From f285d4b0d9018a5b256bcbb729c08bc87f09549e Mon Sep 17 00:00:00 2001
From: Mike Crute <mike@crute.us>
Date: Tue, 26 Sep 2023 21:02:27 -0700
Subject: echo: add header merge middleware

---
 echo/echo_default.go             |  1 +
 echo/middleware/header_merger.go | 39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)
 create mode 100644 echo/middleware/header_merger.go

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)
+		}
+	}
+}
-- 
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