[kotlin] [multiplatform] [jvm-ktor] Fix formdata file upload

[kalinjul]

The currently generated code for multipart/form-data does not compile for binary parameters.
See #9423, #19644 and #18094. It also fails when there's a list of files to be uploaded.

This PR includes 2 changes:

1. KotlinClientCodegen was changed to respect container types (to fix lists of files).
2. KotlinClientCodegen will now set the dataType to Ktor's FormPart instead of InputProvider
3. The api.mustache was changed accordingly.

No. 2 has some implications:

• The parameter name must be passed by the caller (When creating a FormPart like FormPart("document", inputProvider)
• The headers are still not set correctly, e.g. mime type and filename have to be set by the caller.

This may not be the most convenient solution to upload files, but it should suit every application as it leaves FormPart creation to the caller.
I think this would be a good improvement to the current state.

Sample Yaml:

openapi: 3.0.1
info:
  title: Test API
  version: '1'
paths:
  /some/path/method/:
    post:
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                singledocument:
                  type: "string"
                  format: "binary"
                documents:
                  type: "array"
                  items:
                    type: "string"
                    format: "binary"
                isAccepted:
                  type: "boolean"
      responses:
        204:
          description: 'Processed.'

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[wing328]

thanks for the PR

using the spec provided, I did a test with

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -g kotlin -i /tmp/kotlin-form.yaml -o /tmp/kotlin-form/ --library multiplatform --additional-properties dateLibrary=kotlinx-datetime

but i don't see any change in the code (only changes in the doc - DefaultApi.md)

am I testing your change with the right configurations/options?

[kalinjul]

That seems strange, i get the following diff vs. the generated code using master:

--- out_master/src/commonMain/kotlin/org/openapitools/client/apis/DefaultApi.kt 2025-04-09 10:37:15
+++ out/src/commonMain/kotlin/org/openapitools/client/apis/DefaultApi.kt        2025-04-09 10:33:53
@@ -49,15 +49,15 @@
      * @param isAccepted  (optional)
      * @return void
      */
-    open suspend fun somePathMethodPost(singledocument: io.ktor.client.request.forms.InputProvider? = null, documents: io.ktor.client.request.forms.InputProvider? = null, isAccepted: kotlin.Boolean? = null): HttpResponse<Unit> {
+    open suspend fun somePathMethodPost(singledocument: io.ktor.client.request.forms.FormPart<io.ktor.client.request.forms.InputProvider>? = null, documents: kotlin.collections.List<io.ktor.client.request.forms.FormPart<io.ktor.client.request.forms.InputProvider>>? = null, isAccepted: kotlin.Boolean? = null): HttpResponse<Unit> {
 
         val localVariableAuthNames = listOf<String>()
 
         val localVariableBody = 
             formData {
-                singledocument?.apply { append("singledocument", singledocument) }
+                singledocument?.apply { append(singledocument) }
                 documents?.onEach {
-                    append("documents[]", it)
+                    append(it)
                 }
                 isAccepted?.apply { append("isAccepted", isAccepted) }
             }

Used the same command with the yaml from this PR

[kalinjul]

@wing328 : could you restart the ci, i solved the merge conflicts and somehow the ci has an error now..

[wing328]

restarted (by closing and reopening the PR)

[kalinjul]

Here we go. Don't know what the issue was, but rebasing again seems to have solved it.

[wing328]

good

i'm able to spot some differences with your fix

--- a/src/commonMain/kotlin/org/openapitools/client/apis/DefaultApi.kt
+++ b/src/commonMain/kotlin/org/openapitools/client/apis/DefaultApi.kt
@@ -49,15 +49,15 @@ open class DefaultApi : ApiClient {
      * @param isAccepted  (optional)
      * @return void
      */
-    open suspend fun somePathMethodPost(singledocument: io.ktor.client.request.forms.InputProvider? = null, documents: io.ktor.client.request.forms.InputProvider? = null, isAccepted: kotlin.Boolean? = null): HttpResponse<Unit> {
+    open suspend fun somePathMethodPost(singledocument: io.ktor.client.request.forms.FormPart<io.ktor.client.request.forms.InputProvider>? = null, documents: kotlin.collections.List<io.ktor.client.request.forms.FormPart<io.ktor.client.request.forms.InputProvider>>? = null, isAccepted: kotlin.Boolean? = null): HttpResponse<Unit> {

         val localVariableAuthNames = listOf<String>()

         val localVariableBody =
             formData {
-                singledocument?.apply { append("singledocument", singledocument) }
+                singledocument?.apply { append(singledocument) }
                 documents?.onEach {
-                    append("documents[]", it)
+                    append(it)
                 }
                 isAccepted?.apply { append("isAccepted", isAccepted) }
             }

and build is fine

BUILD SUCCESSFUL in 1m 48s
18 actionable tasks: 18 executed