[BUG][PowerShell] Parameter of type array passed in pipeline: only first element is processed

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The issue affects functions New-PSUsersWithArrayInput and New-PSUsersWithListInput in PSPetStore API.
The parameter $User has type PsCustomObject[] and has the attribute ValueFromPipeline = $true.
The body of the function consists of a single Process {} block that ends with a return statement.

In PowerShell the Process {} block is executed once for each input object that reaches the function, so the actual behaviour is the serialization the first element, the execution of Invoke-PSApiClient and a return to the caller.

openapi-generator version

$  openapi-generator --version
openapi-generator-cli 7.8.0
  commit : 6bdc452
  built  : -999999999-01-01T00:00:00+18:00
  source : https://github.com/openapitools/openapi-generator
  docs   : https://openapi-generator.tech/

OpenAPI declaration file content or url

user.yml

Generation Details

Standard generation of PSPetStore client library

Steps to reproduce

Import-Module -Name PSPetstore
$a = Initialize-PSUser -Id 1 -Username foo
$b = Initialize-PSUser -Id 2 -Username bar
$c = @($a, $b)
$c.GetType()
$DebugPreference = 'Continue'
$c | New-PSUsersWithArrayInput

Both New-PSUsersWithArrayInput and Invoke-PSApiClient show a single object being processed (*):

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Object[]                                 System.Array

DEBUG: Calling method: New-PSUsersWithArrayInput
DEBUG: Parameter Value
DEBUG: --------- -----
DEBUG: User      @{id=1; username=foo; firstName=; lastName=; email=; password=; phone=; userStatus=;
DEBUG:           objectWithNoDeclaredProps=; objectWithNoDeclaredPropsNullable=; anyTypeProp=; anyTypePropNullable=}
DEBUG: Calling method: Invoke-PSApiClient
DEBUG: Parameter        Value
DEBUG: ---------        -----
DEBUG: Method           POST
DEBUG: Uri              /user/createWithArray
DEBUG: Accepts
DEBUG: ContentTypes     application/json
DEBUG: Body             [
DEBUG:                    {
DEBUG:                      "id": 1,
DEBUG:                      "username": "foo",
DEBUG:                      "firstName": "",
DEBUG:                      "lastName": "",
DEBUG:                      "email": "",
DEBUG:                      "password": "",
DEBUG:                      "phone": "",
DEBUG:                      "userStatus": null,
DEBUG:                      "objectWithNoDeclaredProps": null,
DEBUG:                      "objectWithNoDeclaredPropsNullable": null,
DEBUG:                      "anyTypeProp": null,
DEBUG:                      "anyTypePropNullable": null
DEBUG:                    }
DEBUG:                  ]
DEBUG: HeaderParameters {}
DEBUG: QueryParameters  {}
DEBUG: FormParameters   {}
DEBUG: CookieParameters {}
DEBUG: ReturnType
DEBUG: IsBodyNullable   False

(*) The function Out-DebugParameter was modified by adding the parameter -Wrap to Format-Table in order to show the full body.

Related issues/PRs

Array context not maintained when converting a parameter to JSON #18427

Suggest a fix

The main product of the function produced by api.mustache is a JSON serialization of the input, and this requires the whole input object to be passed at once to ConvertTo-JSON. Actually, in PowerShell 5 the behavior of ConvertTo-JSON is such that the output may not be correct when the input object is passed from the pipeline.

IMHO there are two options:

1. Remove the ValueFromPipeline attribute when the main parameter is an array type.
2. Use the Process {}block to reconstruct the input array, and pass this variable to ConvertTo-Json in the End {}block of the function. This option shall ensure that the serialization of the reconstructed collection is the same as the original one.

Start with the generated PowerShell functions New-PSUsersWithArrayInput and New-PSUsersWithListInput and the api.mustache template, using user.yml and the reproduction commands to observe pipeline processing. Check how the Process block and return statement interact with Invoke-PSApiClient. Done means array input is serialized and sent as a complete collection rather than only its first element.