feat(livenet): add memory size check depending on live image size

For writeable live images, the memory must hold the complete image.
That means it must be at least the size of the image plus some RAM
necessary to run processes.
With too small RAM, the OOM Killer steps in and makes the boot hang.
This patch lets the system go into the emergency shell instead.

The default minimum RAM after subtracting the live image size is set to 1G.
The parameter rd.minmem.liverw is added to modify this.

modules.d/90livenet/livenetroot.sh

@@ -16,6 +16,26 @@ netroot="$2"
 liveurl="${netroot#livenet:}"
 info "fetching $liveurl"
 
+if getargbool 0 'rd.writable.fsimg'; then
+
+    minmem=$(getarg rd.minmem.liverw)
+    minmem=${minmem:-1024}
+
+    memsize=$(($(sed -n 's/MemTotal: *\([[:digit:]]*\).*/\1/p' /proc/meminfo) / 1024))
+    imgsize=$(($(curl -sI "$liveurl" | sed -n 's/Content-Length: *\([[:digit:]]*\).*/\1/p') / (1024 * 1024)))
+
+    if [ $((memsize - imgsize)) -lt $minmem ]; then
+        sed -i "N;/and attach it to a bug report./s/echo$/echo\n\
+        echo \n\
+        echo 'Warning!!!'\n\
+        echo 'The memory size of your system is too small for this live image.'\n\
+        echo 'Expect killed processes due to out of memory conditions.'\n\
+        echo \n/" /usr/bin/dracut-emergency
+
+        emergency_shell
+    fi
+fi
+
 imgfile=
 #retry until the imgfile is populated with data or the max retries
 i=1