support/scripts: add helper to hardlink-or-copy

When preparing the legal-info, the source archives are copied in the
legal-info/ output directory. When the archives are big, it can take
quite a bit of time and unnecessarily uses disk space. When the
legal-info output directory is on the same filesystem as the BR2_DL_DIR,
we can easily reduce copy time and disk usage by just using hardlins
instead of copying. However, the BR2_DL_DIR may be on a different
filesystem, so we must fallback to copying in this case

Introduce a helper script that copies a source file into a destination
directory, by first attempting to hard-link, and falling back to a
plain copy in case the hardlink fails.

In case the destination already exists, it is forcibly removed first, to
avoid clobering any existing target file (and especially any hardlink to
it), since cp -f does not remove the destination file, but clobbers it.

In some situations, it will be necessary that the destination file is
named differently than the source, so if a third argument is specified,
it is treated as the basename of the destination file.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

support/scripts/hardlink-or-copy

@@ -0,0 +1,35 @@
+#!/bin/bash
+
+# Try to hardlink a file into a directory, fallback to copy on failure.
+#
+# Hardlink-or-copy the source file in the first argument into the
+# destination directory in the second argument, using the basename in
+# the third argument as basename for the destination file. If the third
+# argument is missing, use the basename of the source file as basename
+# for the destination file.
+#
+# In either case, remove the destination prior to doing the
+# hardlink-or-copy.
+#
+# Note that this is NOT an atomic operation.
+
+set -e
+
+main() {
+    local src_file="${1}"
+    local dst_dir="${2}"
+    local dst_file="${3}"
+
+    if [ -n "${dst_file}" ]; then
+        dst_file="${dst_dir}/${dst_file}"
+    else
+        dst_file="${dst_dir}/${src_file##*/}"
+    fi
+
+    mkdir -p "${dst_dir}"
+    rm -f "${dst_file}"
+    ln -f "${src_file}" "${dst_file}" 2>/dev/null \
+    || cp -f "${src_file}" "${dst_file}"
+}
+
+main "${@}"