tail: improve inotify handling of symlinks

Previous behavior failed to read contents of a (re)appearing file,
when symlinked by tail's watched file.  Also we now diagnose other
edge cases when running in inotify mode, where an initially
missing or regular file changes to a symlink.

* src/tail.c (main): If any arg is a symlink, use polling mode.
(recheck): Diagnose the edge case where a symlink appears during
inotify processing.
* tests/tail-2/symlink.sh: Test the fix. Mention the edge cases.
* tests/local.mk: Reference the new test.
* NEWS: Mention the fix.
Reported by: Ondrej Oprala

2 files changed, 80 insertions, 1 deletions

diff --git a/tests/local.mk b/tests/local.mk
index 59bf07f01..2dd7e2063 100644
--- a/tests/local.mk
+++ b/tests/local.mk
@@ -388,7 +388,8 @@ all_tests =					\
   tests/misc/uniq-perf.sh			\
   tests/misc/xattr.sh				\
   tests/tail-2/wait.sh				\
-  tests/tail-2/retry.sh			\
+  tests/tail-2/retry.sh				\
+  tests/tail-2/symlink.sh				\
   tests/chmod/c-option.sh			\
   tests/chmod/equal-x.sh			\
   tests/chmod/equals.sh				\
diff --git a/tests/tail-2/symlink.sh b/tests/tail-2/symlink.sh
new file mode 100755
index 000000000..bca0797fa
--- /dev/null
+++ b/tests/tail-2/symlink.sh
@@ -0,0 +1,78 @@
+#!/bin/sh
+# Ensure tail tracks symlinks properly.
+
+# Copyright (C) 2013 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 <http://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ tail
+
+# Function to check the expected line count in 'out'.
+# Called via retry_delay_().  Sleep some time - see retry_delay_() - if the
+# line count is still smaller than expected.
+wait4lines_ ()
+{
+  local delay=$1
+  local elc=$2   # Expected line count.
+  [ "$( wc -l < out )" -ge "$elc" ] || { sleep $delay; return 1; }
+}
+
+# Ensure changing targets of cli specified symlinks are handled.
+# Prior to v8.22, inotify would fail to recognize changes in the targets.
+# Clear 'out' so that we can check its contents without races.
+:>out                           || framework_failure_
+ln -nsf target symlink          || framework_failure_
+timeout 10 tail -s.1 -F symlink >out 2>&1 & pid=$!
+retry_delay_ wait4lines_ .1 6 1 || fail=1  # Wait for "cannot open..."
+echo "X" > target               || fail=1
+retry_delay_ wait4lines_ .1 6 3 || fail=1  # Wait for the expected output.
+kill $pid
+wait $pid
+# Expect 3 lines in the output file.
+[ $( wc -l < out ) = 3 ]   || { fail=1; cat out; }
+grep -F 'cannot open' out  || { fail=1; cat out; }
+grep -F 'has appeared' out || { fail=1; cat out; }
+grep '^X$' out             || { fail=1; cat out; }
+rm -f target out           || framework_failure_
+
+# Ensure we correctly handle the source symlink itself changing.
+# I.E. that we don't operate solely on the targets.
+# Clear 'out' so that we can check its contents without races.
+:>out                           || framework_failure_
+echo "X1" > target1             || framework_failure_
+ln -nsf target1 symlink         || framework_failure_
+timeout 10 tail -s.1 -F symlink >out 2>&1 & pid=$!
+retry_delay_ wait4lines_ .1 6 1 || fail=1  # Wait for the expected output.
+ln -nsf target2 symlink         || framework_failure_
+retry_delay_ wait4lines_ .1 6 2 || fail=1  # Wait for "become inaccess..."
+echo "X2" > target2             || fail=1
+retry_delay_ wait4lines_ .1 6 4 || fail=1  # Wait for the expected output.
+kill $pid
+wait $pid
+# Expect 4 lines in the output file.
+[ $( wc -l < out ) = 4 ]    || { fail=1; cat out; }
+grep -F 'become inacce' out || { fail=1; cat out; }
+grep -F 'has appeared' out  || { fail=1; cat out; }
+grep '^X1$' out             || { fail=1; cat out; }
+grep '^X2$' out             || { fail=1; cat out; }
+rm -f target1 target2 out   || framework_failure_
+
+# Note other symlink edge cases are currently just diagnosed
+# rather than being handled. I.E. if you specify a missing item,
+# or existing file that later change to a symlink, if inotify
+# is in use, you'll get a diagnostic saying that link will
+# no longer be tailed.
+
+Exit $fail