fix pyr tiff write to buffer

now we have a test for it heh

libvips/foreign/tiff.c

@@ -314,24 +314,11 @@ openout_buffer_read( thandle_t st, tdata_t data, tsize_t size )
 {
 	VipsTiffOpenoutBuffer *buffer = (VipsTiffOpenoutBuffer *) st;
 
-	off_t write_point;
-	size_t available;
-	unsigned char *from;
+#ifdef DEBUG
+	printf( "openout_buffer_read: %zd bytes\n", size );
+#endif /*DEBUG*/
 
-	write_point = vips_dbuf_tell( &buffer->dbuf ); 
-	from = vips_dbuf_get_write( &buffer->dbuf, &available );
-	vips_dbuf_seek( &buffer->dbuf, write_point, SEEK_SET ); 
-
-	if( available < size ) { 
-		vips_error( "openout_buffer_read", 
-			"%s", _( "read beyond end of buffer" ) );
-		return( 0 );
-	}
-
-	memcpy( data, from, size ); 
-
-	vips_dbuf_seek( &buffer->dbuf, size, SEEK_CUR );                                 
-	return( size );
+	return( vips_dbuf_read( &buffer->dbuf, data, size ) );
 }
 
 static tsize_t
@@ -365,8 +352,26 @@ openout_buffer_seek( thandle_t st, toff_t position, int whence )
 	VipsTiffOpenoutBuffer *buffer = (VipsTiffOpenoutBuffer *) st;
 
 #ifdef DEBUG
-	printf( "openout_buffer_seek: position %zd, whence %d\n", 
+	printf( "openout_buffer_seek: position %zd, whence %d ",
 		position, whence );
+	switch( whence ) {
+	case SEEK_SET:
+		printf( "set" ); 
+		break;
+
+	case SEEK_END:
+		printf( "end" ); 
+		break;
+
+	case SEEK_CUR:
+		printf( "cur" ); 
+		break;
+
+	default:
+		printf( "unknown" ); 
+		break;
+	}
+	printf( "\n" ); 
 #endif /*DEBUG*/
 
 	vips_dbuf_seek( &buffer->dbuf, position, whence );

libvips/include/vips/dbuf.h

@@ -64,6 +64,7 @@ typedef struct _VipsDbuf {
 void vips_dbuf_destroy( VipsDbuf *buf );
 void vips_dbuf_init( VipsDbuf *buf );
 gboolean vips_dbuf_allocate( VipsDbuf *dbuf, size_t size );
+size_t vips_dbuf_read( VipsDbuf *dbuf, unsigned char *data, size_t size );
 unsigned char *vips_dbuf_get_write( VipsDbuf *dbuf, size_t *size );
 gboolean vips_dbuf_write( VipsDbuf *dbuf, 
 	const unsigned char *data, size_t size );

libvips/iofuncs/dbuf.c

@@ -69,7 +69,7 @@ gboolean
 vips_dbuf_minimum_size( VipsDbuf *dbuf, size_t size )
 {
 	if( size > dbuf->allocated_size ) { 
-		size_t new_allocated_size = 3 * (16 + size) / 2;
+		const size_t new_allocated_size = 3 * (16 + size) / 2;
 
 		unsigned char *new_data;
 
@@ -102,25 +102,26 @@ vips_dbuf_allocate( VipsDbuf *dbuf, size_t size )
 }
 
 /**
- * vips_dbuf_null_terminate:
+ * vips_dbuf_read:
  * @dbuf: the buffer
+ * @data: read to this area
+ * @size: read up to this many bytes
  *
- * Make sure the byte after the last data byte is `\0`. This extra byte is not
- * included in the data size and the write point is not moved.
+ * Up to @size bytes are read from the buffer and copied to @data. The number
+ * of bytes transferred is returned.
  *
- * This makes it safe to treat the dbuf contents as a C string. 
- * 
- * Returns: %FALSE on out of memory, %TRUE otherwise.
+ * Returns: the number of bytes transferred.
  */
-static gboolean
-vips_dbuf_null_terminate( VipsDbuf *dbuf )
+size_t
+vips_dbuf_read( VipsDbuf *dbuf, unsigned char *data, size_t size )
 {
-	if( !vips_dbuf_minimum_size( dbuf, dbuf->data_size + 1 ) )
-		return( FALSE );
+	const size_t available = dbuf->data_size - dbuf->write_point;
+	const size_t copied = VIPS_MIN( size, available );
 
-	dbuf->data[dbuf->data_size] = 0;
+	memcpy( data, dbuf->data + dbuf->write_point, copied );
+	dbuf->write_point += copied;
 
-	return( TRUE ); 
+	return( copied );
 }
 
 /**
@@ -141,14 +142,14 @@ unsigned char *
 vips_dbuf_get_write( VipsDbuf *dbuf, size_t *size )
 {
 	unsigned char *write = dbuf->data + dbuf->write_point;
-	size_t length = dbuf->data + dbuf->allocated_size - write;
+	const size_t available = dbuf->allocated_size - dbuf->write_point;
 
-	memset( write, 0, length ); 
+	memset( write, 0, available ); 
 	dbuf->write_point = dbuf->allocated_size;
 	dbuf->data_size = dbuf->allocated_size;
 
 	if( size )
-		*size = length;
+		*size = available;
 
 	return( write ); 
 }
@@ -310,6 +311,28 @@ vips_dbuf_tell( VipsDbuf *dbuf )
 	return( dbuf->write_point ); 
 }
 
+/**
+ * vips_dbuf_null_terminate:
+ * @dbuf: the buffer
+ *
+ * Make sure the byte after the last data byte is `\0`. This extra byte is not
+ * included in the data size and the write point is not moved.
+ *
+ * This makes it safe to treat the dbuf contents as a C string. 
+ * 
+ * Returns: %FALSE on out of memory, %TRUE otherwise.
+ */
+static gboolean
+vips_dbuf_null_terminate( VipsDbuf *dbuf )
+{
+	if( !vips_dbuf_minimum_size( dbuf, dbuf->data_size + 1 ) )
+		return( FALSE );
+
+	dbuf->data[dbuf->data_size] = 0;
+
+	return( TRUE ); 
+}
+
 /**
  * vips_dbuf_steal:
  * @dbuf: the buffer