AdapterIO: add FlushBefore() to strip the beginning of the MallocIO object

by default, AdapterIO is initialized with a BMallocIO object, which will
be extended indefinitely. Flushing regularly is necessary to avoid
excessive memory usage. Tested in StreamRadio.

Change-Id: I9f3142c0a2300ad44dc54ccf6932d41c9526320b
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7302
Reviewed-by: Adrien Destugues <[email protected]>
Tested-by: Commit checker robot <[email protected]>

headers/private/media/experimental/AdapterIO.h

@@ -62,6 +62,8 @@ public:
 			void					SeekCompleted();
 			status_t				SetBuffer(BPositionIO* buffer);
 
+			status_t				FlushBefore(off_t position);
+
 			BInputAdapter*			BuildInputAdapter();
 
 protected:

src/kits/media/experimental/AdapterIO.cpp

@@ -46,6 +46,28 @@ public:
 		return B_OK;
 	}
 
+	status_t FlushBefore(off_t position, BPositionIO* buffer, const void* oldBuffer,
+		size_t oldLength)
+	{
+		AutoWriteLocker _(fLock);
+		off_t relative = _PositionToRelative(position);
+		if (relative < 0)
+			return B_OK;
+		if (relative > (off_t)oldLength)
+			return B_BAD_VALUE;
+		status_t status = buffer->WriteAt(0, (void*)((addr_t)oldBuffer + relative),
+			oldLength - relative);
+		if (status < B_OK)
+			return status;
+		status = buffer->Seek(fBuffer->Position() - relative, SEEK_SET);
+		if (status < B_OK)
+			return status;
+		fBackPosition -= relative;
+		fStartOffset += relative;
+		SetBuffer(buffer);
+		return B_OK;
+	}
+
 	status_t EvaluatePosition(off_t position, off_t totalSize)
 	{
 		if (position < 0)
@@ -182,6 +204,11 @@ public:
 		return ((flags & B_MEDIA_SEEKABLE) == B_MEDIA_SEEKABLE);
 	}
 
+	const BPositionIO* Buffer() const
+	{
+		return fBuffer;
+	}
+
 private:
 
 	off_t _PositionToRelative(off_t position) const
@@ -382,6 +409,16 @@ BAdapterIO::SetBuffer(BPositionIO* buffer)
 }
 
 
+status_t
+BAdapterIO::FlushBefore(off_t position)
+{
+	BMallocIO* buffer = new BMallocIO();
+	BMallocIO* oldBuffer = (BMallocIO*)fBuffer->Buffer();
+	fBuffer->FlushBefore(position, buffer, oldBuffer->Buffer(), oldBuffer->BufferLength());
+	return B_OK;
+}
+
+
 BInputAdapter*
 BAdapterIO::BuildInputAdapter()
 {