fix: harden high-fanout delivery under load

server/db/mysql/invalid_connection_test.go

@@ -0,0 +1,55 @@
+//go:build mysql
+
+package mysql
+
+import (
+	"context"
+	"database/sql/driver"
+	"errors"
+	"testing"
+
+	ms "github.com/go-sql-driver/mysql"
+)
+
+func TestRetryInvalidConnectionRetriesOnce(t *testing.T) {
+	attempts := 0
+	err := retryInvalidConnection("test", func() error {
+		attempts++
+		if attempts == 1 {
+			return ms.ErrInvalidConn
+		}
+		return nil
+	})
+	if err != nil || attempts != 2 {
+		t.Fatalf("err=%v attempts=%d", err, attempts)
+	}
+}
+
+func TestRetryInvalidConnectionDoesNotRetryOtherErrors(t *testing.T) {
+	want := errors.New("write failed")
+	attempts := 0
+	err := retryInvalidConnection("test", func() error {
+		attempts++
+		return want
+	})
+	if !errors.Is(err, want) || attempts != 1 {
+		t.Fatalf("err=%v attempts=%d", err, attempts)
+	}
+}
+
+func TestInstrumentedQueryDiscardsInvalidConnection(t *testing.T) {
+	conn := instrumentedConn{conn: invalidQueryConn{}}
+	_, err := conn.QueryContext(context.Background(), "SELECT 1", nil)
+	if !errors.Is(err, driver.ErrBadConn) {
+		t.Fatalf("err=%v want=%v", err, driver.ErrBadConn)
+	}
+}
+
+type invalidQueryConn struct{}
+
+func (invalidQueryConn) Prepare(string) (driver.Stmt, error) { return nil, driver.ErrSkip }
+func (invalidQueryConn) Close() error                        { return nil }
+func (invalidQueryConn) Begin() (driver.Tx, error)           { return nil, driver.ErrSkip }
+func (invalidQueryConn) QueryContext(context.Context, string, []driver.NamedValue) (driver.Rows, error) {
+	return nil, ms.ErrInvalidConn
+}