With the trait in place the claim still meant nothing while every caller
named dr_decode's free functions: a second decoder would have had to be
threaded through the scan, the thumbnail ladder, import, the viewer,
export, merge and repairs at the moment it arrived.
Each of those now takes a &dyn Decoder and reads headers, previews,
orientation and sensor data through it, including the header budget a
remote fetch asks for (header_bytes) and where it finds the embedded
preview (locate_preview). Only the places that start a job name
dr_decode::default(): the thumbnail, sweep and thumbnail-sweep threads,
the viewer's open handlers, and the request structs a job is handed
(BatchRequest, MergeRequest, the import Request, the repairs Toolkit),
so a caller can be given another decoder by changing what it is handed.
The default is rawler through the same free functions as before, so
nothing a user sees changes. The trait gains Debug as a supertrait so
request structs that derive Debug can carry one.
FR-RAW-2 says a second decoder may be added for broader camera coverage
without changing callers, and D2 names LibRaw as that second decoder.
Nothing tested the claim: dr_decode was one decoder reached through free
functions, so adding another would have meant editing every caller at
the moment there was most pressure not to.
Decoder is an object-safe trait over bytes: header_bytes, metadata,
orientation, locate_preview, preview and decode. Rawler implements it by
delegating to the existing free functions, so behaviour is unchanged,
and dr_decode::default() hands it out as a &'static dyn Decoder, which
is what the places that start work will name. JPEG recognition, decoding
and completeness checks stay free functions: they are not a RAW
decoder's to vary.
Nothing in the trait takes a path or a SourceRef; the decoder states how
much of a file it needs and where its preview sits, and the caller's
storage fetches that.
dtourolle