tail: return inotify resources where possible

Each user has a maximum number of inotify watches,
so handle the cases where we exhaust these resources.

* src/tail.c (tail_forever_inotify): Ensure we inotify_rm_watch()
the watch for an inode, when replacing with a new watch for a name.
Return all used inotify resources when reverting to polling.
Revert to polling upon first indication of inotify resource exhaustion.
Revert to polling on any inotify resource exhaustion.
Diagnose resource exhaustion correctly in all cases.
Avoid redundant reinsertion in the hash for unchanged watches
(where only attributes of the file are changed).
* tests/tail-2/retry.sh: Avoid false failure when reverting to polling.
* tests/tail-2/inotify-rotate.sh: Likewise.
* tests/tail-2/symlink.sh: Likewise.
* tests/tail-2/inotify-rotate-resources.sh: New test to check
that we're calling inotify_rm_watch() for replaced files.
* tests/local.mk: Reference the new test.
* NEWS: Mention the bug fix.
* THANKS.in: Thanks for reporting and problem identification.

1 files changed, 10 insertions, 3 deletions

diff --git a/tests/tail-2/symlink.sh b/tests/tail-2/symlink.sh
index 300cb139b..274df9587 100755
--- a/tests/tail-2/symlink.sh
+++ b/tests/tail-2/symlink.sh
@@ -19,6 +19,13 @@
 . "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
 print_ver_ tail
 
+# Function to count number of lines from tail
+# while ignoring transient errors due to resource limits
+countlines_ ()
+{
+  grep -Ev 'inotify (resources exhausted|cannot be used)' out | wc -l
+}
+
 # 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.
@@ -26,7 +33,7 @@ wait4lines_ ()
 {
   local delay=$1
   local elc=$2   # Expected line count.
-  [ "$( wc -l < out )" -ge "$elc" ] || { sleep $delay; return 1; }
+  [ "$(countlines_)" -ge "$elc" ] || { sleep $delay; return 1; }
 }
 
 # Ensure changing targets of cli specified symlinks are handled.
@@ -41,7 +48,7 @@ retry_delay_ wait4lines_ .1 6 3 || fail=1  # Wait for the expected output.
 kill $pid
 wait $pid
 # Expect 3 lines in the output file.
-[ $( wc -l < out ) = 3 ]   || { fail=1; cat out; }
+[ "$(countlines_)" = 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; }
@@ -62,7 +69,7 @@ retry_delay_ wait4lines_ .1 6 4 || fail=1  # Wait for the expected output.
 kill $pid
 wait $pid
 # Expect 4 lines in the output file.
-[ $( wc -l < out ) = 4 ]    || { fail=1; cat out; }
+[ "$(countlines_)" = 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; }