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Re: qemu modularization of qemu-5.1 vs libvirt domcapabilities cache?



On 8/20/20 10:54 AM, Mark Mielke wrote:
On Thu, Aug 20, 2020 at 11:48 AM Christian Ehrhardt <christian ehrhardt canonical com <mailto:christian ehrhardt canonical com>> wrote:

    On Thu, Aug 20, 2020 at 5:15 PM Mark Mielke <mark mielke gmail com
    <mailto:mark mielke gmail com>> wrote:
     > On Thu, Aug 20, 2020 at 8:55 AM Christian Ehrhardt
    <christian ehrhardt canonical com <mailto:christian ehrhardt canonical com>>
    wrote:
     >> This was meant for upgrades, but if libvirt would define a known path in
     >> there like /var/run/qemu/last_packaging_change the packages could easily
     >> touch it on any install/remove/update as Daniel suggested and libvirt could
     >> check this path like it does with the date of the qemu binary already.
     > Earlier in this thread - I think one or two of us had asked about the
    timestamp on the directory that contains the modules.
     > I'm wondering if a "last_packaging_change" provides any value over and
    above the timestamp of the directory itself? Wouldn't the directory be best
    - as it would work automatically for both distro packaging as well as custom
    installs?
    Sure, if
    - "list of files in module dir"
    - "stat dates of files in module dir"
    would be checked by libvirt that would also be a no-op for packaging
    and thereby land faster.


Why is the list of files and stat dates needed? Any change done by RPM to add or remove a file from the directory, should cause the mtime for the directory to be updated. It doesn't really matter what the change is - it only matters that the change is detected.

The case for needing to check the files *in* the directory, would be a concern about the modules keeping the same name, but being updated in place. I'm doubtful that this ever occurs for real, as it would cause breakage for running programs. Existing running programs would mmap() or similar the binaries into memory, and cannot be updated in place. Instead, the inode remains alive, and a new file is created with the same name, to replace the old file, and once all file descriptors are closed - the inode is deleted.

So, unless there is a hierarchical directory structure - I believe checking the modules directory timestamp is near 100% accuracy for whether modules were added, removed, or updated using "safe" deployment practices either by RPM or "make install".

I wrote a small test program that checks mtime (also tried ctime) and it only changes on the directory when files are added and deleted. There is no change to either if a file in the directory has been updated. It would be really convenient to check only the directory to see if its' contents have changed but I'm not aware of how to do that other than with something like inotify. Suggestions welcomed.

Regards,
Jim
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

int main(int argc, int **argv)
{
    struct stat sb;
    time_t curtime;

    if (stat("/home/jfehlig/virt/temp", &sb) < 0) {
        printf("stat failed with error %s\n", strerror(errno));
        exit(1);
    }

    curtime = sb.st_mtime;
    printf("Got initial mtime on directory, looking for changes...\n");
    while (1) {
        sleep(5);
        if (stat("/home/jfehlig/virt/temp", &sb) < 0) {
            printf("stat failed with error %s\n", strerror(errno));
            exit(1);
        }

        if (curtime != sb.st_mtime) {
            printf("The directory has changed!\n");
            curtime = sb.st_mtime;
        }
    }

    exit(0);
}

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