Enhance nscd's inotify support (Bug 14906).

In bug 14906 the user complains that the inotify support in nscd
is not sufficient when it comes to detecting changes in the
configurationfiles that should be watched for the various databases.

The current nscd implementation uses inotify to watch for changes in
the configuration files, but adds watches only for IN_DELETE_SELF and
IN_MODIFY. These watches are insufficient to cover even the most basic
uses by a system administrator. For example using emacs or vim to edit
a configuration file should trigger a reload but it might not if
the editors use move to atomically update the file. This atomic update
changes the inode and thus removes the notification on the file (as
inotify is based on inodes). Thus the inotify support in nscd for
configuration files is insufficient to account for the average use
cases of system administrators and users.

The inotify support is significantly enhanced and described here:
https://www.sourceware.org/ml/libc-alpha/2015-02/msg00504.html

Tested on x86_64 with and without inotify support.

1 files changed, 19 insertions, 9 deletions

diff --git a/nscd/cache.c b/nscd/cache.c
index ea9f72311b..f0200c303c 100644
--- a/nscd/cache.c
+++ b/nscd/cache.c
@@ -272,28 +272,38 @@ prune_cache (struct database_dyn *table, time_t now, int fd)
       while (runp != NULL)
 	{
 #ifdef HAVE_INOTIFY
-	  if (runp->inotify_descr == -1)
+	  if (runp->inotify_descr[TRACED_FILE] == -1)
 #endif
 	    {
 	      struct stat64 st;
 
 	      if (stat64 (runp->fname, &st) < 0)
 		{
+		  /* Print a diagnostic that the traced file was missing.
+		     We must not disable tracing since the file might return
+		     shortly and we want to reload it at the next pruning.
+		     Disabling tracing here would go against the configuration
+		     as specified by the user via check-files.  */
 		  char buf[128];
-		  /* We cannot stat() the file, disable file checking if the
-		     file does not exist.  */
-		  dbg_log (_("cannot stat() file `%s': %s"),
+		  dbg_log (_("checking for monitored file `%s': %s"),
 			   runp->fname, strerror_r (errno, buf, sizeof (buf)));
-		  if (errno == ENOENT)
-		    table->check_file = 0;
 		}
 	      else
 		{
-		  if (st.st_mtime != table->file_mtime)
+		  /* This must be `!=` to catch cases where users turn the
+		     clocks back and we still want to detect any time difference
+		     in mtime.  */
+		  if (st.st_mtime != runp->mtime)
 		    {
-		      /* The file changed.  Invalidate all entries.  */
+		      dbg_log (_("monitored file `%s` changed (mtime)"),
+			       runp->fname);
+		      /* The file changed. Invalidate all entries.  */
 		      now = LONG_MAX;
-		      table->file_mtime = st.st_mtime;
+		      runp->mtime = st.st_mtime;
+#ifdef HAVE_INOTIFY
+		      /* Attempt to install a watch on the file.  */
+		      install_watches (runp);
+#endif
 		    }
 		}
 	    }