Prevents files to be used as input of dagger.#FS

Errors out in the PreRun phase of the clientfilesystemreader task since
otherwise, dagger execution would hang
Fixes #1977

Signed-off-by: Marcos Lilljedahl <marcosnils@gmail.com>

tests/plan.bats

@@ -84,6 +84,15 @@ setup() {
   assert_output --partial 'path "/foobar" does not exist'
 }
 
+
+@test "plan/client/filesystem/read/fs/invalid" {
+  cd "$TESTDIR/plan/client/filesystem/read/fs/invalid"
+
+  run "$DAGGER" "do" -p . test
+  assert_failure
+  assert_output --partial 'test.txt" is not a directory'
+}
+
 @test "plan/client/filesystem/read/fs/relative" {
   cd "$TESTDIR/plan/client/filesystem/read/fs/relative"
 

tests/plan/client/filesystem/read/fs/invalid/test.cue

@@ -0,0 +1,11 @@
+package main
+
+import (
+	"dagger.io/dagger"
+)
+
+dagger.#Plan & {
+	client: filesystem: "../rootfs/test.txt": read: contents: dagger.#FS
+	actions: test: {
+	}
+}