tail: ignore -f for piped-stdin, as POSIX requires

* src/tail.c (main): Tailing a pipe "forever" is not useful,
and POSIX specifies that tail ignore the -f when there is no
file argument and stdin is a FIFO or pipe.  So we do that.
In addition, GNU tail excludes "-" arguments from the list of files
to tail forever, when the associated file descriptor is connected
to a FIFO or pipe.  Before this change, ":|tail -f" would hang.
Reported by Ren Yang and Ulrich Drepper.
* tests/tail-2/pipe-f: Test for this.
* tests/tail-2/pipe-f2: Ensure tail doesn't exit early for a fifo.
* tests/Makefile.am (TESTS): Add these tests.
* NEWS (POSIX conformance): Mention it.

1 files changed, 15 insertions, 1 deletions

diff --git a/src/tail.c b/src/tail.c
index 9288007ca..d75d9b3e4 100644
--- a/src/tail.c
+++ b/src/tail.c
@@ -1979,7 +1979,21 @@ main (int argc, char **argv)
   for (i = 0; i < n_files; i++)
     ok &= tail_file (&F[i], n_units);
 
-  if (forever)
+  /* When there is no FILE operand and stdin is a pipe or FIFO
+     POSIX requires that tail ignore the -f option.
+     Since we allow multiple FILE operands, we extend that to say:
+     ignore any "-" operand that corresponds to a pipe or FIFO.  */
+  size_t n_viable = 0;
+  for (i = 0; i < n_files; i++)
+    {
+      if (STREQ (F[i].name, "-") && !F[i].ignore
+          && 0 <= F[i].fd && S_ISFIFO (F[i].mode))
+        F[i].ignore = true;
+      else
+        ++n_viable;
+    }
+
+  if (forever && n_viable)
     {
 #if HAVE_INOTIFY
       /* If the user specifies stdin via a command line argument of "-",