[format] Support lineSep as fallback key of json.line-delimiter

paimon-format/src/main/java/org/apache/paimon/format/json/JsonOptions.java

@@ -52,6 +52,7 @@ public class JsonOptions {
             ConfigOptions.key("json.line-delimiter")
                     .stringType()
                     .defaultValue("\n")
+                    .withFallbackKeys("lineSep")
                     .withDescription("The line delimiter for JSON format");
 
     /** Enum for handling null keys in JSON maps. */

paimon-format/src/test/java/org/apache/paimon/format/json/JsonFileFormatTest.java

@@ -592,6 +592,65 @@ public void testWithCustomLineDelimiters() throws IOException {
         }
     }
 
+    @Test
+    public void testJsonLineDelimiterFallbackKey() throws IOException {
+        RowType rowType = DataTypes.ROW(DataTypes.INT().notNull(), DataTypes.STRING());
+
+        Options options = new Options();
+        options.set("lineSep", "|");
+
+        assertThat(options.get(JsonOptions.LINE_DELIMITER)).isEqualTo("|");
+
+        // the writer uses the delimiter given by the fallback key
+        String json =
+                writeToJson(
+                        options,
+                        rowType,
+                        GenericRow.of(1, BinaryString.fromString("Alice")),
+                        "test_line_sep");
+        assertThat(json).endsWith("|").doesNotContain("\n");
+
+        // the reader splits a '|' delimited file into rows
+        Path testFile = new Path(parent, "test_line_sep_read_" + UUID.randomUUID() + ".json");
+        try (PositionOutputStream out = fileIO.newOutputStream(testFile, false)) {
+            out.write(
+                    "{\"f0\":1,\"f1\":\"Alice\"}|{\"f0\":2,\"f1\":\"Bob\"}|"
+                            .getBytes(StandardCharsets.UTF_8));
+        }
+        FileFormat format =
+                new JsonFileFormat(new FileFormatFactory.FormatContext(options, 1024, 1024));
+        List<InternalRow> result = new ArrayList<>();
+        try (RecordReader<InternalRow> reader =
+                format.createReaderFactory(rowType, rowType, new ArrayList<>())
+                        .createReader(
+                                new FormatReaderContext(
+                                        fileIO,
+                                        testFile,
+                                        fileIO.getFileSize(testFile),
+                                        null,
+                                        null))) {
+            InternalRowSerializer serializer = new InternalRowSerializer(rowType);
+            reader.forEachRemaining(row -> result.add(serializer.copy(row)));
+        }
+        assertThat(result).hasSize(2);
+        assertThat(result.get(0).getInt(0)).isEqualTo(1);
+        assertThat(result.get(0).getString(1).toString()).isEqualTo("Alice");
+        assertThat(result.get(1).getInt(0)).isEqualTo(2);
+        assertThat(result.get(1).getString(1).toString()).isEqualTo("Bob");
+
+        List<InternalRow> roundTrip =
+                writeThenRead(
+                        options,
+                        rowType,
+                        Arrays.asList(
+                                GenericRow.of(1, BinaryString.fromString("Alice")),
+                                GenericRow.of(2, BinaryString.fromString("Bob"))),
+                        "test_line_sep_round_trip");
+        assertThat(roundTrip).hasSize(2);
+        assertThat(roundTrip.get(0).getString(1).toString()).isEqualTo("Alice");
+        assertThat(roundTrip.get(1).getString(1).toString()).isEqualTo("Bob");
+    }
+
     @Test
     public void testVectorTypeReadWrite() throws IOException {
         RowType rowType = DataTypes.ROW(DataTypes.INT(), DataTypes.VECTOR(3, DataTypes.FLOAT()));