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authorPeter Bex <peter@more-magic.net>2017-11-09 21:26:48 +0100
committerPeter Bex <peter@more-magic.net>2017-11-09 21:26:48 +0100
commite7bf6cc5f6301e5d4be4d877b57039512379a1ec (patch)
treeeeea0de6aef1bf824ee63c46272c1776222034d9 /scsh-process.scm
parent6ffe2658fb703ddac9b7801b2f67ab0ea597472d (diff)
downloadscsh-process-e7bf6cc5f6301e5d4be4d877b57039512379a1ec.tar.gz
Temporarily mask sigchld to avoid race condition in fork
We do a fork, and immediately afterwards we insert a scsh-process object into the pending processes table with the pid of the newly created child process. However, sigchld may arrive in between the fork and the insertion of the scsh-process object into the pending process table. The sigchld handler will reap the child process and attempt to update the pending scsh-process object, but it can't find it yet because the insertion of this object into the table happens after the fork, thus losing the reaped child's exit status. Then, if the program waits for the process after that, we enter an infinite busy waiting loop that expects a sigchld to arrive and update the object through the table, but that never happens because the child has already been reaped.
Diffstat (limited to 'scsh-process.scm')
-rw-r--r--scsh-process.scm49
1 files changed, 29 insertions, 20 deletions
diff --git a/scsh-process.scm b/scsh-process.scm
index 4153c00..4e6708d 100644
--- a/scsh-process.scm
+++ b/scsh-process.scm
@@ -221,26 +221,35 @@
(exit 0)))
(define (fork #!optional thunk continue-threads?)
- (let* ((maybe-reinstall-deadlock-workaround!
- (lambda ()
- (cond-expand
- (has-thread-killer
- (unless continue-threads? (install-deadlock-workaround!)))
- (else (void)))))
- (thunk (and thunk (lambda ()
- (clear-scsh-pending-processes!)
- (maybe-reinstall-deadlock-workaround!)
- (thunk))))
- (pid (cond-expand
- (has-thread-killer
- (process-fork thunk (not continue-threads?)))
- (else ;; Ignore both args if thunk is #f, so #f won't be applied
- (if thunk (process-fork thunk) (process-fork))))))
- (cond ((zero? pid)
- (clear-scsh-pending-processes!)
- (maybe-reinstall-deadlock-workaround!)
- #f)
- (else (add-scsh-pending-process! pid)))))
+ (dynamic-wind
+ ;; If we're really unlucky, sigchld might be delivered
+ ;; immediately after forking, but before we added the child's
+ ;; pid to the pending processes table. This means we'll lose
+ ;; the event and won't mark the process as finished, resulting
+ ;; in an endless loop in process-wait. So, mask it.
+ (lambda () (signal-mask! signal/chld))
+ (lambda ()
+ (let* ((maybe-reinstall-deadlock-workaround!
+ (lambda ()
+ (cond-expand
+ (has-thread-killer
+ (unless continue-threads? (install-deadlock-workaround!)))
+ (else (void)))))
+ (thunk (and thunk (lambda ()
+ (clear-scsh-pending-processes!)
+ (maybe-reinstall-deadlock-workaround!)
+ (thunk))))
+ (pid (cond-expand
+ (has-thread-killer
+ (process-fork thunk (not continue-threads?)))
+ (else ;; Ignore both args if thunk is #f, so #f won't be applied
+ (if thunk (process-fork thunk) (process-fork))))))
+ (cond ((zero? pid)
+ (clear-scsh-pending-processes!)
+ (maybe-reinstall-deadlock-workaround!)
+ #f)
+ (else (add-scsh-pending-process! pid)))))
+ (lambda () (signal-unmask! signal/chld))))
(define %fork fork)