Update ld-precache part

Signed-off-by: Daniel Llewellyn <daniel@bowlhat.net>

docs/lib/linker-cache.rst

@@ -46,5 +46,12 @@ prefixed with ``$SNAP/``:
     :start-after: ### START environ
     :end-before: ### END environ
 
+Add the layout definition for the ``ld.so.cache`` file:
+
+.. literalinclude:: ../../examples/linker-cache/snapcraft.yaml
+    :language: yaml
+    :start-after: ### START layout
+    :end-before: ### END layout
+
 This library expects you to have added
 the :ref:`Extensible Hooks <extensible_hooks>` customisations, too.

examples/linker-cache/snapcraft.yaml

@@ -27,3 +27,9 @@ parts:
 environment:
   FINAL_BINARY: $SNAP/usr/bin/my-app
 ### END environ
+
+### START layout
+layout:
+  /etc/ld.so.cache:
+    bind-file: $SNAP_DATA/etc/ld.so.cache
+### END layout

snapcraft-assets/ld-precache/bin/build-ld-cache

@@ -1,20 +1,23 @@
-#!/bin/bash -ex
-
-# copying the conf dir from real /etc into $SNAP_DATA
-mkdir -p "$SNAP_DATA/etc/ld.so.conf.d"
-cp -r /etc/ld.so.conf.d/* "$SNAP_DATA/etc/ld.so.conf.d"
-cp /etc/ld.so.conf "$SNAP_DATA/etc/ld.so.conf"
+#!/bin/bash -e
 
 # delete empty entries in the LD_LIBRARY_PATH
 # i.e. change "/a/b/c:/1/2/3::/other" into "/a/b/c:/1/2/3:/other"
 # if we don't do this, then ldconfig gets confused with "" as arguments of dirs
 # to add to the cache
 LD_LIBRARY_PATH="${LD_LIBRARY_PATH//::/:}"
+# Remove leading empty element if it exists
+LD_LIBRARY_PATH="${LD_LIBRARY_PATH#:}"
+# Remove trailing empty element if it exists
+LD_LIBRARY_PATH="${LD_LIBRARY_PATH%:}"
 
-# run ldconfig on our LD_LIBRARY_PATH lib dirs
 IFS=':' read -ra PATHS <<< "$LD_LIBRARY_PATH"
+# The fake-etc directory is to allow ldconfig to unlink the
+# ld.so.cache file without breaking the bind-file layout on
+# the real ld.so.cache file.
 mkdir -p "$SNAP_DATA/fake-etc"
 ldconfig -v -C "$SNAP_DATA/fake-etc/ld.so.cache" -f "$SNAP_DATA/etc/ld.so.conf" "${PATHS[@]}"
 
-# install the new ld.so.cache to the global location by just copying it
-cp "$SNAP_DATA/fake-etc/ld.so.cache" "$SNAP_DATA/etc/ld.so.cache"
+# install the new ld.so.cache to the global location by
+# copying it. We must not unlink the global file or the snap
+# 'layout' configuration will break.
+cp "$SNAP_DATA/fake-etc/ld.so.cache" "$SNAP_DATA/etc/ld.so.cache"

snapcraft-assets/ld-precache/bin/check-ld-cache

@@ -19,12 +19,8 @@ else
     # TODO: make this less of a hack?
     if ldd "$FINAL_BINARY" | grep "=> not found" | grep -q "=> not found"; then
         # regenerate the cache first
-        LD_LIBRARY_PATH=$ORIGINAL_LD_LIBRARY_PATH
+        LD_LIBRARY_PATH="$ORIGINAL_LD_LIBRARY_PATH"
         export LD_LIBRARY_PATH
-        # "$SNAP/bin/build-ld-cache"
-        # # unset to continue executing the next command in the chain
-        # LD_LIBRARY_PATH=""
-        # export LD_LIBRARY_PATH
     fi
 fi