common/rc: introduce _random_file() helper

Currently, we use "ls ... | sort -R | head -n1" (or tail) to choose a
random file in a directory.It sorts the files with "ls", sort it randomly
and pick the first line, which wastes the "ls" sort.

Also, using "sort -R | head -n1" is inefficient. For example, in a
directory with 1000000 files, it takes more than 15 seconds to pick a file.

  $ time bash -c "ls -U | sort -R | head -n 1 >/dev/null"
  bash -c "ls -U | sort -R | head -n 1 >/dev/null"  15.38s user 0.14s system 99% cpu 15.536 total

  $ time bash -c "ls -U | shuf -n 1 >/dev/null"
  bash -c "ls -U | shuf -n 1 >/dev/null"  0.30s user 0.12s system 138% cpu 0.306 total

So, we should just use "ls -U" and "shuf -n 1" to choose a random file.
Introduce _random_file() helper to do it properly.

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/common/rc b/common/rc
index 7005f4bf62..68d2ad041e 100644
--- a/common/rc
+++ b/common/rc
@@ -5229,6 +5229,13 @@ _require_unshare() {
 		_notrun "unshare: command not found, should be in util-linux"
 }
 
+# Return a random file in a directory. A directory is *not* followed
+# recursively.
+_random_file() {
+	local basedir=$1
+	echo "$basedir/$(ls -U $basedir | shuf -n 1)"
+}
+
 init_rc
 
 ################################################################################