tail: without inotify, ensure prompt output

This was seen with a NetBSD failure in the pipe-f2.sh test
due to it clearing the EOF indication on a fifo, and thus
blocking in the subsequent read.  That was discussed at:
https://lists.gnu.org/r/coreutils/2026-04/msg00129.html
https://bugs.gnu.org/81923
Note the NetBSD issue may be addressed in NetBSD 12 as
discussed in the bug above.

This is more defensive/consistent in the case
where a subsequent read on a file blocks.
We might be operating on non fifos after all.

* src/tail.c (tail_forever): Flush output before we might block.
* tests/tail/pipe-f2.sh: Test non inotify path on all platforms.
* NEWS: Mention the improvement.

NEWS

@@ -17,6 +17,10 @@ GNU coreutils NEWS                                    -*- outline -*-
 
   'od' allocates half the memory when -v,--output-duplicates is specified.
 
+  'tail' ensures prompt output when inotify is not being used, and when
+  initial reads finish, but subsequent reads might block, which was seen
+  e.g. with fifos on NetBSD 10.
+
 ** Build-related
 
   gzip-compressed tarballs are no longer distributed,

src/tail.c

@@ -1177,6 +1177,10 @@ tail_forever (struct File_spec *f, int n_files, double sleep_interval)
 
   static bool debugged;
 
+  /* Flush initial output, in case reading blocks below.  */
+  if (fflush (stdout) < 0)
+    write_error ();
+
   while (true)
     {
       /* Use blocking I/O as an optimization, when it's easy.  */

tests/tail/pipe-f2.sh

@@ -21,7 +21,6 @@ print_ver_ tail
 
 mkfifo_or_skip_ fifo
 
-echo 1 > fifo &
 echo 1 > exp || framework_failure_
 
 # Terminate any background tail process
@@ -30,18 +29,22 @@ cleanup_() { kill $pid 2>/dev/null && wait $pid; }
 # Speedup the non inotify case
 fastpoll='-s.1 --max-unchanged-stats=1'
 
-timeout 10 tail $fastpoll -f fifo > out & pid=$!
-
 check_tail_output() { sleep $1; test -s out; }
 
-# Wait 12.7s for tail to write something.
-retry_delay_ check_tail_output .1 7 || fail=1
+for mode in '' '---disable-inotify'; do
+  rm -f out || framework_failure_
+  echo 1 > fifo &
+  timeout 10 tail $fastpoll -f $mode fifo > out & pid=$!
 
-compare exp out || fail=1
+  # Wait 12.7s for tail to write something.
+  retry_delay_ check_tail_output .1 7 || fail=1
 
-# Ensure tail is still running
-kill -0 $pid || fail=1
+  compare exp out || fail=1
 
-cleanup_
+  # Ensure tail is still running
+  kill -0 $pid || fail=1
+
+  cleanup_
+done
 
 Exit $fail