7f7a7d6605
I want to use FFI to link with a C library containing the actual block cache, io engine and crc32 code.
55 lines
1.5 KiB
Scheme
55 lines
1.5 KiB
Scheme
(library
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(block-io)
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(export metadata-block-size
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open-metadata
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with-metadata
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read-block)
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(import (rnrs)
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(fmt fmt))
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;;;---------------------------------------------------
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;;; TODO:
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;;; - implement a little block cache.
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;;; - writes
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;;; - zero blocks
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;;; - prefetching
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;;;---------------------------------------------------
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(define metadata-block-size 4096)
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(define (open-metadata path)
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(open-file-input-port path (file-options) (buffer-mode none)))
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(define-syntax with-metadata
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(syntax-rules ()
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((_ (port path) body ...)
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(let ((port (open-metadata path)))
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(dynamic-wind
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(lambda () #f)
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(lambda () body ...)
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(lambda () (close-port port)))))))
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;; FIXME: return our own condition?
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(define (io-error msg)
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(raise (condition
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(make-error)
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(make-message-condition msg))))
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;;; Returns a boolean indicating success
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(define (read-exact! port offset len bv start)
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(set-port-position! port offset)
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(let ((nr (get-bytevector-n! port bv start len)))
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(and (not (eof-object? nr))
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(= len nr))))
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;;; Returns a 4k bytevector or #f
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(define (read-exact port offset len)
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(let ((bv (make-bytevector len)))
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(if (read-exact! port offset len bv 0) bv #f)))
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(define (read-block port b)
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(or (read-exact port (* b metadata-block-size) metadata-block-size)
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(io-error (fmt #f (dsp "Unable to read metadata block: ") (num b))))))
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