From 429438e56441447ebc64c335cf30e66604404418 Mon Sep 17 00:00:00 2001
From: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Date: Tue, 28 Apr 2026 00:37:00 -0700
Subject: [PATCH] fix(sender): capture Write errors in SendCalendarReply iMIP
 body (#1128)

The multipart body for an iMIP calendar reply was built with direct
Write / Fprint calls whose errors were discarded. A short write or
buffer error there produces a malformed reply that Google Calendar
and Outlook silently reject without recording the RSVP. Capture the
error from each of the eight writes/closes in the iMIP body block
and return early.

No behavior change on the success path.
---
 sender/sender.go | 32 ++++++++++++++++++++++++--------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/sender/sender.go b/sender/sender.go
index caba62d..225b295 100644
--- a/sender/sender.go
+++ b/sender/sender.go
@@ -800,8 +800,12 @@ func SendCalendarReply(account *config.Account, to []string, subject, plainBody
 		return nil, err
 	}
 	qp := quotedprintable.NewWriter(plainPart)
-	fmt.Fprint(qp, plainBody)
-	qp.Close()
+	if _, err := fmt.Fprint(qp, plainBody); err != nil {
+		return nil, err
+	}
+	if err := qp.Close(); err != nil {
+		return nil, err
+	}
 
 	// text/calendar inline part (Outlook/Mac Mail use this)
 	calHeader := textproto.MIMEHeader{}
@@ -811,10 +815,16 @@ func SendCalendarReply(account *config.Account, to []string, subject, plainBody
 	if err != nil {
 		return nil, err
 	}
-	calPart.Write([]byte(clib.WrapBase64(base64.StdEncoding.EncodeToString(icsData))))
+	if _, err := calPart.Write([]byte(clib.WrapBase64(base64.StdEncoding.EncodeToString(icsData)))); err != nil {
+		return nil, err
+	}
 
-	altWriter.Close()
-	altPart.Write(altMsg.Bytes())
+	if err := altWriter.Close(); err != nil {
+		return nil, err
+	}
+	if _, err := altPart.Write(altMsg.Bytes()); err != nil {
+		return nil, err
+	}
 
 	// .ics file attachment (Gmail uses this)
 	attachHeader := textproto.MIMEHeader{}
@@ -825,10 +835,16 @@ func SendCalendarReply(account *config.Account, to []string, subject, plainBody
 	if err != nil {
 		return nil, err
 	}
-	attachPart.Write([]byte(clib.WrapBase64(base64.StdEncoding.EncodeToString(icsData))))
+	if _, err := attachPart.Write([]byte(clib.WrapBase64(base64.StdEncoding.EncodeToString(icsData)))); err != nil {
+		return nil, err
+	}
 
-	outerWriter.Close()
-	msg.Write(outerMsg.Bytes())
+	if err := outerWriter.Close(); err != nil {
+		return nil, err
+	}
+	if _, err := msg.Write(outerMsg.Bytes()); err != nil {
+		return nil, err
+	}
 
 	// Send via SMTP
 	addr := fmt.Sprintf("%s:%d", smtpServer, smtpPort)