Allow replacing a file with a compatibility symlink

This means we can do stuff like moving /etc/os-release into /usr/lib
and leaving a compatibility symlink where the file was.

Previously baserock-system-config-sync would fail with the following:

    ERROR: found two different types for 'os-release': regular and
    symlink

One of the 'failure' test cases now passes, and I've added a specific
test for our 'os-release' case to the 'symblinks' test case too.

1 files changed, 11 insertions, 2 deletions

diff --git a/baserock-system-config-sync/baserock-system-config-sync b/baserock-system-config-sync/baserock-system-config-sync
index 7b7c697..2cd9807 100755
--- a/baserock-system-config-sync/baserock-system-config-sync
+++ b/baserock-system-config-sync/baserock-system-config-sync
@@ -54,8 +54,17 @@ check_same_type() {
     type1="$(file_type $1)"
     type2="$(file_type $2)"
     if [ "$type1" != "$type2" ]; then
-        die "ERROR: found two different types for '$3':" \
-            "$type1 and $type2"
+        if [ "$type1" == symlink ] || [ "$type2" == symlink ]; then
+            # We allow moving content and leaving a compatibility symlink, as
+            # long as the content of the file didn't change.
+            if ! cmp -s "$1" "$2"; then
+                die "ERROR: $3: replacing a file with a symlink is only " \
+                    "supported if there are no changes to the content."
+            fi
+        else
+            die "ERROR: found two different types for '$3':" \
+                "$type1 and $type2"
+        fi
     fi
 }