Add 'cwd' option to run_sandbox() functions.

Also, set it correctly when running an App Container image.

2 files changed, 39 insertions, 10 deletions

diff --git a/sandboxlib/chroot.py b/sandboxlib/chroot.py
index 3fe1857..157b9fe 100644
--- a/sandboxlib/chroot.py
+++ b/sandboxlib/chroot.py
@@ -13,21 +13,41 @@
 # with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 
-'''Execute command in a sandbox, using 'chroot'.'''
+'''Execute command in a sandbox, using os.chroot().
 
+The code would be simpler if we just used the 'chroot' program, but it's not
+always practical to do that. First, it may not be installed. Second, we can't
+set the working directory of the program inside the chroot, unless we assume
+that the sandbox contains a shell and we do some hack like running
+`/bin/sh -c "cd foo && command"`. It's better to call the kernel directly.
 
+'''
+
+
+import os
 import subprocess
+import sys
 
 import sandboxlib
 
 
-def run_sandbox(rootfs_path, command, extra_env=None):
+def run_sandbox(rootfs_path, command, cwd=None, extra_env=None):
     if type(command) == str:
         command = [command]
 
-    env = sandboxlib.BASE_ENVIRONMENT.copy()
-    if extra_env is not None:
-        env.update(extra_env)
+    env = sandboxlib.environment_vars(extra_env)
+
+    pid = os.fork()
+    if pid == 0:
+        # Child process. It's a bit messy that we create a child process and
+        # then a second child process, but it saves duplicating stuff from the
+        # 'subprocess' module.
+
+        # FIXME: you gotta be root for this one.
+        os.chroot(rootfs_path)
 
-    # FIXME: you gotta be root for this one.
-    subprocess.call(['chroot', rootfs_path] + command)
+        result = subprocess.call(command, cwd=cwd, env=env)
+        os._exit(result)
+    else:
+        # Parent process. Wait for child to exit.
+        os.waitpid(pid, 0)
diff --git a/sandboxlib/linux_user_chroot.py b/sandboxlib/linux_user_chroot.py
index 8957191..7af7608 100644
--- a/sandboxlib/linux_user_chroot.py
+++ b/sandboxlib/linux_user_chroot.py
@@ -21,10 +21,19 @@ import subprocess
 import sandboxlib
 
 
-def run_sandbox(rootfs_path, command, extra_env=None):
+def run_sandbox(rootfs_path, command, cwd=None, extra_env=None):
     if type(command) == str:
         command = [command]
-        
+
+    linux_user_chroot = 'linux-user-chroot'
+
+    linux_user_chroot_args = []
+
+    if cwd is not None:
+        linux_user_chroot_args.extend(['--chdir', cwd])
+
     env = sandboxlib.environment_vars(extra_env)
 
-    subprocess.call(['linux-user-chroot', rootfs_path] + command, env=env)
+    argv = (
+        [linux_user_chroot] + linux_user_chroot_args + [rootfs_path] + command)
+    subprocess.call(argv, env=env)