objectstore: Object.read() for read only access

Provide a way to read the current contents of the object, in a way
the follows the copy-on-write semantics: If `base` is set but the
object has not yet been written to, the `base` content will be
exposed. If no base is set or the object has been written to, the
current (temporary) tree will be exposed. In either way it is done
via a bind mount so it is assured that the contents indeed can only
be read from, but not written to.
The code also currently make sure that there is no write operation
started as long as there is at least one reader.
Additionally, also introduce checks that the object is intact, i.e.
not cleaned up, for all operations that require such a state.

osbuild/objectstore.py

@@ -54,6 +54,7 @@ def umount(target, lazy=True):
 class Object:
     def __init__(self, store: "ObjectStore"):
         self._init = True
+        self._readers = 0
         self._base = None
         self._workdir = None
         self._tree = None
@@ -62,6 +63,8 @@ class Object:
 
     def init(self) -> None:
         """Initialize the object with content of its base"""
+        self._check_writable()
+        self._check_readers()
         if self._init:
             return
 
@@ -99,9 +102,23 @@ class Object:
 
     def write(self) -> str:
         """Return a path that can be written to"""
+        self._check_writable()
+        self._check_readers()
         self.init()
         return self._tree
 
+    @contextlib.contextmanager
+    def read(self) -> str:
+        self._check_writable()
+        with self.tempdir("mount") as target:
+            mount(self._path, target)
+            try:
+                self._readers += 1
+                yield target
+            finally:
+                umount(target)
+                self._readers -= 1
+
     def store_tree(self, destination: str):
         """Store the tree at destination and reset itself
 
@@ -109,6 +126,8 @@ class Object:
         target already exist, does nothing. Afterwards it
         resets itself and can be used as if it was new.
         """
+        self._check_writable()
+        self._check_readers()
         self.init()
         with suppress_oserror(errno.ENOTEMPTY, errno.EEXIST):
             os.rename(self._tree, destination)
@@ -122,20 +141,40 @@ class Object:
         self._init = not self._base
 
     def cleanup(self):
+        self._check_readers()
         if self._workdir:
             self._workdir.cleanup()
             self._workdir = None
 
+    def _check_readers(self):
+        """Internal: Raise a ValueError if there are readers"""
+        if self._readers:
+            raise ValueError("Read operation is ongoing")
+
+    def _check_writable(self):
+        """Internal: Raise a ValueError if not writable"""
+        if not self._workdir:
+            raise ValueError("Object is not writable")
+
     @contextlib.contextmanager
     def _open(self):
         """Open the directory and return the file descriptor"""
-        fd = os.open(self._path, os.O_DIRECTORY)
-        try:
-            yield fd
-        finally:
-            os.close(fd)
+        with self.read() as path:
+            fd = os.open(path, os.O_DIRECTORY)
+            try:
+                yield fd
+            finally:
+                os.close(fd)
+
+    def tempdir(self, suffix=None):
+        workdir = self._workdir.name
+        if suffix:
+            suffix = "-" + suffix
+        return tempfile.TemporaryDirectory(dir=workdir,
+                                           suffix=suffix)
 
     def __enter__(self):
+        self._check_writable()
         return self
 
     def __exit__(self, exc_type, exc_val, exc_tb):