From e9dde230247ec5525d4c3803ff99c97e0dc8e348 Mon Sep 17 00:00:00 2001
From: Francis Lavoie <lavofr@gmail.com>
Date: Tue, 4 Jan 2022 12:10:11 -0500
Subject: headers: Fix `+` in Caddyfile to properly append rather than set
 (#4506)

---
 caddytest/integration/caddyfile_adapt/header.txt | 15 +++++++++++++++
 modules/caddyhttp/headers/caddyfile.go           |  2 +-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/caddytest/integration/caddyfile_adapt/header.txt b/caddytest/integration/caddyfile_adapt/header.txt
index 223839e..34a044d 100644
--- a/caddytest/integration/caddyfile_adapt/header.txt
+++ b/caddytest/integration/caddyfile_adapt/header.txt
@@ -13,6 +13,10 @@
 	header @images {
 		Cache-Control "public, max-age=3600, stale-while-revalidate=86400"
 	}
+	header {
+		+Link "Foo"
+		+Link "Bar"
+	}
 }
 ----------
 {
@@ -121,6 +125,17 @@
 											]
 										}
 									}
+								},
+								{
+									"handler": "headers",
+									"response": {
+										"add": {
+											"Link": [
+												"Foo",
+												"Bar"
+											]
+										}
+									}
 								}
 							]
 						}
diff --git a/modules/caddyhttp/headers/caddyfile.go b/modules/caddyhttp/headers/caddyfile.go
index c6ea2fb..eec1149 100644
--- a/modules/caddyhttp/headers/caddyfile.go
+++ b/modules/caddyhttp/headers/caddyfile.go
@@ -222,7 +222,7 @@ func applyHeaderOp(ops *HeaderOps, respHeaderOps *RespHeaderOps, field, value, r
 		if ops.Add == nil {
 			ops.Add = make(http.Header)
 		}
-		ops.Add.Set(field[1:], value)
+		ops.Add.Add(field[1:], value)
 
 	case strings.HasPrefix(field, "-"): // delete
 		ops.Delete = append(ops.Delete, field[1:])
-- 
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