Fix writeHeader computation in RollingRandomAccessFileManager

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

@@ -41,11 +41,14 @@
 import java.util.Set;
 import java.util.concurrent.locks.LockSupport;
 import org.apache.logging.log4j.core.config.DefaultConfiguration;
+import org.apache.logging.log4j.core.layout.PatternLayout;
 import org.apache.logging.log4j.core.util.Closer;
 import org.apache.logging.log4j.core.util.FileUtils;
 import org.apache.logging.log4j.core.util.NullOutputStream;
 import org.apache.logging.log4j.util.Strings;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
 
 /**
  * Tests the RollingRandomAccessFileManager class.
@@ -333,4 +336,62 @@ void testRolloverRetainsFileAttributes() throws Exception {
                 .permissions();
         assertEquals(filePermissions, actualFilePermissions);
     }
+
+    @ParameterizedTest
+    @CsvSource({
+            "true,true",
+            "true,false",
+            "false,true",
+            "false,false",
+    })
+    void testWriteHeaderWhetherAppendOrExists(final boolean append, final boolean fileExists) throws Exception {
+        final File file = File.createTempFile("log4j2", "test");
+        if (!fileExists) {
+            file.delete(); // Ensure file doesn't exist
+        } else {
+            // If file exists, write some content to it so it's not empty
+            try (FileOutputStream fos = new FileOutputStream(file)) {
+                fos.write("EXISTING_CONTENT".getBytes());
+            }
+        }
+        file.deleteOnExit();
+
+        final String header = "HEADER";
+        final PatternLayout layout = PatternLayout.newBuilder()
+                .setHeader(header)
+                .build();
+
+        final RollingRandomAccessFileManager manager = RollingRandomAccessFileManager.getRollingRandomAccessFileManager(
+                file.getAbsolutePath(),
+                Strings.EMPTY,
+                append,
+                true,
+                RollingRandomAccessFileManager.DEFAULT_BUFFER_SIZE,
+                new SizeBasedTriggeringPolicy(Long.MAX_VALUE),
+                null,
+                null,
+                layout,
+                null,
+                null,
+                null,
+                null);
+        assertNotNull(manager);
+        manager.close();
+
+        // Verify header was written based on logic: writeHeader = !append || !fileExistedBefore
+        // Note: writeHeader() also checks file.length() == 0, so header is only written if file is empty
+        assertTrue(file.exists(), "File should exist");
+        final byte[] fileContent = Files.readAllBytes(file.toPath());
+        final String content = new String(fileContent);
+        final boolean expectedHeaderWritten = !append || !fileExists;
+        if (expectedHeaderWritten) {
+            assertTrue(content.startsWith(header), "File should start with header when append=" + append
+                    + ", fileExists=" + fileExists + ", content: " + content);
+        } else {
+            // When append=true and fileExists=true, file has existing content, so header should not be written
+            assertTrue(!content.startsWith(header),
+                    "File should not start with header when append=" + append + ", fileExists=" + fileExists
+                            + ", content: " + content);
+        }
+    }
 }

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

@@ -174,8 +174,10 @@ public static RollingRandomAccessFileManager getRollingRandomAccessFileManager(
                             long size = 0;
                             long time = System.currentTimeMillis();
                             RandomAccessFile raf = null;
+                            boolean fileExistedBefore = false;
                             if (fileName != null) {
                                 file = new File(name);
+                                fileExistedBefore = file.exists();
 
                                 if (!append) {
                                     file.delete();
@@ -207,7 +209,7 @@ public static RollingRandomAccessFileManager getRollingRandomAccessFileManager(
                                     return null;
                                 }
                             }
-                            final boolean writeHeader = !append || file == null || !file.exists();
+                            final boolean writeHeader = !append || file == null || !fileExistedBefore;
 
                             final RollingRandomAccessFileManager rrm = new RollingRandomAccessFileManager(
                                     data.getLoggerContext(),

src/changelog/.2.x.x/fix_RollingRandomAccessFileManager_writeHeader.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<entry xmlns="https://logging.apache.org/xml/ns"
+       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+       xsi:schemaLocation="
+           https://logging.apache.org/xml/ns
+           https://logging.apache.org/xml/ns/log4j-changelog-0.xsd"
+       type="fixed">
+  <description format="asciidoc">
+    Fix header write in `RollingRandomAccessFileManager` that was being incorrectly skipped if `append=true` and the file didn't exist before.
+  </description>
+</entry>
+