Trailers sent twice if no body written about gin

[RedCrazyGhost]

Try manual flush

package main

import (
	"net/http"

	"github.com/gin-gonic/gin"
)

func main() {
	app := gin.New()
	app.UseH2C = true

	app.POST("/*path", func(c *gin.Context) {
		c.Header("Trailer", "my-status")
		c.Status(http.StatusOK)
		
		c.Writer.Flush()
		
		c.Header("my-status", "OK")
	})

	app.Run(":8080")
}

  │  ~ ▓▒░ curl --http2-prior-knowledge -X POST -H "content-type: application/grpc" -v http://localhost:8080/foo
* processing: http://localhost:8080/foo
*   Trying [::1]:8080...
* Connected to localhost (::1) port 8080
* h2 [:method: POST]
* h2 [:scheme: http]
* h2 [:authority: localhost:8080]
* h2 [:path: /foo]
* h2 [user-agent: curl/8.2.1]
* h2 [accept: */*]
* h2 [content-type: application/grpc]
* Using Stream ID: 1
> POST /foo HTTP/2
> Host: localhost:8080
> User-Agent: curl/8.2.1
> Accept: */*
> content-type: application/grpc
> 
< HTTP/2 200 
< trailer: my-status
< date: Wed, 31 Jan 2024 17:06:38 GMT
< 
< my-status: OK
* Connection #0 to host localhost left intact

[anuraaga]

Ah forgot to mention it, yes flush works as a workaround. But it seems like a bug to require it, in comparison with normal http servemux, trailers would only be sent once even without flush.