[glibc] nscd: Skip unusable entries in first pass in prune_cache (bug 30800)

[Florian Weimer <fw@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=c00b984fcd53f679ca2dafcd1aee2c89836e6e73

commit c00b984fcd53f679ca2dafcd1aee2c89836e6e73
Author: Florian Weimer <fweimer@redhat.com>
Date:   Tue Aug 29 08:28:31 2023 +0200

    nscd: Skip unusable entries in first pass in prune_cache (bug 30800)
    
    Previously, if an entry was marked unusable for any reason, but had
    not timed out yet, the assert would trigger.
    
    One way to get into such state is if a data change is detected during
    re-validation of an entry.  This causes the entry to be marked as not
    usable.  If exits nscd soon after that, then the clock jumps
    backwards, and nscd restarted, the cache re-validation run after
    startup triggers the removed assert.
    
    The change is more complicated than just the removal of the assert
    because entries marked as not usable should be garbage-collected in
    the second pass.  To make this happen, it is necessary to update some
    book-keeping data.
    
    Reviewed-by: DJ Delorie <dj@redhat.com>

Diff:
---
 nscd/cache.c | 25 +++++++++++--------------
 1 file changed, 11 insertions(+), 14 deletions(-)

diff --git a/nscd/cache.c b/nscd/cache.c
index b4b54d82bb..336ff548cb 100644
--- a/nscd/cache.c
+++ b/nscd/cache.c
@@ -370,8 +370,11 @@ prune_cache (struct database_dyn *table, time_t now, int fd)
 		       serv2str[runp->type], str, dh->timeout);
 	    }
 
-	  /* Check whether the entry timed out.  */
-	  if (dh->timeout < now)
+	  /* Check whether the entry timed out.  Timed out entries
+	     will be revalidated.  For unusable records, it is still
+	     necessary to record that the bucket needs to be scanned
+	     again below.  */
+	  if (dh->timeout < now || !dh->usable)
 	    {
 	      /* This hash bucket could contain entries which need to
 		 be looked at.  */
@@ -383,7 +386,7 @@ prune_cache (struct database_dyn *table, time_t now, int fd)
 	      /* We only have to look at the data of the first entries
 		 since the count information is kept in the data part
 		 which is shared.  */
-	      if (runp->first)
+	      if (runp->first && dh->usable)
 		{
 
 		  /* At this point there are two choices: we reload the
@@ -399,9 +402,6 @@ prune_cache (struct database_dyn *table, time_t now, int fd)
 		    {
 		      /* Remove the value.  */
 		      dh->usable = false;
-
-		      /* We definitely have some garbage entries now.  */
-		      any = true;
 		    }
 		  else
 		    {
@@ -413,18 +413,15 @@ prune_cache (struct database_dyn *table, time_t now, int fd)
 
 		      time_t timeout = readdfcts[runp->type] (table, runp, dh);
 		      next_timeout = MIN (next_timeout, timeout);
-
-		      /* If the entry has been replaced, we might need
-			 cleanup.  */
-		      any |= !dh->usable;
 		    }
 		}
+
+	      /* If the entry has been replaced, we might need cleanup.  */
+	      any |= !dh->usable;
 	    }
 	  else
-	    {
-	      assert (dh->usable);
-	      next_timeout = MIN (next_timeout, dh->timeout);
-	    }
+	    /* Entry has not timed out and is usable.  */
+	    next_timeout = MIN (next_timeout, dh->timeout);
 
 	  run = runp->next;
 	}