From 6464ce86f86fdd8ae0f9a3fc80885eb31b1cd835 Mon Sep 17 00:00:00 2001
From: KarthikeyaKollu <karthikeyakollu8@gmail.com>
Date: Wed, 7 Oct 2026 01:28:51 +0530
Subject: [PATCH] fix: ask Zoho for MODSEQ searches as ESEARCH so an empty
 reply no longer drops the IMAP session (#857)

Zoho answers an empty UID SEARCH MODSEQ with "* SEARCH  (MODSEQ n)", which
go-imap cannot parse; the session dropped on every quiet pass and the mailbox
showed SERVER_UNREACHABLE. Its ESEARCH answer to the same query is well formed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 internal/client/smtpimap/imap/client.go       |  9 +++-
 .../smtpimap/imap/search_modseq_test.go       | 47 +++++++++++++++++++
 2 files changed, 55 insertions(+), 1 deletion(-)
 create mode 100644 internal/client/smtpimap/imap/search_modseq_test.go

diff --git a/internal/client/smtpimap/imap/client.go b/internal/client/smtpimap/imap/client.go
index 078777d73..f40fdd755 100644
--- a/internal/client/smtpimap/imap/client.go
+++ b/internal/client/smtpimap/imap/client.go
@@ -602,7 +602,14 @@ func (c *Client) uidSearch(criteria *imap.SearchCriteria) ([]imap.UID, *errx.Mai
 	if err := c.resumeSyncLocked(); err != nil {
 		return nil, c.handleError(err)
 	}
-	data, err := c.client.UIDSearch(criteria, nil).Wait()
+	// Zoho answers a MODSEQ search that matches nothing with
+	// "* SEARCH  (MODSEQ n)", which go-imap cannot parse, so the session drops
+	// on every quiet pass. Its ESEARCH answer to the same query is well formed.
+	var opts *imap.SearchOptions
+	if criteria.ModSeq != nil && c.client.Caps().Has(imap.CapESearch) {
+		opts = &imap.SearchOptions{ReturnAll: true}
+	}
+	data, err := c.client.UIDSearch(criteria, opts).Wait()
 	if err != nil {
 		return nil, c.handleError(err)
 	}
diff --git a/internal/client/smtpimap/imap/search_modseq_test.go b/internal/client/smtpimap/imap/search_modseq_test.go
new file mode 100644
index 000000000..9292ed60a
--- /dev/null
+++ b/internal/client/smtpimap/imap/search_modseq_test.go
@@ -0,0 +1,47 @@
+package imap
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/emersion/go-imap/v2"
+)
+
+// zohoModSeqSearch answers a CONDSTORE search the way imappro.zoho.in does
+// when nothing changed: the plain form carries a double space go-imap cannot
+// parse, the ESEARCH form is well formed.
+func zohoModSeqSearch(line string) string {
+	fields := strings.Fields(line)
+	if len(fields) < 2 || !strings.Contains(strings.ToUpper(line), "SEARCH") || !strings.Contains(strings.ToUpper(line), "MODSEQ") {
+		return ""
+	}
+	tag := fields[0]
+	if strings.Contains(strings.ToUpper(line), "RETURN") {
+		return `* ESEARCH (TAG "` + tag + `") UID MODSEQ 1791314919556115300` + "\r\n" + tag + " OK Success"
+	}
+	return "* SEARCH  (MODSEQ 1791314919556115300)\r\n" + tag + " OK Success"
+}
+
+// Zoho dropped every SMTP/IMAP mailbox with "server unreachable" on the first
+// quiet sync pass: the empty MODSEQ search answer killed the session.
+func TestSearchChangedSinceOnZohoEmptyResult(t *testing.T) {
+	caps := imap.CapSet{imap.CapIMAP4rev1: {}, imap.CapESearch: {}, imap.CapCondStore: {}}
+	c, wire := interceptingServer(t, caps, zohoModSeqSearch)
+	if err := c.Connect(); err != nil {
+		t.Fatalf("Connect: %v", err)
+	}
+	if _, err := c.SelectForSync("INBOX"); err != nil {
+		t.Fatalf("SelectForSync: %v", err)
+	}
+	uids, err := c.SearchChangedSince(1791314919556115300)
+	if err != nil {
+		t.Fatalf("SearchChangedSince: %v", err)
+	}
+	if len(uids) != 0 {
+		t.Fatalf("uids = %v, want none", uids)
+	}
+	searches := wire.commands("SEARCH")
+	if len(searches) != 1 || !strings.Contains(searches[0], "RETURN (ALL)") {
+		t.Fatalf("modseq search did not use ESEARCH: %v", searches)
+	}
+}