Fix writeHeader computation in RollingRandomAccessFileManager

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

@@ -41,6 +41,7 @@
 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;
@@ -333,4 +334,40 @@ void testRolloverRetainsFileAttributes() throws Exception {
                 .permissions();
         assertEquals(filePermissions, actualFilePermissions);
     }
+
+    @Test
+    void testWriteHeaderWhenFileDoesNotExistBefore() throws IOException {
+        final File file = File.createTempFile("log4j2", "test");
+        file.delete(); // Ensure file doesn't exist
+        file.deleteOnExit();
+
+        final String header = "HEADER";
+        final PatternLayout layout = PatternLayout.newBuilder()
+                .setHeader(header)
+                .build();
+
+        final boolean isAppend = true;
+        final RollingRandomAccessFileManager manager = RollingRandomAccessFileManager.getRollingRandomAccessFileManager(
+                file.getAbsolutePath(),
+                Strings.EMPTY,
+                isAppend,
+                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
+        assertTrue(file.exists(), "File should exist");
+        final byte[] fileContent = Files.readAllBytes(file.toPath());
+        final String content = new String(fileContent);
+        assertTrue(content.startsWith(header), "File should start with header: " + 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 = new File(name).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 writeHeader computation in RollingRandomAccessFileManager. The writeHeader boolean was computed after creating the RandomAccessFile, which creates the file if it doesn't exist. This made file.exists() always return true, causing incorrect header write decisions when append=true and the file didn't exist before.
+  </description>
+</entry>
+