B #3244: Prevent race condition in ln.ssh (#3935)

Don't evaluate presence of the SRC_SNAP_DIR on hypervisor,
but rather on the frontend to prevent race condition.

src/tm_mad/qcow2/ln.ssh

@@ -49,7 +49,6 @@ DST_SNAP_DIR=${DST_PATH}.snap
 DST_FILE=`basename $DST_PATH`
 DST_DIR=`dirname $DST_PATH`
 
-SRC_HOST=`arg_host $SRC`
 SRC_PATH=`arg_path $SRC`
 SRC_SNAP_DIR=${SRC_PATH}.snap
 
@@ -64,21 +63,26 @@ ssh_make_path $DST_HOST $DST_DIR 'ssh'
 #-------------------------------------------------------------------------------
 log "Copying $SRC_PATH in $DST"
 
-CMD=$(cat <<EOT
-set -ex -o pipefail
-mkdir -p "${DST_SNAP_DIR}"
-cd "${DST_SNAP_DIR}"
-
-if [ -d ${SRC_SNAP_DIR} ]; then
+if [ -d "${SRC_SNAP_DIR}" ]; then
+    CP_CMD=$(cat <<EOT
     for F in \$(ls ${SRC_SNAP_DIR}); do
         if [ -f ${SRC_SNAP_DIR}/\$F ]; then
-            cp ${SRC_SNAP_DIR}/\$F .
+            cp ${SRC_SNAP_DIR}/\$F ${DST_SNAP_DIR}
         fi
     done
+EOT
+)
 else
-    cp ${SRC_PATH} 0
+    CP_CMD="cp ${SRC_PATH} ${DST_SNAP_DIR}/0"
 fi
 
+CMD=$(cat <<EOT
+set -ex -o pipefail
+mkdir -p "${DST_SNAP_DIR}"
+cd "${DST_SNAP_DIR}"
+
+${CP_CMD}
+
 rm -f "${DST_FILE}.snap"
 ln -sf "." "${DST_FILE}.snap"