sandbox: Add better error reporting for overlayfs

Let's do some basic checks up front so we catch
trivial errors ourselves instead of having to
debug obscure errors we get back from the kernel
when calling mount().

diff --git a/mkosi/sandbox.py b/mkosi/sandbox.py
index bb10902aef855939db5297cecebbd97afd597b44..22829b90dd017b0403b257ca624c9994970df2c9 100755 (executable)
--- a/mkosi/sandbox.py
+++ b/mkosi/sandbox.py
@@ -946,6 +946,16 @@ class OverlayOperation(FSOperation):
         workdir = chase(oldroot, self.workdir) if self.workdir else None
         dst = chase(newroot, self.dst)
 
+        for p in lowerdirs:
+            if not os.path.exists(p):
+                oserror("mount", p, ENOENT)
+
+        if upperdir and upperdir != "tmpfs" and upperdir != dst and not os.path.exists(upperdir):
+            oserror("mount", upperdir, ENOENT)
+
+        if workdir and not os.path.exists(workdir):
+            oserror("mount", workdir, ENOENT)
+
         with umask(~0o755):
             os.makedirs(os.path.dirname(dst), exist_ok=True)