From 0413ff8003e1fa1cd33b4ea456e0943af948606e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Dani=C3=ABl=20van=20Eeden?= <git@myname.nl>
Date: Tue, 11 Mar 2025 09:02:32 +0100
Subject: [PATCH 1/2] replication: check hostname length

---
 canal/config.go                  |  3 +-
 replication/binlogsyncer.go      | 12 +++++---
 replication/binlogsyncer_test.go | 50 ++++++++++++++++++++++++++++++++
 3 files changed, 60 insertions(+), 5 deletions(-)
 create mode 100644 replication/binlogsyncer_test.go

diff --git a/canal/config.go b/canal/config.go
index 35df54632..583906ac5 100644
--- a/canal/config.go
+++ b/canal/config.go
@@ -105,7 +105,8 @@ type Config struct {
 	// Set Dialer
 	Dialer client.Dialer
 
-	// Set Localhost
+	// Set the hostname that is used when registering as replica. This is similar to `report_host` in MySQL.
+	// This will be truncated if it is longer than 255 characters.
 	Localhost string
 
 	// EventCacheCount is the capacity of the BinlogStreamer internal event channel.
diff --git a/replication/binlogsyncer.go b/replication/binlogsyncer.go
index 3ef4fb63a..612229926 100644
--- a/replication/binlogsyncer.go
+++ b/replication/binlogsyncer.go
@@ -573,13 +573,17 @@ func (b *BinlogSyncer) writeBinlogDumpMariadbGTIDCommand(gset mysql.GTIDSet) err
 	return b.writeBinlogDumpCommand(mysql.Position{Name: "", Pos: 0})
 }
 
-// localHostname returns the hostname that register slave would register as.
+// localHostname returns the hostname that register replica would register as.
+// this gets truncated to 255 bytes.
 func (b *BinlogSyncer) localHostname() string {
-	if len(b.cfg.Localhost) == 0 {
-		h, _ := os.Hostname()
+	h := b.cfg.Localhost
+	if len(h) == 0 {
+		h, _ = os.Hostname()
+	}
+	if len(h) <= 255 {
 		return h
 	}
-	return b.cfg.Localhost
+	return h[:255]
 }
 
 func (b *BinlogSyncer) writeRegisterSlaveCommand() error {
diff --git a/replication/binlogsyncer_test.go b/replication/binlogsyncer_test.go
new file mode 100644
index 000000000..052ce67ad
--- /dev/null
+++ b/replication/binlogsyncer_test.go
@@ -0,0 +1,50 @@
+package replication
+
+import (
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestLocalHostname(t *testing.T) {
+	b := BinlogSyncer{
+		cfg: BinlogSyncerConfig{
+			Localhost: "foobar",
+		},
+	}
+
+	require.Equal(t, "foobar", b.localHostname())
+}
+
+func TestLocalHostname_long(t *testing.T) {
+	b := BinlogSyncer{
+		cfg: BinlogSyncerConfig{
+			Localhost: strings.Repeat("x", 255),
+		},
+	}
+
+	require.Equal(t, 255, len(b.localHostname()))
+}
+
+func TestLocalHostname_toolong(t *testing.T) {
+	b := BinlogSyncer{
+		cfg: BinlogSyncerConfig{
+			Localhost: strings.Repeat("x", 300),
+		},
+	}
+
+	require.Equal(t, 255, len(b.localHostname()))
+}
+
+func TestLocalHostname_os(t *testing.T) {
+	b := BinlogSyncer{
+		cfg: BinlogSyncerConfig{
+			Localhost: "",
+		},
+	}
+
+	h, _ := os.Hostname()
+	require.Equal(t, h, b.localHostname())
+}
\ No newline at end of file

From 0fd57bb53b03a3195f6d8382f4a5247ade50170a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Dani=C3=ABl=20van=20Eeden?= <git@myname.nl>
Date: Tue, 11 Mar 2025 16:57:26 +0100
Subject: [PATCH 2/2] fixup

---
 replication/binlogsyncer_test.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/replication/binlogsyncer_test.go b/replication/binlogsyncer_test.go
index 052ce67ad..9895673e3 100644
--- a/replication/binlogsyncer_test.go
+++ b/replication/binlogsyncer_test.go
@@ -47,4 +47,4 @@ func TestLocalHostname_os(t *testing.T) {
 
 	h, _ := os.Hostname()
 	require.Equal(t, h, b.localHostname())
-}
\ No newline at end of file
+}