echo: add header merge middlewareecho/v0.12.0

2 files changed, 40 insertions, 0 deletions

diff --git a/echo/echo_default.go b/echo/echo_default.go
index 33ad015..067e52f 100644
--- a/echo/echo_default.go
+++ b/echo/echo_default.go
@@ -163,6 +163,7 @@ func (w *EchoWrapper) Configure(c EchoConfig) error {
         w.configureCompression(&c)
 
         w.Use(glmw.StrictSecure())
+        w.Use(glmw.MergeHeaders())
 
         w.configureCORS(&c)
         w.configureCSP(&c)
diff --git a/echo/middleware/header_merger.go b/echo/middleware/header_merger.go
new file mode 100644
index 0000000..706ca29
--- /dev/null
+++ b/echo/middleware/header_merger.go
@@ -0,0 +1,39 @@
+package middleware
+
+import (
+        "strings"
+
+        "github.com/labstack/echo/v4"
+)
+
+func mergeHeaders(c echo.Context) {
+        hdr := c.Response().Header()
+
+        // echo duplicates the Vary header a few places so deduplicate them and
+        // write it out as a comma separated list instead of as many duplicate
+        // headers.
+        if vh := hdr.Values(echo.HeaderVary); len(vh) > 1 {
+                seen := map[string]bool{}
+                out := []string{}
+                for _, v := range vh {
+                        if _, ok := seen[v]; !ok {
+                                seen[v] = true
+                                out = append(out, v)
+                        }
+                }
+                hdr.Set(echo.HeaderVary, strings.Join(out, ", "))
+        }
+}
+
+// MergeHeaders merges together specific sets of duplicated headers. It
+// doesn't try to be a general solution to header duplication because
+// that would be too error prone. Currently this only works for the Vary
+// header.
+func MergeHeaders() echo.MiddlewareFunc {
+        return func(next echo.HandlerFunc) echo.HandlerFunc {
+                return func(c echo.Context) error {
+                        c.Response().Before(func() { mergeHeaders(c) })
+                        return next(c)
+                }
+        }
+}