element: Make "--sysroot" take a bare directory

i.e. instead of taking a directory that must contain "root" and
"scratch", and treating "root" as the root, use the directory directly.

In element.py:
* __sandbox takes the `bare_sandbox` arg, to pass into the sandbox's constructor

In sandbox.py:
* If bare_sandbox, `_root` is the passed-in directory, and `__scratch`
  is None.
* Trying to use `__scratch` when bare_sandbox is True is a bug.

In _mount.py:
* Don't get the value of `__scratch` if it's not needed.

This is part of #539

3 files changed, 22 insertions, 8 deletions

diff --git a/buildstream/element.py b/buildstream/element.py
index 83d7f9705..27c5014a3 100644
--- a/buildstream/element.py
+++ b/buildstream/element.py
@@ -1318,7 +1318,9 @@ class Element(Plugin):
     @contextmanager
     def _prepare_sandbox(self, scope, directory, deps='run', integrate=True):
         # bst shell and bst checkout require a local sandbox.
-        with self.__sandbox(directory, config=self.__sandbox_config, allow_remote=False) as sandbox:
+        bare_directory = True if directory else False
+        with self.__sandbox(directory, config=self.__sandbox_config, allow_remote=False,
+                            bare_directory=bare_directory) as sandbox:
 
             # Configure always comes first, and we need it.
             self.configure_sandbox(sandbox)
@@ -2156,12 +2158,14 @@ class Element(Plugin):
     #    stderr (fileobject): The stream for stderr for the sandbox
     #    config (SandboxConfig): The SandboxConfig object
     #    allow_remote (bool): Whether the sandbox is allowed to be remote
+    #    bare_directory (bool): Whether the directory is bare i.e. doesn't have
+    #                           a separate 'root' subdir
     #
     # Yields:
     #    (Sandbox): A usable sandbox
     #
     @contextmanager
-    def __sandbox(self, directory, stdout=None, stderr=None, config=None, allow_remote=True):
+    def __sandbox(self, directory, stdout=None, stderr=None, config=None, allow_remote=True, bare_directory=False):
         context = self._get_context()
         project = self._get_project()
         platform = Platform.get_platform()
@@ -2192,6 +2196,7 @@ class Element(Plugin):
                                               stdout=stdout,
                                               stderr=stderr,
                                               config=config,
+                                              bare_directory=bare_directory,
                                               allow_real_directory=not self.BST_VIRTUAL_DIRECTORY)
             yield sandbox
 
@@ -2201,7 +2206,7 @@ class Element(Plugin):
 
             # Recursive contextmanager...
             with self.__sandbox(rootdir, stdout=stdout, stderr=stderr, config=config,
-                                allow_remote=allow_remote) as sandbox:
+                                allow_remote=allow_remote, bare_directory=False) as sandbox:
                 yield sandbox
 
             # Cleanup the build dir
diff --git a/buildstream/sandbox/_mount.py b/buildstream/sandbox/_mount.py
index aff9e8a2d..c0f26c8d7 100644
--- a/buildstream/sandbox/_mount.py
+++ b/buildstream/sandbox/_mount.py
@@ -31,7 +31,6 @@ from .._fuse import SafeHardlinks
 #
 class Mount():
     def __init__(self, sandbox, mount_point, safe_hardlinks, fuse_mount_options=None):
-        scratch_directory = sandbox._get_scratch_directory()
         # Getting _get_underlying_directory() here is acceptable as
         # we're part of the sandbox code. This will fail if our
         # directory is CAS-based.
@@ -51,6 +50,7 @@ class Mount():
         #        a regular mount point within the parent's redirected mount.
         #
         if self.safe_hardlinks:
+            scratch_directory = sandbox._get_scratch_directory()
             # Redirected mount
             self.mount_origin = os.path.join(root_directory, mount_point.lstrip(os.sep))
             self.mount_base = os.path.join(scratch_directory, utils.url_directory_name(mount_point))
diff --git a/buildstream/sandbox/sandbox.py b/buildstream/sandbox/sandbox.py
index 6aea4b54b..eabe57d75 100644
--- a/buildstream/sandbox/sandbox.py
+++ b/buildstream/sandbox/sandbox.py
@@ -98,15 +98,23 @@ class Sandbox():
         self.__config = kwargs['config']
         self.__stdout = kwargs['stdout']
         self.__stderr = kwargs['stderr']
+        self.__bare_directory = kwargs['bare_directory']
 
         # Setup the directories. Root and output_directory should be
         # available to subclasses, hence being single-underscore. The
         # others are private to this class.
-        self._root = os.path.join(directory, 'root')
+        # If the directory is bare, it probably doesn't need scratch
+        if self.__bare_directory:
+            self._root = directory
+            self.__scratch = None
+            os.makedirs(self._root, exist_ok=True)
+        else:
+            self._root = os.path.join(directory, 'root')
+            self.__scratch = os.path.join(directory, 'scratch')
+            for directory_ in [self._root, self.__scratch]:
+                os.makedirs(directory_, exist_ok=True)
+
         self._output_directory = None
-        self.__scratch = os.path.join(directory, 'scratch')
-        for directory_ in [self._root, self.__scratch]:
-            os.makedirs(directory_, exist_ok=True)
         self._vdir = None
 
         # This is set if anyone requests access to the underlying
@@ -333,6 +341,7 @@ class Sandbox():
     # Returns:
     #    (str): The sandbox scratch directory
     def _get_scratch_directory(self):
+        assert not self.__bare_directory, "Scratch is not going to work with bare directories"
         return self.__scratch
 
     # _get_output()