osbuild.py: separate tree from build root

There's no reason to conflate the two. This allows us to build on
something other than a tmpfs.

osbuild

@@ -17,7 +17,7 @@ def run_interactive(pipeline_path, input_dir, output_dir):
     with open(pipeline_path) as f:
         pipeline = json.load(f)
 
-    with osbuild.BuildRoot("/run/osbuild") as buildroot:
+    with osbuild.BuildRoot() as buildroot, osbuild.tmpfs() as tree:
         for i, stage in enumerate(pipeline["stages"], start=1):
             name = stage["name"]
             options = stage.get("options", {})
@@ -26,7 +26,7 @@ def run_interactive(pipeline_path, input_dir, output_dir):
             print("Inspect with:")
             print(f"\t# nsenter -a --wd=/root -t `machinectl show {buildroot.machine_name} -p Leader --value`")
             print()
-            buildroot.run_stage(name, options, input_dir)
+            buildroot.run_stage(name, tree, options, input_dir)
 
         assembler = pipeline.get("assembler")
         if assembler:
@@ -37,7 +37,7 @@ def run_interactive(pipeline_path, input_dir, output_dir):
             print("Inspect with:")
             print(f"\t# nsenter -a --wd=/root -t `machinectl show {buildroot.machine_name} -p Leader --value`")
             print()
-            buildroot.run_assembler(name, options, output_dir)
+            buildroot.run_assembler(name, tree, options, input_dir, output_dir)
 
 
 if __name__ == "__main__":

osbuild.py

@@ -8,7 +8,8 @@ import tempfile
 
 __all__ = [
     "StageFailed",
-    "BuildRoot"
+    "BuildRoot",
+    "tmpfs"
 ]
 
 
@@ -23,17 +24,41 @@ class StageFailed(Exception):
         self.returncode = returncode
 
 
-class BuildRoot:
-    def __init__(self, path=None):
-        self.buildroot = tempfile.mkdtemp(prefix="osbuild-buildroot-", dir=path)
-        self.buildroot_mounted = False
-        self.tree = tempfile.mkdtemp(prefix="osbuild-tree-", dir=path)
-        self.tree_mounted = False
+class tmpfs:
+    def __init__(self, path="/run/osbuild"):
+        self.root = tempfile.mkdtemp(prefix="osbuild-tmpfs-", dir=path)
+        self.mounted = False
         try:
-            subprocess.run(["mount", "-o", "bind,ro", "/", self.buildroot], check=True)
-            self.tree_mounted = True
-            subprocess.run(["mount", "-t", "tmpfs", "tmpfs", self.tree], check=True)
-            self.buildroot_mounted = True
+            subprocess.run(["mount", "-t", "tmpfs", "tmpfs", self.root], check=True)
+            self.mounted = True
+        except subprocess.CalledProcessError:
+            self.unmount()
+
+    def unmount(self):
+        if not self.root:
+            return
+        if self.mounted:
+            subprocess.run(["umount", "--lazy", self.root], check=True)
+        os.rmdir(self.root)
+        self.root = None
+
+    def __del__(self):
+        self.unmount()
+
+    def __enter__(self):
+        return self.root
+
+    def __exit__(self, exc_type, exc_value, exc_tb):
+        self.unmount()
+
+
+class BuildRoot:
+    def __init__(self, path="/run/osbuild"):
+        self.root = tempfile.mkdtemp(prefix="osbuild-buildroot-", dir=path)
+        self.mounted = False
+        try:
+            subprocess.run(["mount", "-o", "bind,ro", "/", self.root], check=True)
+            self.mounted = True
         except subprocess.CalledProcessError:
             self.unmount()
             raise
@@ -43,19 +68,15 @@ class BuildRoot:
         # us is '_', because all other characters used by
         # TemporaryDirectory() are allowed. Replace it with 'L's
         # (TemporaryDirectory() only uses lower-case characters)
-        self.machine_name = os.path.basename(self.buildroot).replace("_", "L")
+        self.machine_name = os.path.basename(self.root).replace("_", "L")
 
     def unmount(self):
-        if self.tree:
-            if self.tree_mounted:
-                subprocess.run(["umount", "--lazy", self.tree], check=True)
-            os.rmdir(self.tree)
-            self.tree = None
-        if self.buildroot:
-            if self.buildroot_mounted:
-                subprocess.run(["umount", "--lazy", self.buildroot], check=True)
-            os.rmdir(self.buildroot)
-            self.buildroot = None
+        if not self.root:
+            return
+        if self.mounted:
+            subprocess.run(["umount", "--lazy", self.root], check=True)
+        os.rmdir(self.root)
+        self.root = None
 
     def run(self, argv, binds=[], readonly_binds=[], *args, **kwargs):
         return subprocess.run([
@@ -65,7 +86,7 @@ class BuildRoot:
             "--link-journal=no",
             "--volatile=yes",
             f"--machine={self.machine_name}",
-            f"--directory={self.buildroot}",
+            f"--directory={self.root}",
             f"--bind-ro=/etc/pki",
             f"--bind={libdir}/osbuild-run:/run/osbuild/osbuild-run",
             *[f"--bind={b}" for b in binds],
@@ -73,7 +94,7 @@ class BuildRoot:
             "/run/osbuild/osbuild-run",
         ] + argv, *args, **kwargs)
 
-    def run_stage(self, name, options={}, input_dir=None):
+    def run_stage(self, name, tree, options={}, input_dir=None):
         options = {
             **options,
             "tree": "/run/osbuild/tree",
@@ -81,7 +102,7 @@ class BuildRoot:
         }
 
         robinds = [f"{libdir}/stages/{name}:/run/osbuild/{name}"]
-        binds = [f"{self.tree}:/run/osbuild/tree"]
+        binds = [f"{tree}:/run/osbuild/tree"]
         if input_dir:
             robinds.append(f"{input_dir}:/run/osbuild/input")
             options["input_dir"] = "/run/osbuild/input"
@@ -91,7 +112,7 @@ class BuildRoot:
         except subprocess.CalledProcessError as error:
             raise StageFailed(name, error.returncode)
 
-    def run_assembler(self, name, options, input_dir=None, output_dir=None):
+    def run_assembler(self, name, tree, options={}, input_dir=None, output_dir=None):
         if output_dir and not os.path.exists(output_dir):
             os.makedirs(output_dir)
 
@@ -102,7 +123,7 @@ class BuildRoot:
             "output_dir": None
         }
         robinds = [
-            f"{self.tree}:/run/osbuild/tree",
+            f"{tree}:/run/osbuild/tree",
             f"{libdir}/assemblers/{name}:/run/osbuild/{name}"
         ]
         binds = []