f77a812941
We have many places where an ObjectFile subclass will take the DataExtractor representing the entire binary, create a subsection of that in a new DataExtractor for processing. For instance, an object file might have symbol table entries with offsets into the string table. A common code pattern is to create a DataExtractor representing the string table, and then pulling out the c-strings based on those offsets from the string table DataExtractor. When code does this, it creates a new DataExtractor, copies the Endianness and Wordsize from the original, copies the DataBufferSP from the original, and specifies a new start and offset into the DataBuffer. However, if the binary is actaully stored in a VirtualDataExtractor, this code pattern loses the correct virtual-to-physical table translation and will not work correctly. This new method simplifies this common pattern, and correctly takes a subset of a VirtualDataExtractor. The current implementation only allows a subset of a VirtualDataExtractor that is contained within a single virtual entry (LookupTable entry) and returns a DataExtractor with the corret offsets calculated from the LookupTable. If we need to a VirtualDataExtractor to create a Subset DataExtractor representing multiple separate virtual ranges of data, we'll need to copy over the LookupTable entries that cover all the bytes, and update them to be relative to the new VirtualDataExtractor. It's a bit of work, and it's not needed right now, so I'm not tackling that. I am working on a larger PR which needs this new method. This PR contains a unit test that uses it. rdar://148939795
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