From dd0d82ad0f56590d1396ad4f12c6379c996adec8 Mon Sep 17 00:00:00 2001
From: "sentry[bot]" <39604003+sentry[bot]@users.noreply.github.com>
Date: Wed, 15 Apr 2026 15:06:59 +0000
Subject: [PATCH 1/2] Normalize iCal line endings to CRLF

---
 internal/app.go | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/internal/app.go b/internal/app.go
index d5faaa4..3512f15 100644
--- a/internal/app.go
+++ b/internal/app.go
@@ -226,7 +226,12 @@ func (a *App) handleIcal(ctx *gin.Context) {
 		return
 	}
 
-	response := []byte(cleaned.Serialize())
+	// Normalize line endings to CRLF as required by RFC 5545.
+	// First replace any existing CRLF with LF to avoid doubling the \r,
+	// then replace all LF with CRLF.
+	serialized := strings.ReplaceAll(cleaned.Serialize(), "\r\n", "\n")
+	serialized = strings.ReplaceAll(serialized, "\n", "\r\n")
+	response := []byte(serialized)
 	ctx.Header("Content-Type", "text/calendar")
 	ctx.Header("Content-Length", fmt.Sprintf("%d", len(response)))
 

From 821bbf11ad1bdf182e354316f6f7b32ce21e8b98 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Wed, 15 Apr 2026 15:17:21 +0000
Subject: [PATCH 2/2] Add test for CRLF normalization and extract helper
 function

Agent-Logs-Url: https://github.com/TUM-Dev/CalendarProxy/sessions/6214dd01-8799-45e9-a021-70d21545a01a

Co-authored-by: kordianbruck <298860+kordianbruck@users.noreply.github.com>
---
 internal/app.go      | 15 +++++++++------
 internal/app_test.go | 27 +++++++++++++++++++++++++++
 2 files changed, 36 insertions(+), 6 deletions(-)

diff --git a/internal/app.go b/internal/app.go
index 3512f15..740daa1 100644
--- a/internal/app.go
+++ b/internal/app.go
@@ -226,12 +226,7 @@ func (a *App) handleIcal(ctx *gin.Context) {
 		return
 	}
 
-	// Normalize line endings to CRLF as required by RFC 5545.
-	// First replace any existing CRLF with LF to avoid doubling the \r,
-	// then replace all LF with CRLF.
-	serialized := strings.ReplaceAll(cleaned.Serialize(), "\r\n", "\n")
-	serialized = strings.ReplaceAll(serialized, "\n", "\r\n")
-	response := []byte(serialized)
+	response := []byte(normalizeCRLF(cleaned.Serialize()))
 	ctx.Header("Content-Type", "text/calendar")
 	ctx.Header("Content-Length", fmt.Sprintf("%d", len(response)))
 
@@ -240,6 +235,14 @@ func (a *App) handleIcal(ctx *gin.Context) {
 	}
 }
 
+// normalizeCRLF normalizes line endings in s to CRLF (\r\n) as required by RFC 5545.
+// It first replaces any existing CRLF with LF to avoid doubling the \r,
+// then replaces all LF with CRLF.
+func normalizeCRLF(s string) string {
+	s = strings.ReplaceAll(s, "\r\n", "\n")
+	return strings.ReplaceAll(s, "\n", "\r\n")
+}
+
 // handleGetCourses returns a list of all courses that are currently offered on campus.
 // This is used to populate the dropdown in the landing page for hiding courses.
 func (a *App) handleGetCourses(ctx *gin.Context) {
diff --git a/internal/app_test.go b/internal/app_test.go
index 638d85a..8837eeb 100644
--- a/internal/app_test.go
+++ b/internal/app_test.go
@@ -171,6 +171,33 @@ func TestLocationReplacement(t *testing.T) {
 	}
 }
 
+func TestNormalizeCRLF(t *testing.T) {
+	testData, app := getTestData(t, "duplication.ics")
+	calendar, err := app.getCleanedCalendar([]byte(testData), map[string]bool{})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	serialized := normalizeCRLF(calendar.Serialize())
+
+	// RFC 5545 requires CRLF line endings; there must be no bare \n.
+	if !strings.Contains(serialized, "\r\n") {
+		t.Error("serialized calendar should contain CRLF line endings")
+	}
+	for i, ch := range serialized {
+		if ch == '\n' && (i == 0 || serialized[i-1] != '\r') {
+			t.Error("serialized calendar contains a bare LF (\\n) without a preceding CR (\\r)")
+			break
+		}
+	}
+
+	// Content-Length must equal byte length of the normalized output.
+	response := []byte(serialized)
+	if len(response) == 0 {
+		t.Error("serialized calendar response must not be empty")
+	}
+}
+
 func TestCourseFiltering(t *testing.T) {
 	testData, app := getTestData(t, "coursefiltering.ics")