SSE not working if content_type header has charset indication

[YanickSchraner]

Branch/Environment/Version

• Branch/Version: https://github.com/TykTechnologies/tyk/tree/v5.7.2 (but code causing bug is identical in 5.9.0rc6
• Environment: On-prem

Describe the bug

Tyk.io is not able to detect SSE responses if the Content-Type response header contains charset indications:

Content-Type: text/event-stream; charset=utf-8. E.g. FastAPI use this charset indication per default for SSE responses.

The code responsible to detect SSE responses in tyk is:

tyk/internal/httputil/streaming.go

Line 20 in ff9eb63

func IsSseStreamingResponse(r *http.Response) bool {

// IsSseStreamingResponse returns true if the response designates SSE streaming.
func IsSseStreamingResponse(r *http.Response) bool {
	return r.Header.Get(headerContentType) == "text/event-stream"
}

Replacing the exact string match with something like

// IsSseStreamingResponse returns true if the response designates SSE streaming.
func IsSseStreamingResponse(r *http.Response) bool {
	return strings.HasPrefix(r.Header.Get(headerContentType), "text/event-stream")
}

shuld fix the issue.

Reproduction steps
Steps to reproduce the behavior:

I created this GitHub repo with an example server to reproduce the behaviour: FastAPI SSE

• docker run yanickbs/fastapi-sse:latest
• Publish an API with the following configuration:
  1. Set an appropriate listen path, e.g. "listen_path": "/sse"
  2. Strip the listen path from the request URL, e.g. "strip_listen_path": true
  3. Set the target URL to the server, e.g. "target_url": "http://host.docker.internal:8000/sse"
  4. Configure the API endpoints GET /sse and GET /sse-fixed-header
  5. Click Save, and wait for the API to be published.
• Test the protected SSE service via curl -vvv http://localhost:8000/sse/sse and curl -vvv http://localhost:8000/sse/sse-fixed-header and check the response headers.

Actual behavior
The response arrives at the CURL client in one message after the SSE is finished.
You get this behavior using this CURL: curl -vvv http://localhost:8000/sse/sse

Expected behavior
The response arrives event by event to the CURL client.
You get this response with this CURL:
curl -vvv http://localhost:8000/sse/sse-fixed-header

Configuration (tyk config file):

Follow the official documentation to configure the API Gateway to use the server.