readdir/telldir/seekdir problem (i think)
Julian Elischer
julian at freebsd.org
Thu Apr 23 03:36:25 UTC 2015
On 4/23/15 11:20 AM, Julian Elischer wrote:
> I'm debugging a problem being seen with samba 3.6.
>
> basically telldir/seekdir/readdir don't seem to work as advertised..
On the intergooglewebs I see multiple previous reports of similar
problems, some going back 30 years, but
we supposedly have the fixes for those in -current.
added data point.. if I comment out the 'unlink' lines all the files
are enumerated,
so it's something similar to the previously reported unlink-vs-readdir
bugs. but different.
If I insert a rewinddir() after each set of deletes it all works right
but that means the
system is continuously rereading all the prior directory blocks..
It goes wrong on at least UFS and ZFS. maybe others.
I'm not totally sure it can be fixed. If teh underlying directory
blocs are
reshuffled or changed and things are moved, it may just be impossible
to keep any information as to where you are in the directory without
adding more state.
> here's a little test program
>
> #include <stdio.h>
> #include <sys/types.h>
> #include <dirent.h>
> #include <fcntl.h>
> #include <errno.h>
> #include <string.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <sysexits.h>
> #include <err.h>
>
> #define CHUNKSIZE 5
> #define TOTALFILES 40
>
> static void
> SeekDir(DIR *dirp, long loc)
> {
> printf("Seeking back to location %ld\n", loc);
> seekdir(dirp, loc);
> }
>
> static long
> TellDir(DIR *dirp)
> {
> long loc;
>
> loc = telldir(dirp);
> printf("telldir assigned location %ld\n", loc);
> return (loc);
> }
>
> int
> main(int argc, char *argv[])
> {
> DIR *dirp;
> int i;
> int j;
> long offset = 0, prev_offset = 0;
> char *files[100];
> char filename[100];
> int fd;
> struct dirent *dp = NULL;
>
> if (chdir("./test2") != 0) {
> err(EX_OSERR, "chdir");
> }
>
> /*****************************************************/
> /* Put a set of sample files in the target directory */
> /*****************************************************/
>
> for (i=1; i < TOTALFILES ; i++)
> {
> sprintf(filename, "file-%d", i);
> fd = open(filename, O_CREAT, 0666);
> if (fd == -1) {
> err(EX_OSERR, "open");
> }
> close(fd);
> }
> dirp = opendir(".");
> offset = TellDir(dirp);
> for (i = 0; i < 20; i++)
> files[i] = malloc(20);
>
> /*******************************************************/
> /* enumerate and delete small sets of files, one group */
> /* at a time. */
> /*******************************************************/
> do {
>
> /*****************************************/
> /* Read in up to CHUNKSIZE file names */
> /* i will be the number of files we hold */
> /*****************************************/
> for (i = 0; i < CHUNKSIZE; i++) {
> if ((dp = readdir(dirp)) != NULL) {
> strcpy(files[i], dp->d_name);
>
> printf("readdir (%ld) returned file %s\n",
> offset, files[i]);
>
> prev_offset = offset;
> offset = TellDir(dirp);
>
> } else {
> printf("readdir returned null\n");
> break;
> }
> }
>
>
> /****************************************************************/
> /* Simuate the last entry not fitting into our (samba's)
> buffer */
> /* If we read someting in on the last slot, push it
> back */
> /* Pretend it didn't fit. This is approximately what SAMBA
> does.*/
> /****************************************************************/
> if (dp != NULL) {
> /* Step back */
> SeekDir(dirp, prev_offset);
> offset = TellDir(dirp);
> i--;
> printf("file %s returned\n", files[i]);
> }
>
> /*****************************************/
> /* i is the number of names we have left.*/
> /* Delete them. */
> /*****************************************/
> for (j = 0; j < i; j++) {
> if (*files[j] == '.') {
> printf ("skipping %s\n", files[j]);
> } else {
> printf("Unlinking file %s\n", files[j]);
> if (unlink(files[j]) != 0) {
> err(EX_OSERR, "unlink");
> }
> }
> }
> } while (dp != NULL);
>
> closedir(dirp);
> //chdir("..");
>
> }
>
> The program is simulating what Samba does when fails. (doing a
> recursive delete of a directory)
> What it does is reads a chunk of names using readdir() until it's
> (small) buffer s full,
> then it uses seekdir() to seek back before the last entry it read,
> (which fails to fit),
> theortically leaving it for the next read.
> It then deletes the entries it found and repeats the cycle.
>
> Eventually it should have found all the files in the directory and
> deleted them.
> Except that it doesn't.
>
> What actually happens is that some files are not enumerated, even
> though
> the seekdir() should have made the readdir() find them.
> for added fun. the FIRST seekdir appears to work. but all subsequent
> ones don't.
>
> It behaves this way in -current , all the way back to at least 8.0.
>
> if there's a bug in my program please let me know, but samba has the
> same problem.. e.g. on freeNAS.
>
> to use the program make a directory called "./test2" and then run it
> in the current directory..
> It fills it with files and then tried to (fails) delete them in
> small batches.
>
> here's some (annotated) output:
>
> ./testit
> telldir assigned location 0
> readdir (0) returned file .
> telldir assigned location 1
> readdir (1) returned file ..
> telldir assigned location 2
> readdir (2) returned file file-1
> telldir assigned location 3
> readdir (3) returned file file-2
> telldir assigned location 4
> readdir (4) returned file file-3
> telldir assigned location 5
> >>>>> here we pretend the buffer was full and put the file
> >>>>> marker back so that it will get read next time
> Seeking back to location 4
> telldir assigned location 4
> file file-3 returned
> skipping .
> skipping ..
> Unlinking file file-1
> Unlinking file file-2
> readdir (4) returned file file-3
> >>>>> hey it worked (this time)
> telldir assigned location 5
> readdir (5) returned file file-4
> telldir assigned location 6
> readdir (6) returned file file-5
> telldir assigned location 7
> readdir (7) returned file file-6
> telldir assigned location 8
> readdir (8) returned file file-7
> telldir assigned location 9
> >>>>> OK do it again.. pretend file-7 didn't fit..
> >>>>> set the pointer back so we re-read it next time.
>
> Seeking back to location 8
> telldir assigned location 8
> file file-7 returned
> Unlinking file file-3
> Unlinking file file-4
> Unlinking file file-5
> Unlinking file file-6
> >>>>> OK lets go get file-7 again
>
> readdir (8) returned file file-9
> >>>>> WTF? what happened to file-7 ?
>
> telldir assigned location 9
> readdir (9) returned file file-10
> telldir assigned location 10
>
>
>
>
>
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