Add trailing comma feature

Currently the behavior allows there to be a single extraneous data column
that is empty. This is normally harmless, however, external tools that
expect the same number of data columns as header columns will complain
when this extraneous column is encountered.

The fix is to make this feature explicit and allow for it to be disabled.

This patch retains backwards compatibility by enabling the feature by
default.

Author: nickbabcock

src/main/java/com/fasterxml/jackson/dataformat/csv/CsvParser.java

@@ -67,6 +67,15 @@ public enum Feature
          * @since 2.7
          */
         IGNORE_TRAILING_UNMAPPABLE(false),
+
+        /**
+         * Feature that allows there to be a trailing single extraneous data
+         * column that is empty. When this feature is disabled, any extraneous
+         * column, regardless of content will cause an exception to be thrown.
+         * Disabling this feature is only useful when
+         * IGNORE_TRAILING_UNMAPPABLE is also disabled.
+         */
+        ALLOW_TRAILING_COMMA(true),
         ;
 
         final boolean _defaultState;
@@ -730,7 +739,7 @@ protected JsonToken _handleExtraColumn(String value) throws IOException
         // 14-Mar-2012, tatu: As per [dataformat-csv#1], let's allow one specific case
         // of extra: if we get just one all-whitespace entry, that can be just skipped
         _state = STATE_SKIP_EXTRA_COLUMNS;
-        if (_columnIndex == _columnCount) {
+        if (_columnIndex == _columnCount && Feature.ALLOW_TRAILING_COMMA.enabledIn(_formatFeatures)) {
             value = value.trim();
             if (value.isEmpty()) {
                 // if so, need to verify we then get the end-of-record;

src/test/java/com/fasterxml/jackson/dataformat/csv/deser/TrailingCommaTest.java

@@ -0,0 +1,39 @@
+package com.fasterxml.jackson.dataformat.csv.deser;
+
+import com.fasterxml.jackson.annotation.JsonPropertyOrder;
+import com.fasterxml.jackson.databind.JsonMappingException;
+import com.fasterxml.jackson.databind.MappingIterator;
+import com.fasterxml.jackson.dataformat.csv.CsvMapper;
+import com.fasterxml.jackson.dataformat.csv.CsvParser;
+import com.fasterxml.jackson.dataformat.csv.CsvSchema;
+import com.fasterxml.jackson.dataformat.csv.ModuleTestBase;
+
+public class TrailingCommaTest extends ModuleTestBase {
+    final CsvMapper MAPPER = mapperForCsv();
+
+    @JsonPropertyOrder({ "a", "b" })
+    static class StringPair {
+        public String a, b;
+    }
+
+    public void testDisallowTrailingComma() throws Exception
+    {
+        final String INPUT = "s,t\nd,e,\n";
+        final CsvSchema schema = MAPPER.schemaFor(StringPair.class);
+
+        MappingIterator<StringPair> it = MAPPER.readerFor(StringPair.class)
+                .with(schema)
+                .without(CsvParser.Feature.ALLOW_TRAILING_COMMA)
+                .readValues(INPUT);
+
+        it.nextValue();
+        try {
+            it.nextValue();
+            fail("Should not have passed");
+        } catch (JsonMappingException e) {
+            verifyException(e, "Too many entries: expected at most 2 (value #2 (0 chars) \"\")");
+        }
+
+        it.close();
+    }
+}