tail: consistently output all data for truncated files

Generally if logs are truncated, they're truncated to 0 length,
so output all existing data when our heuristic determines truncation.
Note with inotify, truncate() and write() are often determined
independently and so all data would be written if that was the case.

* src/tail.c (check_fspec): Reset file offset to 0 upon truncation.
(tail_forever): Likewise.
(recheck): Add a FIXME for the related issue where tail may lose
data due to tail discounting older log files too early.
* tests/tail-2/truncate.sh: A new test.
* tests/local.mk: Reference the new test.
* NEWS: Mention the fix.

1 files changed, 22 insertions, 8 deletions

diff --git a/src/tail.c b/src/tail.c
index bc1d04a8f..c9736ca5b 100644
--- a/src/tail.c
+++ b/src/tail.c
@@ -1025,7 +1025,7 @@ recheck (struct File_spec *f, bool blocking)
       if (f->fd == -1)
         {
           error (0, 0,
-                 _("%s has appeared;  following end of new file"),
+                 _("%s has appeared;  following new file"),
                  quote (pretty_name (f)));
         }
       else
@@ -1036,7 +1036,7 @@ recheck (struct File_spec *f, bool blocking)
           /* File has been replaced (e.g., via log rotation) --
              tail the new one.  */
           error (0, 0,
-                 _("%s has been replaced;  following end of new file"),
+                 _("%s has been replaced;  following new file"),
                  quote (pretty_name (f)));
         }
     }
@@ -1055,6 +1055,12 @@ recheck (struct File_spec *f, bool blocking)
         }
     }
 
+  /* FIXME: When a log is rotated, daemons tend to log to the
+     old file descriptor until the new file is present and
+     the daemon is sent a signal.  Therefore tail may miss entries
+     being written to the old file.  Perhaps we should keep
+     the older file open and continue to monitor it until
+     data is written to a new file.  */
   if (new_file)
     {
       /* Start at the beginning of the file.  */
@@ -1193,13 +1199,16 @@ tail_forever (struct File_spec *f, size_t n_files, double sleep_interval)
               /* reset counter */
               f[i].n_unchanged_stats = 0;
 
+              /* XXX: This is only a heuristic, as the file may have also
+                 been truncated and written to if st_size >= size
+                 (in which case we ignore new data <= size).  */
               if (S_ISREG (mode) && stats.st_size < f[i].size)
                 {
                   error (0, 0, _("%s: file truncated"), name);
-                  last = i;
-                  xlseek (fd, stats.st_size, SEEK_SET, name);
-                  f[i].size = stats.st_size;
-                  continue;
+                  /* Assume the file was truncated to 0,
+                     and therefore output all "new" data.  */
+                  xlseek (fd, 0, SEEK_SET, name);
+                  f[i].size = 0;
                 }
 
               if (i != last)
@@ -1330,12 +1339,17 @@ check_fspec (struct File_spec *fspec, int wd, int *prev_wd)
       return;
     }
 
+  /* XXX: This is only a heuristic, as the file may have also
+     been truncated and written to if st_size >= size
+     (in which case we ignore new data <= size).
+     Though in the inotify case it's more likely we'll get
+     separate events for truncate() and write().  */
   if (S_ISREG (fspec->mode) && stats.st_size < fspec->size)
     {
       error (0, 0, _("%s: file truncated"), name);
       *prev_wd = wd;
-      xlseek (fspec->fd, stats.st_size, SEEK_SET, name);
-      fspec->size = stats.st_size;
+      xlseek (fspec->fd, 0, SEEK_SET, name);
+      fspec->size = 0;
     }
   else if (S_ISREG (fspec->mode) && stats.st_size == fspec->size
            && timespec_cmp (fspec->mtime, get_stat_mtime (&stats)) == 0)