).

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch zibet27/fix-internal-class-message-leak

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[coderabbitai]

Actionable comments posted: 1

🧹 Nitpick comments (1)

ktor-server/ktor-server-plugins/ktor-server-status-pages/common/test/io/ktor/server/plugins/statuspages/StatusPagesTest.kt (1)

631-647: Please cover the default no-leak path too.

Because this handler replaces the body, the test doesn't verify the actual leak fix in the default engine response. A companion case that triggers CannotTransformContentToTypeException through the receive path without StatusPages would lock in the production 415/no-message behavior as well.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In
`@ktor-server/ktor-server-plugins/ktor-server-status-pages/common/test/io/ktor/server/plugins/statuspages/StatusPagesTest.kt`
around lines 631 - 647, Add a companion test that exercises the default
(no-StatusPages) path for CannotTransformContentToTypeException to ensure the
engine returns the production 415/no-message behavior; specifically, create a
setup that does not install StatusPages, has a routing POST that triggers throw
CannotTransformContentToTypeException(typeOf<String>()), issues client.post("/")
and asserts response.status is HttpStatusCode.UnsupportedMediaType and
response.bodyAsText() is empty (or no message) so the leak fix is verified in
the default receive path.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In
`@ktor-server/ktor-server-plugins/ktor-server-status-pages/common/test/io/ktor/server/plugins/statuspages/StatusPagesTest.kt`:
- Line 630: Rename the test function testCannotTransformContentToTypeException
to a descriptive backtick-style Kotlin test name (e.g., `fun \`cannot transform
content to target type returns appropriate status\`()`) to follow the
repository's **/test/**/*.kt naming convention; update the function declaration
(testCannotTransformContentToTypeException) and any references to it so the test
framework runs the new backtick name without changing behavior.

---

Nitpick comments:
In
`@ktor-server/ktor-server-plugins/ktor-server-status-pages/common/test/io/ktor/server/plugins/statuspages/StatusPagesTest.kt`:
- Around line 631-647: Add a companion test that exercises the default
(no-StatusPages) path for CannotTransformContentToTypeException to ensure the
engine returns the production 415/no-message behavior; specifically, create a
setup that does not install StatusPages, has a routing POST that triggers throw
CannotTransformContentToTypeException(typeOf<String>()), issues client.post("/")
and asserts response.status is HttpStatusCode.UnsupportedMediaType and
response.bodyAsText() is empty (or no message) so the leak fix is verified in
the default receive path.

🪄 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: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: ae4f0fc2-8550-4a20-bafa-7e36299097e3

📥 Commits

Reviewing files that changed from the base of the PR and between 12de7fb and 0a59138.

📒 Files selected for processing (3)

• ktor-server/ktor-server-core/common/src/io/ktor/server/engine/BaseApplicationEngine.kt
• ktor-server/ktor-server-core/common/src/io/ktor/server/engine/DefaultEnginePipeline.kt
• ktor-server/ktor-server-plugins/ktor-server-status-pages/common/test/io/ktor/server/plugins/statuspages/StatusPagesTest.kt

[coderabbitai]

⚠️ Potential issue | 🟡 Minor

Use a descriptive backtick test name.

This new case should follow the repository's **/test/**/*.kt naming convention.

✏️ Rename suggestion

-    fun testCannotTransformContentToTypeException() = testApplication {
+    fun `handles CannotTransformContentToTypeException via StatusPages`() = testApplication {

As per coding guidelines `**/test/**/*.kt`: Use descriptive test names in backticks following the pattern: `describe what is being tested`.

📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change

	fun testCannotTransformContentToTypeException() = testApplication {
	fun `handles CannotTransformContentToTypeException via StatusPages`() = testApplication {

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In
`@ktor-server/ktor-server-plugins/ktor-server-status-pages/common/test/io/ktor/server/plugins/statuspages/StatusPagesTest.kt`
at line 630, Rename the test function testCannotTransformContentToTypeException
to a descriptive backtick-style Kotlin test name (e.g., `fun \`cannot transform
content to target type returns appropriate status\`()`) to follow the
repository's **/test/**/*.kt naming convention; update the function declaration
(testCannotTransformContentToTypeException) and any references to it so the test
framework runs the new backtick name without changing behavior.

[osipxd]

Please check failing tests