`bibfetch::cover(&agent, isbn, cover_id, size)` asks Open Library's covers
store by the ISBN's thirteen-digit form, then its ten-digit form, then by a
cover id a lookup record named, and returns the image or `Ok(None)`. The
answer is checked before it is trusted: JPEG or PNG by content type and by
magic bytes, between 1 KB and 10 MB, at most three redirects, none of them
off HTTPS. One request at a time per process; a 429 or 503 is waited out
for its Retry-After, up to a minute.
`idnorm::isbn` converts between the two forms, and only for an ISBN whose
check digit agrees. Lookup records carry `cover_id` from the edition record
and the search index. The CLI gains `bibfetch cover <isbn> -o <file>`.
`10.1093/<zwsp>oso/<zwsp>` is exactly sixteen bytes, the length of
`https://doi.org/`, so testing that prefix against it sliced into the middle
of the third zero-width space and brought the process down. Found by running
citefind over the corpus: a real book prints its DOI that way, because the
typesetter marked where the line could break.
Both prefix tests — DOI and arXiv — now ask for the head rather than cutting
one, so a byte length that is not a character boundary is an answer of no
rather than a panic.
And an invisible character is not part of an identifier. Zero-width spaces
and joiners, the soft hyphen, a byte-order mark left at the front of a
fragment: text out of a PDF or an EPUB is full of them, none of them can be
seen, and a DOI carrying one does not resolve. They are dropped before
anything is read.
`10.1093/<zwsp>oso/<zwsp>` is exactly sixteen bytes, which is the length of
`https://doi.org/`, so testing that prefix against it sliced into the middle
of the third zero-width space and brought the process down. Found by running
citefind over the corpus: the DOI is printed that way in a real book, because
the typesetter marked where the line could break.
Both prefix tests — DOI and arXiv — now ask for the head rather than cutting
one, so a byte length that is not a character boundary is an answer of no
rather than a panic.
And an invisible character is not part of an identifier. Zero-width spaces,
joiners, the soft hyphen, a byte-order mark left at the front of a fragment:
text out of a PDF or an EPUB is full of them, they cannot be seen, and a DOI
carrying one does not resolve. They are dropped before anything is read.
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