truncate: don't hang with --reference=FIFO

* NEWS: Mention the fix.
* src/truncate.c (main): Open the reference with O_NONBLOCK.
* tests/truncate/truncate-fifo.sh: Also cover --reference=FIFO.

NEWS

@@ -58,6 +58,9 @@ GNU coreutils NEWS                                    -*- outline -*-
   with an error.
   [This bug was present in "the beginning".]
 
+  'truncate --reference=R' no longer hangs when R is a FIFO with no readers.
+  [bug introduced in coreutils-8.17]
+
   'unexpand -t' no longer overflows a heap buffer, for tab values > SIZE_MAX/16,
   or with multi-byte blank characters longer than the tab value.
   [bugs introduced in coreutils-9.11]

src/truncate.c

@@ -326,7 +326,8 @@ main (int argc, char **argv)
         file_size = sb.st_size;
       else
         {
-          int ref_fd = open (ref_file, O_RDONLY);
+          /* O_NONBLOCK so we do not hang forever on a FIFO with no writer.  */
+          int ref_fd = open (ref_file, O_RDONLY | O_NONBLOCK);
           if (0 <= ref_fd)
             {
               off_t file_end = lseek (ref_fd, 0, SEEK_END);

tests/truncate/truncate-fifo.sh

@@ -21,7 +21,13 @@ print_ver_ truncate
 
 mkfifo_or_skip_ fifo
 
+# Output FIFO must not hang.
 timeout 10 truncate -s0 fifo
 test "$?" = 124 && fail=1
 
+# Reference FIFO must not hang either.
+touch file || framework_failure_
+timeout 10 truncate --reference=fifo file 2>/dev/null
+test "$?" = 124 && fail=1
+
 Exit $fail