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01/02: nar: 'finalize-store-file' follows proper store lock protocol.
From: |
guix-commits |
Subject: |
01/02: nar: 'finalize-store-file' follows proper store lock protocol. |
Date: |
Thu, 7 May 2020 06:56:12 -0400 (EDT) |
reepca pushed a commit to branch master
in repository guix.
commit 37edbc91e34fb5658261e637e6ffccdb381e5271
Author: Caleb Ristvedt <address@hidden>
AuthorDate: Wed May 6 11:48:21 2020 -0500
nar: 'finalize-store-file' follows proper store lock protocol.
* guix/nar.scm (finalize-store-file): check for deletion token when
acquiring
lock, write deletion token and delete lock file before releasing lock.
---
guix/nar.scm | 26 +++++++++++++++++++++++++-
1 file changed, 25 insertions(+), 1 deletion(-)
diff --git a/guix/nar.scm b/guix/nar.scm
index 29636aa..0a6f59b 100644
--- a/guix/nar.scm
+++ b/guix/nar.scm
@@ -82,10 +82,28 @@
REFERENCES and DERIVER. When LOCK? is true, acquire exclusive locks on TARGET
before attempting to register it; otherwise, assume TARGET's locks are already
held."
+ ;; TODO: make this reusable
+ (define (acquire-lock file)
+ (let ((port (lock-file file)))
+ ;; There is an inherent race condition between opening the lock file and
+ ;; attempting to acquire the lock on it, and because we like deleting
+ ;; these lock files when we release them, only the first successful
+ ;; acquisition on a given lock file matters. To make it easier to tell
+ ;; when an acquisition is and isn't the first, the first to acquire it
+ ;; writes a deletion token (arbitrary character) prior to releasing the
+ ;; lock.
+ (if (zero? (stat:size (stat port)))
+ port
+ ;; if FILE is non-empty, that's because it contains the deletion
+ ;; token, so we aren't the first to acquire it. So try again!
+ (begin
+ (close port)
+ (acquire-lock file)))))
+
(with-database %default-database-file db
(unless (path-id db target)
(let ((lock (and lock?
- (lock-file (string-append target ".lock")))))
+ (acquire-lock (string-append target ".lock")))))
(unless (path-id db target)
;; If FILE already exists, delete it (it's invalid anyway.)
@@ -102,6 +120,12 @@ held."
#:deriver deriver))
(when lock?
+ (delete-file (string-append target ".lock"))
+ ;; Write the deletion token to inform anyone who acquires the lock
+ ;; on this particular file next that they aren't the first to
+ ;; acquire it, so they should retry.
+ (display "d" lock)
+ (force-output lock)
(unlock-file lock))))))
(define (temporary-store-file)