MemoryMappedFileAppender ignores immediateFlush=true due to hard-coded false #4480

Expected: immediateFlush="true" on MemoryMappedFile forces the mapped buffer after every event.
Actual: the appender's own flag is always false (getImmediateFlush() returns false); only the manager receives the configured true.
Affects: 2.25.3 and 2.26.1, 2.x and main heads.
Reproducer: configure <MemoryMappedFile immediateFlush="true">, read getImmediateFlush() from the appender after startup.

Original report:

Description

MemoryMappedFileAppender does not honor the configured immediateFlush option.

In MemoryMappedFileAppender.Builder.build(), the configured value is correctly passed to MemoryMappedFileManager,

final MemoryMappedFileManager manager =
    MemoryMappedFileManager.getFileManager(
        fileName,
        append,
        isImmediateFlush(),
        actualRegionLength,
        advertiseURI,
        layout);

However, the MemoryMappedFileAppender constructor receives a hard-coded false.

return new MemoryMappedFileAppender(
    name,
    layout,
    getFilter(),
    manager,
    fileName,
    isIgnoreExceptions(),
    false,  // ignores the configured immediateFlush value
    advertise ? getConfiguration().getAdvertiser() : null,
    getPropertyArray());

As a result, setting immediateFlush=true does not enable per-event flushing, contrary to the documented behavior.

Suggested fix

Replace the hard-coded false with isImmediateFlush():

 return new MemoryMappedFileAppender(
     name,
     layout,
     getFilter(),
     manager,
     fileName,
     isIgnoreExceptions(),
-    false,
+    isImmediateFlush(),
     advertise ? getConfiguration().getAdvertiser() : null,
     getPropertyArray());

Related issues and pull requests

PR #4325 addresses a similar problem in ConsoleAppender (also see #4262, #4327), in which a hard-coded constructor argument causes an appender to ignore its configured immediateFlush value.