[Pkg-xen-devel] [PATCH] debian/scripts: Optimize scripts

[Elliott Mitchell]

Well.  I continued playing with `xen-utils-wrapper` to ensure things
work like they were supposed to.  Apparently the `exec` command does
*NOT* behave in the obvious fashion.  On failure, the shell exits instead
of acting like the command failed.  As such an invocation along the lines
of below is needed.  I'm pretty sure this does actually work to
distinctly reduce the overhead of the wrapper while retaining
functionality.


From: Elliott Mitchell <ehem+debian at m5p.com>
Date: Sun, 20 Sep 2020 21:28:53 -0700
Subject: [PATCH] debian/scripts: Optimize scripts

Fewer fork()s and execve()s quickly add up to significant savings.  I'm
concerned Debian is slowly headed towards recreating SunOS^WSolaris
5.7^W2.7^W7 and the layers and layers of scripts which killed
performance.

As these runtime scripts are heavily used, avoid all uses of external
programs by them.

Signed-off-by: Elliott Mitchell <ehem+debian at m5p.com>
---
 debian/scripts/xen-utils-wrapper | 42 +++++++++++++++++++++++++++++---
 debian/scripts/xen-version       |  6 +++--
 2 files changed, 42 insertions(+), 6 deletions(-)

diff --git a/debian/scripts/xen-utils-wrapper b/debian/scripts/xen-utils-wrapper
index 4d27a62d33..4750156d71 100755
--- a/debian/scripts/xen-utils-wrapper
+++ b/debian/scripts/xen-utils-wrapper
@@ -1,6 +1,40 @@
-#!/bin/sh -e
+#!/bin/sh
 
-COMMAND="$(basename $0)"
-DIR=$(/usr/lib/xen-common/bin/xen-dir)
+# This portion is fast-path, avoid external programs, NOTE: NOT -e HERE!
+
+COMMAND="${0##*/}"
+
+DIR=/sys/hypervisor/version
+read major 2> /dev/null < $DIR/major
+read minor 2> /dev/null < $DIR/minor
+VERSION="$major.$minor"
+
+DIR="/usr/lib/xen-$VERSION/bin"
+
+[ -x "$DIR/$COMMAND" ] && exec "$DIR/$COMMAND" "$@"
+
+
+# Certainly didn't work, now report failures, slower-path
+set -e
+
+error() {
+	err="$1"
+	shift
+	echo "ERROR: " "$@" >&2
+	exit $err
+}
+
+[ -e "/sys/hypervisor/type" ] || error 1 "Can't find hypervisor information in sysfs!"
+
+read type < /sys/hypervisor/type
+if [ "$type" != xen ]; then
+	[ -n "$type" ] || error 1 "Can't read hypervisor type from sysfs!"
+	error 1 "Hypervisor is not xen but '$type'!"
+fi
+
+if [ ! -d "/usr/lib/xen-$VERSION" ]; then
+	error 127 "Can't find version $VERSION of xen utils (maybe xen-utils-$VERSION was already removed before rebooting out of Xen $VERSION), bailing out!"
+fi
+
+error 127 "Unable to execute $COMMAND"
 
-exec "$DIR/bin/$COMMAND" "$@"
diff --git a/debian/scripts/xen-version b/debian/scripts/xen-version
index 492070a43b..5c42ad7351 100755
--- a/debian/scripts/xen-version
+++ b/debian/scripts/xen-version
@@ -6,10 +6,12 @@ error() {
 }
 
 if [ -e "/sys/hypervisor/type" ]; then
-    type="$(cat /sys/hypervisor/type)"
+    read type < /sys/hypervisor/type
     if [ "$type" = xen ]; then
         DIR=/sys/hypervisor/version
-        VERSION="$(cat $DIR/major).$(cat $DIR/minor)"
+        read major < $DIR/major
+        read minor < $DIR/minor
+        VERSION="$major.$minor"
     elif [ -z "$type" ]; then
         error "Can't read hypervisor type from sysfs!"
     else
-- 
2.20.1




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