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Diffstat (limited to 'guix/store/deduplication.scm')
| -rw-r--r-- | guix/store/deduplication.scm | 148 |
1 files changed, 148 insertions, 0 deletions
diff --git a/guix/store/deduplication.scm b/guix/store/deduplication.scm new file mode 100644 index 0000000000..4b4ac01f64 --- /dev/null +++ b/guix/store/deduplication.scm @@ -0,0 +1,148 @@ +;;; GNU Guix --- Functional package management for GNU +;;; Copyright © 2017 Caleb Ristvedt <caleb.ristvedt@cune.org> +;;; Copyright © 2018 Ludovic Courtès <ludo@gnu.org> +;;; +;;; This file is part of GNU Guix. +;;; +;;; GNU Guix is free software; you can redistribute it and/or modify it +;;; under the terms of the GNU General Public License as published by +;;; the Free Software Foundation; either version 3 of the License, or (at +;;; your option) any later version. +;;; +;;; GNU Guix is distributed in the hope that it will be useful, but +;;; WITHOUT ANY WARRANTY; without even the implied warranty of +;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +;;; GNU General Public License for more details. +;;; +;;; You should have received a copy of the GNU General Public License +;;; along with GNU Guix. If not, see <http://www.gnu.org/licenses/>. + +;;; This houses stuff we do to files when they arrive at the store - resetting +;;; timestamps, deduplicating, etc. + +(define-module (guix store deduplication) + #:use-module (guix hash) + #:use-module (guix build utils) + #:use-module (guix base16) + #:use-module (srfi srfi-11) + #:use-module (rnrs io ports) + #:use-module (ice-9 ftw) + #:use-module (guix serialization) + #:export (nar-sha256 + deduplicate)) + +;; Would it be better to just make WRITE-FILE give size as well? I question +;; the general utility of this approach. +(define (counting-wrapper-port output-port) + "Some custom ports don't implement GET-POSITION at all. But if we want to +figure out how many bytes are being written, we will want to use that. So this +makes a wrapper around a port which implements GET-POSITION." + (let ((byte-count 0)) + (make-custom-binary-output-port "counting-wrapper" + (lambda (bytes offset count) + (set! byte-count + (+ byte-count count)) + (put-bytevector output-port bytes + offset count) + count) + (lambda () + byte-count) + #f + (lambda () + (close-port output-port))))) + +(define (nar-sha256 file) + "Gives the sha256 hash of a file and the size of the file in nar form." + (let-values (((port get-hash) (open-sha256-port))) + (let ((wrapper (counting-wrapper-port port))) + (write-file file wrapper) + (force-output wrapper) + (force-output port) + (let ((hash (get-hash)) + (size (port-position wrapper))) + (close-port wrapper) + (values hash size))))) + +(define (tempname-in directory) + "Gives an unused temporary name under DIRECTORY. Not guaranteed to still be +unused by the time you create anything with that name, but a good shot." + (let ((const-part (string-append directory "/.tmp-link-" + (number->string (getpid))))) + (let try ((guess-part + (number->string (random most-positive-fixnum) 16))) + (if (file-exists? (string-append const-part "-" guess-part)) + (try (number->string (random most-positive-fixnum) 16)) + (string-append const-part "-" guess-part))))) + +(define* (get-temp-link target #:optional (link-prefix (dirname target))) + "Like mkstemp!, but instead of creating a new file and giving you the name, +it creates a new hardlink to TARGET and gives you the name. Since +cross-filesystem hardlinks don't work, the temp link must be created on the +same filesystem - where in that filesystem it is can be controlled by +LINK-PREFIX." + (let try ((tempname (tempname-in link-prefix))) + (catch 'system-error + (lambda () + (link target tempname) + tempname) + (lambda (args) + (if (= (system-error-errno args) EEXIST) + (try (tempname-in link-prefix)) + (throw 'system-error args)))))) + +;; There are 3 main kinds of errors we can get from hardlinking: "Too many +;; things link to this" (EMLINK), "this link already exists" (EEXIST), and +;; "can't fit more stuff in this directory" (ENOSPC). + +(define (replace-with-link target to-replace) + "Atomically replace the file TO-REPLACE with a link to TARGET. Note: TARGET +and TO-REPLACE must be on the same file system." + (let ((temp-link (get-temp-link target (dirname to-replace)))) + (rename-file temp-link to-replace))) + +(define-syntax-rule (false-if-system-error (errors ...) exp ...) + "Given ERRORS, a list of system error codes to ignore, evaluates EXP... and +return #f if any of the system error codes in the given list are thrown." + (catch 'system-error + (lambda () + exp ...) + (lambda args + (if (member (system-error-errno args) (list errors ...)) + #f + (apply throw args))))) + +(define* (deduplicate path hash #:key (store %store-directory)) + "Check if a store item with sha256 hash HASH already exists. If so, +replace PATH with a hardlink to the already-existing one. If not, register +PATH so that future duplicates can hardlink to it. PATH is assumed to be +under STORE." + (let* ((links-directory (string-append store "/.links")) + (link-file (string-append links-directory "/" + (bytevector->base16-string hash)))) + (mkdir-p links-directory) + (if (file-is-directory? path) + ;; Can't hardlink directories, so hardlink their atoms. + (for-each (lambda (file) + (unless (member file '("." "..")) + (deduplicate file (nar-sha256 file) + #:store store))) + (scandir path)) + (if (file-exists? link-file) + (false-if-system-error (EMLINK) + (replace-with-link link-file path)) + (catch 'system-error + (lambda () + (link path link-file)) + (lambda args + (let ((errno (system-error-errno args))) + (cond ((= errno EEXIST) + ;; Someone else put an entry for PATH in + ;; LINKS-DIRECTORY before we could. Let's use it. + (false-if-system-error (EMLINK) + (replace-with-link path link-file))) + ((= errno ENOSPC) + ;; There's not enough room in the directory index for + ;; more entries in .links, but that's fine: we can + ;; just stop. + #f) + (else (apply throw args)))))))))) |