In some situations, EXTRA-PORTS may not work as described, because it copies
file descriptor contents in an arbitrary order. For example, suppose
that (map fileno EXTRA-PORTS) is (7 6 5 4 3). If the underlying file
originally stored in fd N is represented by F(N), it will assign
3 <-- F(7)
4 <-- F(6)
5 <-- F(5)
6 <-- F(6)
7 <-- F(7)
In other words, the copying of earlier FDs in EXTRA-PORTS may overwrite later
FDs in EXTRA-PORTS.
Because the process of properly and safely copying those FDs involves some
complexity, we've split it into a separate procedure named PRESERVE-PORTS.
* modules/shepherd/service.scm (reconfigure-ports): new procedure.
(exec-command): use it.
---
modules/shepherd/service.scm | 154 +++++++++++++++++++++++++----------
1 file changed, 113 insertions(+), 41 deletions(-)
Toggle diff (183 lines)
diff --git a/modules/shepherd/service.scm b/modules/shepherd/service.scm
index 68553d4..68993ac 100644
--- a/modules/shepherd/service.scm
+++ b/modules/shepherd/service.scm
@@ -1434,6 +1434,88 @@ FILE."
(list->vector (map (lambda (group) (group:gid (getgr group)))
supplementary-groups)))
+(define* (reconfigure-fds ports-or-fds #:optional (base 0))
+ "Duplicate the FDs (fd0 fd1 ... fdN) corresponding to the N ports or FDs in
+EXTRA-PORTS into the FD range (0 1 ... N), clearing their FD_CLOEXEC flag at
+the same time. This will work regardless of the numeric values of fd1
+... fdN. File descriptors outside of the range 0..N will not be affected.
+This may fail if there are zero unused file descriptors."
+ ;; If we view each FD as a node, and fd n at index k of FDS as an edge from
+ ;; fd n to fd k, then we have a rather special type of graph. Because of
+ ;; how the edges must be specified, it has the property that no node can
+ ;; have more than one parent, like a tree, but unlike a tree it is possible
+ ;; to have cycles (combined with the prior restriction, this means a given
+ ;; node can be part of at most one cycle). I don't know of a good name for
+ ;; that kind of graph - maybe "rootless tree"? Anyway, our approach is to,
+ ;; for each unique FD in FDS, do a traversal that both finds the cyclic
+ ;; path, if it exists, and sets every FD that isn't part of the cycle, then
+ ;; finally resolve the cycle using a temporary fd.
+
+ (define fds (map (lambda (x)
+ (if (port? x) (fileno x) x))
+ ports-or-fds))
+ (define max-fd (apply max 0 fds))
+ (define fd->targets
+ (let ((vec (make-vector (+ 1 max-fd) '())))
+ (for-each (lambda (source-fd dest-fd)
+ (vector-set! vec source-fd
+ (cons dest-fd
+ (vector-ref vec source-fd))))
+ fds
+ (iota (length fds) base))
+ vec))
+ (define visited (make-vector (+ 1 max-fd) #f))
+
+ (define (rotate-fds! fds)
+ ;; (fdval1 fdval2 fdval3) --> (fdval3 fdval1 fdval2)
+ (match (reverse fds)
+ ((fd0)
+ ;; Clear close-on-exec flag as if it were dup2'ed.
+ (fcntl fd0 F_SETFD 0))
+ ((fd0 . (and rest (fd1 . _)))
+ (let ((tmp-fd (dup->fdes fd0)))
+ (let loop ((fds rest)
+ (prev fd0))
+ (match fds
+ ((fd . rest)
+ (dup2 fd prev)
+ (loop rest fd))
+ (()
+ (dup2 tmp-fd prev)
+ (close-fdes tmp-fd))))))))
+
+ (define (top-visit fd)
+ (let ((cycle (visit fd fd)))
+ (when cycle
+ (rotate-fds! cycle))))
+
+ (define (visit fd cycle-start-fd)
+ (if (vector-ref visited fd)
+ #f
+ (begin
+ (vector-set! visited fd #t)
+ (let loop ((targets (vector-ref fd->targets fd))
+ (cycle-tail #f))
+ (match targets
+ ((target . rest)
+ (cond
+ ((= target cycle-start-fd)
+ (loop rest (list fd)))
+ ((> target max-fd) ;; Has no targets, no need to visit.
+ (dup2 fd target)
+ (loop rest cycle-tail))
+ (else
+ (let ((maybe-cycle-tail (visit target cycle-start-fd)))
+ (if maybe-cycle-tail
+ (loop rest (cons fd maybe-cycle-tail))
+ (begin
+ (dup2 fd target)
+ (loop rest cycle-tail)))))))
+ (()
+ cycle-tail))))))
+
+ (for-each top-visit fds))
+
(define* (exec-command command
#:key
(user #f)
@@ -1479,48 +1561,38 @@ false."
(chdir directory)
(environ environment-variables)
- ;; Close all the file descriptors except stdout and stderr.
- (let ((max-fd (max-file-descriptors)))
+ ;; Redirect stdin.
+ (catch-system-error (close-fdes 0))
+ ;; Make sure file descriptor zero is used, so we don't end up reusing
+ ;; it for something unrelated, which can confuse some packages.
+ (dup2 (if input-port
+ (fileno input-port)
+ (open-fdes "/dev/null" O_RDONLY))
+ 0)
- ;; Redirect stdin.
- (catch-system-error (close-fdes 0))
- ;; Make sure file descriptor zero is used, so we don't end up reusing
- ;; it for something unrelated, which can confuse some packages.
- (dup2 (if input-port
- (fileno input-port)
- (open-fdes "/dev/null" O_RDONLY))
- 0)
+ (when (or log-port log-file)
+ (catch #t
+ (lambda ()
+ ;; Redirect stdout and stderr to use LOG-FILE.
+ (catch-system-error (close-fdes 1))
+ (catch-system-error (close-fdes 2))
+ (dup2 (if log-file
+ (open-fdes log-file (logior O_CREAT O_WRONLY O_APPEND)
+ #o640)
+ (fileno log-port))
+ 1)
+ (dup2 1 2)
+
+ ;; Make EXTRA-PORTS available starting from file descriptor 3.
+ ;; This clears their FD_CLOEXEC flag.
+ (reconfigure-fds extra-ports 3))
- (when (or log-port log-file)
- (catch #t
- (lambda ()
- ;; Redirect stout and stderr to use LOG-FILE.
- (catch-system-error (close-fdes 1))
- (catch-system-error (close-fdes 2))
- (dup2 (if log-file
- (open-fdes log-file (logior O_CREAT O_WRONLY O_APPEND)
- #o640)
- (fileno log-port))
- 1)
- (dup2 1 2)
-
- ;; Make EXTRA-PORTS available starting from file descriptor 3.
- ;; This clears their FD_CLOEXEC flag.
- (let loop ((fd 3)
- (ports extra-ports))
- (match ports
- (() #t)
- ((port rest ...)
- (catch-system-error (close-fdes fd))
- (dup2 (fileno port) fd)
- (loop (+ 1 fd) rest)))))
-
- (lambda (key . args)
- (when log-file
- (format (current-error-port)
- "failed to open log-file ~s:~%" log-file))
- (print-exception (current-error-port) #f key args)
- (primitive-exit 1))))
+ (lambda (key . args)
+ (when log-file
+ (format (current-error-port)
+ "failed to open log-file ~s:~%" log-file))
+ (print-exception (current-error-port) #f key args)
+ (primitive-exit 1))))
;; setgid must be done *before* setuid, otherwise the user will
;; likely no longer have permissions to setgid.
@@ -1558,7 +1630,7 @@ false."
(format (current-error-port)
"exec of ~s failed: ~a~%"
program (strerror (system-error-errno args)))
- (primitive-exit 1)))))))
+ (primitive-exit 1))))))
(define %precious-signals
;; Signals that the shepherd process handles.
--
2.40.1
