Re: tail -f problem

[Jim Meyering]

Pádraig Brady wrote:
> Jim Meyering wrote:
>> Ulrich Drepper wrote:
>>> Jim Meyering wrote:
>>>> Thanks for the report!
>>
>> Ulrich and Ren reported that the command,
>>
>>   echo foobar | tail -c3 -f
>>
>> would block indefinitely, while POSIX says that it must not.
>
> This seems to be handled already (see line 1926):
>
> echo foobar | POSIXLY_CORRECT="" tail -c3 -f
>
> Also there is the stdin_cmdline_arg bit below this new chunk.
> Is that now rendered redundant?
>
> It seems that these 3 chunks should be merged.

Good point.  I missed the POSIXLY_CORRECT-handling code.

Here's an adjustment that updates the documentation, too.
The stdin_cmdline_arg part is different enough (just checking
for nameless args) and uses the result of the prior loop, so
I think it can remain, unmodified.

Thanks for the review!

Once I push this, I'll update from gnulib
and make another snapshot.

Jim

>From 43b14745e98846d18c998022b9d3b1f47215dd80 Mon Sep 17 00:00:00 2001
From: Jim Meyering <address@hidden>
Date: Tue, 8 Sep 2009 08:26:22 +0200
Subject: [PATCH] tail: make the new piped-stdin test as portable as the old one
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

* src/tail.c (main): Adapt piped-stdin test to use the same isapipe,
test as was used in the preceding POSIXLY_CORRECT condition.
Remove the now-subsumed POSIXLY_CORRECT test.
Reported by Pádraig Brady.
* doc/coreutils.texi (tail invocation): Document this change.
* NEWS (Changes in behavior): Reclassify, clarify.
---
 NEWS               |   10 ++++++----
 doc/coreutils.texi |    5 +++--
 src/tail.c         |   31 +++++++------------------------
 3 files changed, 16 insertions(+), 30 deletions(-)

diff --git a/NEWS b/NEWS
index 7d4f7e2..7805fe5 100644
--- a/NEWS
+++ b/NEWS
@@ -46,11 +46,13 @@ GNU coreutils NEWS                                    -*- 
outline -*-
   cp --reflink accepts a new "auto" parameter which falls back to
   a standard copy if creating a copy-on-write clone is not possible.

-** POSIX conformance
+** Changes in behavior

-  tail -f now ignores "-" when stdin is a pipe or FIFO, per POSIX.
-  Now, :|tail -f terminates immediately.  Before, it would block indefinitely.
-  [the old behavior dates back to the original implementation]
+  tail -f now ignores "-" when stdin is a pipe or FIFO.
+  tail-with-no-args now ignores -f unconditionally when stdin is a pipe or 
FIFO.
+  Before, it would ignore -f only when no file argument was specified,
+  and then only when POSIXLY_CORRECT was set.  Now, :|tail -f - terminates
+  immediately.  Before, it would block indefinitely.


 * Noteworthy changes in release 7.5 (2009-08-20) [stable]
diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 86394a1..16ff613 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -2782,9 +2782,10 @@ tail invocation
 The option values @samp{descriptor} and @samp{name} may be specified only
 with the long form of the option, not with @option{-f}.

address@hidden POSIXLY_CORRECT
-If @env{POSIXLY_CORRECT} is set, the @option{-f} option is ignored if
+The @option{-f} option is ignored if
 no @var{file} operand is specified and standard input is a FIFO or a pipe.
+Likewise, the @option{-f} option has no effect for any
+operand specified as @samp{-}, when standard input is a FIFO or a pipe.

 @item -F
 @opindex -F
diff --git a/src/tail.c b/src/tail.c
index 63874bb..81cbae0 100644
--- a/src/tail.c
+++ b/src/tail.c
@@ -1922,28 +1922,6 @@ main (int argc, char **argv)
       static char *dummy_stdin = (char *) "-";
       n_files = 1;
       file = &dummy_stdin;
-
-      /* POSIX says that -f is ignored if no file operand is specified
-         and standard input is a pipe.  However, the GNU coding
-         standards say that program behavior should not depend on
-         device type, because device independence is an important
-         principle of the system's design.
-
-         Follow the POSIX requirement only if POSIXLY_CORRECT is set.  */
-
-      if (forever && getenv ("POSIXLY_CORRECT"))
-        {
-          struct stat st;
-          int is_a_fifo_or_pipe =
-            (fstat (STDIN_FILENO, &st) != 0 ? -1
-             : S_ISFIFO (st.st_mode) ? 1
-             : HAVE_FIFO_PIPES == 1 ? 0
-             : isapipe (STDIN_FILENO));
-          if (is_a_fifo_or_pipe < 0)
-            error (EXIT_FAILURE, errno, _("standard input"));
-          if (is_a_fifo_or_pipe)
-            forever = false;
-        }
     }

   {
@@ -1986,8 +1964,13 @@ main (int argc, char **argv)
   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))
+      bool is_a_fifo_or_pipe =
+        (STREQ (F[i].name, "-")
+         && !F[i].ignore
+         && 0 <= F[i].fd
+         && (S_ISFIFO (F[i].mode)
+             || (HAVE_FIFO_PIPES != 1 && isapipe (F[i].fd))));
+      if (is_a_fifo_or_pipe)
         F[i].ignore = true;
       else
         ++n_viable;
--
1.6.4.2.419.gab238