Re: [Libguestfs] [PATCH nbdkit 2/2] file: Allow a file descriptor to be passed to the plugin
On Wed, Aug 17, 2022 at 04:38:11PM +0100, Richard W.M. Jones wrote:
This allows you to pass in a file descriptor to the plugin, eg:
$ exec 6<>/var/tmp/fedora-36.img
$ ./nbdkit file fd=6 --run 'nbdinfo $uri'
Note this was previously possible on most platforms using /dev/fd/<N>,
but not all platforms that have file descriptors support this
(although it is POSIX) and it seems better to make this explicit.
The parenthetical feels odd here. /dev/fd/<N> is not POSIX; what IS a
POSIX feature is the ability to pass an open file descriptor by
inheritance.
@@ -202,6 +208,17 @@ file_config (const char *key, const char
*value)
if (!directory)
return -1;
}
+ else if (strcmp (key, "fd") == 0) {
+ if (mode != mode_none) goto wrong_mode;
+ mode = mode_filedesc;
+ assert (filedesc == -1);
+ if (nbdkit_parse_int ("fd", value, &filedesc) == -1)
+ return -1;
+ if (filedesc <= 2) {
+ nbdkit_error ("file descriptor must be >= 3");
Is it worth using STDERR_FILENO instead of open-coding 2? I'll be okay
if your answer is that this is one place where <unistd.h> has too much
indirection ;)
@@ -408,59 +436,69 @@ file_open (int readonly)
...
+ case mode_filedesc:
+ h->fd = dup (filedesc);
+ if (h->fd == -1) {
+ nbdkit_error ("dup fd=%d: %m", filedesc);
+ free (h);
+ return NULL;
+ }
+ file = "<file descriptor>"; /* This is needed for error messages.
*/
+ break;
Is it worth trying to figure out if the FD has O_RDWR privileges and
set h->can_write accordingly, to match...
default: abort ();
}
- h = malloc (sizeof *h);
- if (h == NULL) {
- nbdkit_error ("malloc: %m");
- if (dfd != -1)
- close (dfd);
- return NULL;
- }
-
- flags = O_CLOEXEC|O_NOCTTY;
- if (readonly) {
- flags |= O_RDONLY;
- h->can_write = false;
- }
- else {
- flags |= O_RDWR;
- h->can_write = true;
- }
-
- h->fd = openat (dfd, file, flags);
- if (h->fd == -1 && !readonly) {
- nbdkit_debug ("open O_RDWR failed, falling back to read-only: %s: %m",
- file);
- flags = (flags & ~O_ACCMODE) | O_RDONLY;
- h->fd = openat (dfd, file, flags);
- h->can_write = false;
- }
... what we do for normal files?
Overall looks like a nice addition.
ACK series
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org