tail: honor --pid with fifos

* src/tail.c (tail_file): Open files with O_NONBLOCK
if we might need async processing.
(pipe_bytes): Ignore EAGAIN read() errors.
(pipe_lines): Likewise.
* tests/tail/pid-pipe.sh: Add a new test.
* tests/local.mk: Reference the new test.
* NEWS: Mention the bug fix.
Reported by Berhard Voelker.

NEWS

@@ -47,6 +47,10 @@ GNU coreutils NEWS                                    -*- outline -*-
   where inotify is not used, when all followed files become inaccessible.
   [This bug was present in "the beginning".]
 
+  `tail --follow --pid=PID` will now exit when the PID dies,
+  even in the presence of blocking inputs like unopened fifos.
+  [This bug was present in "the beginning".]
+
   'tail -c 4096 /dev/zero' no longer loops forever.
   [This bug was present in "the beginning".]
 

src/tail.c

@@ -698,7 +698,7 @@ pipe_lines (char const *pretty_filename, int fd, uintmax_t n_lines,
 
   free (tmp);
 
-  if (n_read < 0)
+  if (n_read < 0 && errno != EAGAIN)
     {
       error (0, errno, _("error reading %s"), quoteaf (pretty_filename));
       ok = false;
@@ -826,7 +826,7 @@ pipe_bytes (char const *pretty_filename, int fd, uintmax_t n_bytes,
 
   free (tmp);
 
-  if (n_read < 0)
+  if (n_read < 0 && errno != EAGAIN)
     {
       error (0, errno, _("error reading %s"), quoteaf (pretty_filename));
       ok = false;
@@ -2003,11 +2003,14 @@ tail (char const *filename, int fd, uintmax_t n_units,
    Return true if successful.  */
 
 static bool
-tail_file (struct File_spec *f, uintmax_t n_units)
+tail_file (struct File_spec *f, uintmax_t n_files, uintmax_t n_units)
 {
   int fd;
   bool ok;
 
+  /* Avoid blocking if we may need to process asynchronously.  */
+  bool nonblocking = forever && (nbpids || n_files > 1);
+
   bool is_stdin = (STREQ (f->name, "-"));
 
   if (is_stdin)
@@ -2017,7 +2020,8 @@ tail_file (struct File_spec *f, uintmax_t n_units)
       xset_binary_mode (STDIN_FILENO, O_BINARY);
     }
   else
-    fd = open (f->name, O_RDONLY | O_BINARY);
+    fd = open (f->name, O_RDONLY | O_BINARY
+               | nonblocking ? O_NONBLOCK : 0);
 
   f->tailable = !(reopen_inaccessible_files && fd == -1);
 
@@ -2456,7 +2460,7 @@ main (int argc, char **argv)
   xset_binary_mode (STDOUT_FILENO, O_BINARY);
 
   for (i = 0; i < n_files; i++)
-    ok &= tail_file (&F[i], n_units);
+    ok &= tail_file (&F[i], n_files, n_units);
 
   if (forever && ignore_fifo_and_pipe (F, n_files))
     {

tests/local.mk

@@ -268,6 +268,7 @@ all_tests =					\
   tests/misc/xstrtol.pl				\
   tests/tail/overlay-headers.sh			\
   tests/tail/pid.sh				\
+  tests/tail/pid-pipe.sh			\
   tests/od/od.pl				\
   tests/od/od-endian.sh				\
   tests/od/od-float.sh				\

tests/tail/pid-pipe.sh

@@ -0,0 +1,36 @@
+#!/bin/sh
+# Ensure that "tail --pid=PID fifo" exits responsively
+
+# Copyright (C) 2025 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ tail
+
+mkfifo_or_skip_ fifo
+
+# Terminate any background process
+cleanup_() { kill $pid 2>/dev/null && wait $pid; }
+
+# Speedup the non inotify case
+fastpoll='-s.1 --max-unchanged-stats=1'
+
+sleep 1 & pid=$!
+
+returns_ 124 timeout 10 tail -f $fastpoll --pid=$! fifo && fail=1
+
+cleanup_
+
+Exit $fail