Tweak seekdir, telldir and readdir so that when htere are deletes going on,

as seek to teh last location saved will still work. This is needed for Samba
to be able to correctly handle delete requests from windows. This does not
completely fix seekdir when deletes are present but fixes the worst of the
problems. The real solution must involve some changes to the API for eh VFS
and getdirentries(2).

Obtained from:	Panzura inc
MFC after:	1 week

lib/libc/gen/directory.3

@@ -267,4 +267,27 @@ The invalidation of
 .Fn telldir
 tokens when calling
 .Fn seekdir
-is non-standard.
+is non-standard. This is a compile time option.
+.Pp
+The behaviour of
+.Fn telldir
+and
+.Fn seekdir
+is likely to be wrong if there are parallel unlinks happening
+and the directory is larger than one page.
+There is code to ensure that a
+.Fn seekdir
+to the location given by a 
+.Fn telldir
+immediately before the last 
+.Fn readdir
+will always set the correct location to return the same value as that last
+.Fn readdir
+performed.
+This is enough for some applications which want to "push back the last entry read" E.g. Samba. 
+Seeks back to any other location,
+other than the beginning of the directory,
+may result in unexpected behaviour if deletes are present.
+It is hoped that this situation will be resolved with changes to
+.Fn getdirentries
+and the VFS.

lib/libc/gen/readdir.c

@@ -54,19 +54,25 @@ _readdir_unlocked(dirp, skip)
 	int skip;
 {
 	struct dirent *dp;
+	long initial_seek;
+	long initial_loc = 0;
 
 	for (;;) {
 		if (dirp->dd_loc >= dirp->dd_size) {
 			if (dirp->dd_flags & __DTF_READALL)
 				return (NULL);
+			initial_loc = dirp->dd_loc;
+			dirp->dd_flags &= ~__DTF_SKIPREAD;
 			dirp->dd_loc = 0;
 		}
 		if (dirp->dd_loc == 0 &&
 		    !(dirp->dd_flags & (__DTF_READALL | __DTF_SKIPREAD))) {
+			initial_seek = dirp->dd_seek;
 			dirp->dd_size = _getdirentries(dirp->dd_fd,
 			    dirp->dd_buf, dirp->dd_len, &dirp->dd_seek);
 			if (dirp->dd_size <= 0)
 				return (NULL);
+			_fixtelldir(dirp, initial_seek, initial_loc);
 		}
 		dirp->dd_flags &= ~__DTF_SKIPREAD;
 		dp = (struct dirent *)(dirp->dd_buf + dirp->dd_loc);

lib/libc/gen/rewinddir.c

@@ -51,6 +51,7 @@ rewinddir(dirp)
 
 	if (__isthreaded)
 		_pthread_mutex_lock(&dirp->dd_lock);
+	dirp->dd_flags &= ~__DTF_SKIPREAD; /* current contents are invalid */
 	if (dirp->dd_flags & __DTF_READALL)
 		_filldir(dirp, false);
 	else {

lib/libc/gen/telldir.c

@@ -101,9 +101,21 @@ _seekdir(dirp, loc)
 		return;
 	if (lp->loc_loc == dirp->dd_loc && lp->loc_seek == dirp->dd_seek)
 		return;
+	/* If it's within the same chunk of data, don't bother reloading */
+	if (lp->loc_seek == dirp->dd_seek) {
+		/*
+		 * If we go back to 0 don't make the next readdir
+		 * trigger a call to getdirentries()
+		 */
+		if (lp->loc_loc == 0)
+			dirp->dd_flags |= __DTF_SKIPREAD;
+		dirp->dd_loc = lp->loc_loc;
+		return;
+	}
 	(void) lseek(dirp->dd_fd, (off_t)lp->loc_seek, SEEK_SET);
 	dirp->dd_seek = lp->loc_seek;
 	dirp->dd_loc = 0;
+	dirp->dd_flags &= ~__DTF_SKIPREAD; /* current contents are invalid */
 	while (dirp->dd_loc < lp->loc_loc) {
 		dp = _readdir_unlocked(dirp, 0);
 		if (dp == NULL)
@@ -111,6 +123,27 @@ _seekdir(dirp, loc)
 	}
 }
 
+/*
+ * when we do a read and cross a boundary, any telldir we
+ * just did will have wrong information in it.
+ * We need to move it from "beyond the end of the previous chunk"
+ * to "the beginning of the new chunk"
+ */
+void
+_fixtelldir(DIR *dirp, long oldseek, long oldloc)
+{
+	struct ddloc *lp;
+
+	lp = LIST_FIRST(&dirp->dd_td->td_locq);
+	if (lp != NULL) {
+		if (lp->loc_loc == oldloc &&
+		    lp->loc_seek == oldseek) {
+			lp->loc_seek = dirp->dd_seek;
+			lp->loc_loc = dirp->dd_loc;
+		}
+	}
+}
+
 /*
  * Reclaim memory for telldir cookies which weren't used.
  */

lib/libc/gen/telldir.h

@@ -64,5 +64,6 @@ bool		_filldir(DIR *, bool);
 struct dirent	*_readdir_unlocked(DIR *, int);
 void 		_reclaim_telldir(DIR *);
 void 		_seekdir(DIR *, long);
+void		_fixtelldir(DIR *dirp, long oldseek, long oldloc);
 
 #endif