`ConnectionOptionsOAuth2` unmarshal fails on tenants with legacy string-encoded `customHeaders`

[alexmsenger]

Checklist

• I have looked into the README and have not found a suitable solution or answer.
• I have looked into the documentation and have not found a suitable solution or answer.
• I have searched the issues and have not found a suitable solution or answer.
• I have upgraded to the latest version of this SDK and the issue still persists.
• I have searched the Auth0 Community forums and have not found a suitable solution or answer.
• I agree to the terms within the Auth0 Code of Conduct.

Description

ConnectionOptionsOAuth2.UnmarshalJSON fails with a type assertion error when
the Auth0 Management API returns the customHeaders field as a JSON-encoded
string rather than a map object.

The error message is:

json: cannot unmarshal string into Go struct field of type map[string]string

This occurs on older Auth0 tenants where the customHeaders value was persisted
in a legacy string format. The SDK makes no attempt to handle this variant and
instead surfaces the raw JSON decode error to the caller, which causes the
terraform-provider-auth0 to crash when reading or refreshing an OAuth2
connection resource on such tenants.

Expectation

When the API returns "customHeaders": "<json-encoded-string>", the SDK should
not return an error. The legacy format cannot be reliably decoded without knowing
the original structure, so the field should be surfaced as nil/empty and the
operator can restore the values through their Terraform configuration.

When the API returns "customHeaders": {"key": "value"} (the current format),
the field should continue to be decoded into *map[string]string as normal.

Reproduction

package main

import (
    "encoding/json"
    "fmt"
    "github.com/auth0/go-auth0/management"
)

func main() {
    // Simulates what the Management API returns on an older tenant.
    // customHeaders is a JSON-encoded string instead of a map object.
    payload := `{
        "authorizationURL": "https://example.com/oauth/authorize",
        "tokenURL": "https://example.com/oauth/token",
        "customHeaders": "{\"X-Custom-Header\":\"custom-value\"}"
    }`

    var opts management.ConnectionOptionsOAuth2
    err := json.Unmarshal([]byte(payload), &opts)
    if err != nil {
        fmt.Printf("Error: %v\n", err)
        // Prints: json: cannot unmarshal string into Go struct field of type map[string]string
    }
}

Steps to reproduce in production:

1. Have an Auth0 tenant where an OAuth2 connection was created and had customHeaders set while the SDK persisted them in a legacy string format.
2. Use terraform-provider-auth0 to run terraform refresh or terraform plan against that connection.
3. The provider crashes with the above unmarshal error.

Auth0 Go SDK version

v1.44.0

[developerkunal]

Hey @alexmsenger,

Thank you for opening the issue! We are currently looking into the best way to handle and maintain this behavior. As soon as we have a decision, we will get back to you with an update.