Add pre-exists state to __file (#641)

This allows checking whether a file exists, but erroring out instead of
creating the file if it does not (or is not a regular file).

cdist/conf/type/__file/gencode-local

@@ -25,6 +25,20 @@ type="$(cat "$__object/explorer/type")"
 
 [ "$state_should" = "exists" -a "$type" = "file" ] && exit 0 # nothing to do
 
+if [ "$state_should" = "pre-exists" ]; then
+   if [ -f "$__object/parameter/source" ]; then
+      echo "--source cannot be used with --state pre-exists"
+      exit 1
+   fi
+
+   if [ "$type" = "file" ]; then
+      exit 0 # nothing to do
+   else
+      echo "File \"$destination\" does not exist"
+      exit 1
+   fi
+fi
+
 upload_file=
 create_file=
 if [ "$state_should" = "present" -o "$state_should" = "exists" ]; then

cdist/conf/type/__file/gencode-remote

@@ -59,7 +59,7 @@ set_mode() {
 
 set_attributes=
 case "$state_should" in
-    present|exists)
+    present|exists|pre-exists)
     # Note: Mode - needs to happen last as a chown/chgrp can alter mode by
     #  clearing S_ISUID and S_ISGID bits (see chown(2))
     for attribute in group owner mode; do

cdist/conf/type/__file/man.rst

@@ -23,6 +23,10 @@ symlink
 directory
   replace it with the source file
 
+One exception is that when state is pre-exists, an error is raised if
+the file would have been created otherwise (e.g. it is not present or
+not a regular file).
+
 In any case, make sure that the file attributes are as specified.
 
 
@@ -41,6 +45,9 @@ state
       the file does not exist
    exists
       the file from source but only if it doesn't already exist
+   pre-exists
+      check that the file exists and is a regular file, but do not
+      create or modify it
 
 group
    Group to chgrp to.
@@ -93,6 +100,8 @@ EXAMPLES
     __file /home/frodo/.bashrc --source "/etc/skel/.bashrc" \
        --state exists \
        --owner frodo --mode 0600
+    # Check that the file is present, show an error when it is not
+    __file /etc/somefile --state pre-exists
     # Take file content from stdin
     __file /tmp/whatever --owner root --group root --mode 644 --source - << DONE
         Here goes the content for /tmp/whatever