Server-side warnings for NuGet client

Problem

The NuGet client issues warnings and errors as they are encountered. For example a warning is shown when running nuget pack and package metadata is invalid. Or an error when nuget push fails to authenticate with a remote server.

Currently, NuGet servers can return errors to the NuGet client using standard HTTP error codes. However adding a warning to responses is not possible.

This spec proposes an approach to add server-side warnings support to all HTTP communications.

Solution

The server can return a NuGet-specific HTTP header, X-NuGet-Warning. When encountered in any HTTP response from a NuGet server, the NuGet client will display a warning message.

Note the X-NuGet-Warning header is not mandatory. When not present, no warning messages should be displayed. The header can occur multiple times. For each value of the header, a warning message must be shown

Here's an example where the API key of the user is about to expire. The server responded with a X-NuGet-Warning header that held the full warning text: