Re: [PATCH] tail: flush initial output before possibly blocking

[Jim Meyering]

Jim Meyering wrote:
> Pádraig Brady wrote:
>> Jim Meyering wrote:
>>> I noticed that tail -f "didn't work" when run via ssh.
>>> I.e., it printed nothing, when I expected it to print the
>>> last 10 lines.
>>
>> well spotted.
>>
>> I was wondering about the loop in the test though.
>> Can the `kill -0` ever fail. Even if that's not redundant
>> I'm not sure the file is guaranteed to be written at this stage?
>> How about:
>>
>> diff --git a/tests/tail-2/flush-initial b/tests/tail-2/flush-initial
>> index 2deff84..378440c 100755
>> --- a/tests/tail-2/flush-initial
>> +++ b/tests/tail-2/flush-initial
>> @@ -28,10 +28,11 @@ echo line > in || fail=1
>>  stdbuf --output=1K tail -f in > out &
>>  tail_pid=$!
>>
>> -# Wait for the backgrounded `tail' to start.
>> -while :; do
>> -  env kill -0 $tail_pid && break
>> -  sleep .1
>> +# wait for a second for the file to be flushed
>> +count=0
>> +until test -s out -o $count = 10; do
>> +    count=$((count+1))
>> +    sleep .1
>>  done
>
> You're right.  Thanks!
> Maybe this instead?
>
>     for i in $(seq 10); do
>         test -s out && break
>         sleep .1
>     done
>
> Note that $((...)) isn't as portable as we'd like, and neither
> is test's "-o" operator.  The only use of $((...)) in coreutils
> is in bootstrap.  There are no uses of test's -o operator.

Here's what I'm testing now:

>From 15f26e296b4948edc6c7d33fc1caa6cb50999364 Mon Sep 17 00:00:00 2001
From: Jim Meyering <address@hidden>
Date: Sun, 6 Sep 2009 20:00:14 +0200
Subject: [PATCH 1/2] tests: tail-2/flush-initial: correct race avoidance code
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

* tests/tail-2/flush-initial: Wait for the file to be nonempty,
not for the process.  Based on a suggestion from Pádraig Brady.
---
 tests/tail-2/flush-initial |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/tests/tail-2/flush-initial b/tests/tail-2/flush-initial
index 2deff84..515b29d 100755
--- a/tests/tail-2/flush-initial
+++ b/tests/tail-2/flush-initial
@@ -28,9 +28,10 @@ echo line > in || fail=1
 stdbuf --output=1K tail -f in > out &
 tail_pid=$!

-# Wait for the backgrounded `tail' to start.
-while :; do
-  env kill -0 $tail_pid && break
+# Wait for 1 second for the file to be flushed.
+for i in $(seq 10); do
+  test -s out && break
+  echo sleep .1s
   sleep .1
 done

--
1.6.4.2.409.g85dc3


>From 1deab8819f8058a4f21cba461d5b70b50358481b Mon Sep 17 00:00:00 2001
From: Jim Meyering <address@hidden>
Date: Sun, 6 Sep 2009 18:41:59 +0200
Subject: [PATCH 2/2] build: update gnulib submodule to latest

---
 gnulib |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/gnulib b/gnulib
index c46ae39..5985805 160000
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit c46ae39631773a7fdae7d171cc4ef0bf2123efff
+Subproject commit 598580519cc391a0856e5ff7a15c1634d0ad29bc
--
1.6.4.2.409.g85dc3