Primora/apps/backend/internal/middleware/compression.go

package middleware

import (
	"compress/gzip"
	"io"
	"strings"
	"sync"

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

var gzipPool = sync.Pool{
	New: func() any {
		w, _ := gzip.NewWriterLevel(io.Discard, gzip.DefaultCompression)
		return w
	},
}

type gzipWriter struct {
	gin.ResponseWriter
	writer *gzip.Writer
}

func (g *gzipWriter) Write(data []byte) (int, error) {
	// Compressed output never matches a Content-Length set by a handler —
	// drop it so net/http falls back to chunked encoding.
	g.Header().Del("Content-Length")
	return g.writer.Write(data)
}

func Compression() gin.HandlerFunc {
	return func(c *gin.Context) {
		if !strings.Contains(c.Request.Header.Get("Accept-Encoding"), "gzip") {
			c.Next()
			return
		}

		// Skip compression for small responses or streaming — gzip buffers
		// writes, which breaks SSE flush semantics.
		if c.Request.Method == "HEAD" ||
			c.Request.URL.Path == "/api/v1/health/liveness" ||
			strings.HasSuffix(c.Request.URL.Path, "/stream") {
			c.Next()
			return
		}

		gz := gzipPool.Get().(*gzip.Writer)
		defer gzipPool.Put(gz)
		gz.Reset(c.Writer)
		defer gz.Close()

		c.Header("Content-Encoding", "gzip")
		c.Header("Vary", "Accept-Encoding")
		c.Writer = &gzipWriter{ResponseWriter: c.Writer, writer: gz}
		c.Next()
	}
}