Fix NPE in PatternProcessor for the UNIX pattern

Closes 2346.

The constructor of `PatternProcessor` fails to recognize the `UNIX` and
`UNIX_MILLIS` data patterns and deduce the correct rollover frequency.

This PR maps those two patterns to a rollover frequency of second and
millisecond respectively.

Author: ppkarwasz

log4j-core-test/src/test/java/org/apache/logging/log4j/core/appender/rolling/PatternProcessorTest.java

@@ -16,6 +16,8 @@
  */
 package org.apache.logging.log4j.core.appender.rolling;
 
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
 import java.time.Instant;
@@ -25,10 +27,14 @@
 import java.util.Calendar;
 import java.util.Locale;
 import java.util.TimeZone;
+import java.util.stream.Stream;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.parallel.ResourceAccessMode;
 import org.junit.jupiter.api.parallel.ResourceLock;
 import org.junit.jupiter.api.parallel.Resources;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
 
 /**
  * Tests the PatternProcessor class.
@@ -394,4 +400,17 @@ public void testGetNextTimeDailyReturnsFirstHourOfNextDayInGmtIfZoneIsInvalid()
             TimeZone.setDefault(old);
         }
     }
+
+    static Stream<Arguments> works_with_unix_pattern() {
+        return Stream.of(
+                Arguments.of("%d{UNIX}", RolloverFrequency.EVERY_SECOND),
+                Arguments.of("%d{UNIX_MILLIS}", RolloverFrequency.EVERY_MILLISECOND));
+    }
+
+    @ParameterizedTest
+    @MethodSource
+    void works_with_unix_pattern(final String pattern, final RolloverFrequency expectedFrequency) {
+        final PatternProcessor processor = assertDoesNotThrow(() -> new PatternProcessor(pattern));
+        assertThat(processor.getFrequency()).isEqualTo(expectedFrequency);
+    }
 }

log4j-core/src/main/java/org/apache/logging/log4j/core/appender/rolling/PatternProcessor.java

@@ -325,6 +325,12 @@ protected final void formatFileName(final StringBuilder buf, final Object... obj
     }
 
     private RolloverFrequency calculateFrequency(final String pattern) {
+        if ("UNIX".equals(pattern)) {
+            return RolloverFrequency.EVERY_SECOND;
+        }
+        if ("UNIX_MILLIS".equals(pattern)) {
+            return RolloverFrequency.EVERY_MILLISECOND;
+        }
         if (patternContains(pattern, MILLIS_CHAR)) {
             return RolloverFrequency.EVERY_MILLISECOND;
         }

log4j-core/src/main/java/org/apache/logging/log4j/core/pattern/DatePatternConverter.java

@@ -48,9 +48,7 @@ private abstract static class Formatter {
 
         abstract void formatToBuffer(final Instant instant, StringBuilder destination);
 
-        public String toPattern() {
-            return null;
-        }
+        public abstract String toPattern();
 
         public TimeZone getTimeZone() {
             return TimeZone.getDefault();
@@ -143,6 +141,11 @@ String format(final Instant instant) {
         void formatToBuffer(final Instant instant, final StringBuilder destination) {
             destination.append(instant.getEpochSecond()); // no need for caching
         }
+
+        @Override
+        public String toPattern() {
+            return UNIX_FORMAT;
+        }
     }
 
     private static final class UnixMillisFormatter extends Formatter {
@@ -156,6 +159,11 @@ String format(final Instant instant) {
         void formatToBuffer(final Instant instant, final StringBuilder destination) {
             destination.append(instant.getEpochMillisecond()); // no need for caching
         }
+
+        @Override
+        public String toPattern() {
+            return UNIX_MILLIS_FORMAT;
+        }
     }
 
     private final class CachedTime {

src/changelog/.2.x.x/2346_unix_millis_data_pattern.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<entry xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+       xmlns="http://logging.apache.org/log4j/changelog"
+       xsi:schemaLocation="http://logging.apache.org/log4j/changelog https://logging.apache.org/log4j/changelog-0.1.3.xsd"
+       type="changed">
+  <issue id="2346" link="https://github.com/apache/logging-log4j2/pull/2346"/>
+  <description format="asciidoc">Fix `NullPointerException` in `PatternProcessor` for a `UNIX_MILLIS` pattern.</description>
+</entry>