tee: fix infinite loop when write returns EAGAIN and short write errors

* NEWS: Mention the bug fixes.
* THANKS.in: Add Bernhard M. Wiedemann for reporting the bugs.
* src/iopoll.c (close_wait): Remove function.
(write_wait): Don't call wait_for_nonblocking_write if write is
successful. Handle errors more robustly.
* src/iopoll.h (close_wait): Remove declaration.
* src/tee.c (tee_files): Use close instead of close_wait.
* tests/tee/short-write.sh: New test for the bug.
* tests/tee/write-eagain.sh: Likewise.
* tests/local.mk (all_tests): Add the new tests.
Fixes https://bugs.gnu.org/81060

NEWS

@@ -15,6 +15,13 @@ GNU coreutils NEWS                                    -*- outline -*-
   'shred' no longer blocks when opening a FIFO that has no readers.
   [This bug was present in "the beginning".]
 
+  'tee' no longer loops infinitely after writing all output if a write call sets
+  errno to EAGAIN.
+  [bug introduced in coreutils-9.11]
+
+  'tee' no longer treats short writes as errors.
+  [bug introduced in coreutils-9.11]
+
   'unexpand -t' no longer overflows a heap buffer, for tab values > SIZE_MAX/16.
   [bug introduced in coreutils-9.11]
 

THANKS.in

@@ -86,6 +86,7 @@ Bernd Leibing                       bernd.leibing@rz.uni-ulm.de
 Bernd Melchers                      melchers@cis.fu-berlin.de
 Bernhard Baehr                      bernhard.baehr@gmx.de
 Bernhard Gabler                     bernhard@uni-koblenz.de
+Bernhard M. Wiedemann               bwiedemann@suse.de
 Bernhard Rosenkraenzer              bero@redhat.de
 Bert Deknuydt                       Bert.Deknuydt@esat.kuleuven.ac.be
 Bert Wesarg                         bert.wesarg@googlemail.com

src/iopoll.c

@@ -194,17 +194,6 @@ wait_for_nonblocking_write (int fd)
   return true;
 }
 
-/* wrapper for close() that also waits for FD if non blocking.  */
-
-extern bool
-close_wait (int fd)
-{
-  while (wait_for_nonblocking_write (fd))
-    ;
-  return close (fd) == 0;
-}
-
-
 /* wrapper for write() that also waits for FD if non blocking.  */
 
 extern bool
@@ -212,19 +201,43 @@ write_wait (int fd, void const *buffer, size_t size)
 {
   unsigned char const *buf = buffer;
 
-  while (true)
+  do
     {
-      ssize_t written = write (fd, buf, size);
+      const ssize_t written = write (fd, buf, size);
+      /* POSIX says that calling write with SIZE of zero may detect and
+         return errors.  If no error occurs, or write makes no attempt
+         to detect errors, then write returns zero with no other
+         results.  write_fail will return successfully in this case.  */
+      if (written == 0)
+        {
+          if (size == 0)
+            return true;
+          else
+            {
+              /* If SIZE is greater than zero and write returns zero,
+                 treat it as an error.  Some buggy drivers behave this
+                 way.  See src/dd.c and Gnulib's lib/full-write.c for
+                 more details.  */
+              errno = ENOSPC;
+              return false;
+            }
+        }
+
       if (written < 0)
-        written = 0;
-
-      size -= written;
-      if (size <= 0)  /* everything written */
-        return true;
-
-      if (! wait_for_nonblocking_write (fd))
-        return false;
-
-      buf += written;
+        {
+          /* Return an error if write detected one with a SIZE of zero.
+             Otherwise, if SIZE is greater than zero, fail if it does
+             not become writable.  */
+          if (size == 0 || ! wait_for_nonblocking_write (fd))
+            return false;
+        }
+      else
+        {
+          buf += written;
+          size -= written;
+        }
     }
+  while (0 < size);
+
+  return true;
 }

src/iopoll.h

@@ -5,5 +5,4 @@ int iopoll (int fdin, int fdout, bool block);
 bool iopoll_input_ok (int fdin);
 bool iopoll_output_ok (int fdout);
 
-bool close_wait (int fd);
 bool write_wait (int fd, void const *buffer, size_t size);

src/tee.c

@@ -329,7 +329,7 @@ tee_files (int nfiles, char **files, bool pipe_check)
 
   /* Close the files, but not standard output.  */
   for (int i = 1; i <= nfiles; i++)
-    if (0 <= descriptors[i] && ! close_wait (descriptors[i]))
+    if (0 <= descriptors[i] && close (descriptors[i]) < 0)
       {
         error (0, errno, "%s", quotef (files[i]));
         ok = false;

tests/local.mk

@@ -490,7 +490,9 @@ all_tests =					\
   tests/tac/tac-2-nonseekable.sh		\
   tests/tail/tail.pl				\
   tests/tee/append.sh				\
+  tests/tee/short-write.sh			\
   tests/tee/tee.sh				\
+  tests/tee/write-eagain.sh			\
   tests/test/test-N.sh				\
   tests/test/test-diag.pl			\
   tests/test/test-file.sh			\

tests/tee/short-write.sh

@@ -0,0 +1,33 @@
+#!/bin/sh
+# Test 'tee' when a write is short.
+
+# Copyright (C) 2026 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_ tee
+require_strace_ write
+
+printf 'abcdef' >file1-exp || framework_failure_
+printf 'f' >out-exp || framework_failure_
+
+# In coreutils-9.11, a short write would be treated as an error.
+strace -qqq -o /dev/null --trace-fds=1 -e trace=write \
+  -e inject=write:retval=1:when=1..5 tee file1 >out 2>err <file1-exp || fail=1
+compare file1-exp file1 || fail=1
+compare out-exp out || fail=1
+compare /dev/null err || fail=1
+
+Exit $fail

tests/tee/write-eagain.sh

@@ -0,0 +1,31 @@
+#!/bin/sh
+# Test 'tee' when a write fails with errno set to EAGAIN.
+
+# Copyright (C) 2026 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_ tee
+require_strace_ write
+
+# In coreutils-9.11 the following test would infinite loop.
+echo a >exp || framework_failure_
+timeout 10 strace -qqq -o /dev/null -e trace-fds=3 \
+  -e inject=write:error=EAGAIN:when=1 tee file1 <exp >out 2>err || fail=1
+compare exp file1 || fail=1
+compare exp out || fail=1
+compare /dev/null err || fail=1
+
+Exit $fail