Change how we search for 'linux-user-chroot'

First, the user should get better errors now if it is not found:

    sandboxlib.ProgramNotFound: Did not find 'linux-user-chroot' in
    PATH. Searched '/sbin:/bin:/usr/sbin:/usr/bin'

Second, we explicitly search for the program on each call to
run_sandbox() using the same search code used in
sandbox_module_for_platform(). This shouldn't change anything, but I
think it's better practice to search for the program ourselves than to
assume exec('linux-user-chroot') will do the right thing.

3 files changed, 61 insertions, 24 deletions

diff --git a/sandboxlib/__init__.py b/sandboxlib/__init__.py
index 946ba52..17b29da 100644
--- a/sandboxlib/__init__.py
+++ b/sandboxlib/__init__.py
@@ -26,9 +26,11 @@ import logging
 import os
 import platform
 import pipes
-import shutil
 import subprocess
-import sys
+
+
+class ProgramNotFound(Exception):
+    pass
 
 
 def maximum_possible_isolation():
@@ -133,21 +135,6 @@ def run_sandbox_with_redirection(command, **sandbox_config):
     raise NotImplementedError()
 
 
-def find_program(program_name):
-    # Python 3.3 and newer provide a 'find program in PATH' function. Otherwise
-    # we fall back to the `which` program.
-    if sys.version_info.major >= 3 and sys.version_info.minor >= 3:
-        program_path = shutil.which(program_name)
-    else:
-        try:
-            argv = ['which', program_name]
-            program_path = subprocess.check_output(argv).strip()
-        except subprocess.CalledProcessError as e:
-            logging.debug("Error searching for %s: %s", program_name, e)
-            program_path = None
-    return program_path
-
-
 def sandbox_module_for_platform():
     '''Returns an execution module that will work on the current platform.'''
 
@@ -157,13 +144,12 @@ def sandbox_module_for_platform():
 
     if platform.uname() == 'Linux':
         log.info("Linux detected, looking for 'linux-user-chroot'.")
-        linux_user_chroot_program = find_program('linux-user-chroot')
-        if linux_user_chroot_program is not None:
-            log.info("Found %s, choosing 'linux_user_chroot' module.",
-                     linux_user_chroot_program)
+        try:
+            program = sandboxlib.linux_user_chroot.linux_user_chroot_program()
+            log.info("Found %s, choosing 'linux_user_chroot' module.", program)
             backend = sandboxlib.linux_user_chroot
-        else:
-            log.debug("Did not find 'linux-user-chroot' program in PATH.")
+        except sandboxlib.utils.ProgramNotFound as e:
+            log.debug("Did not find 'linux-user-chroot': %s", e)
 
     if backend is None:
         log.info("Choosing 'chroot' sandbox module.")
@@ -260,3 +246,4 @@ import sandboxlib.chroot
 import sandboxlib.linux_user_chroot
 
 import sandboxlib.load
+import sandboxlib.utils
diff --git a/sandboxlib/linux_user_chroot.py b/sandboxlib/linux_user_chroot.py
index a5f84e0..3397a1a 100644
--- a/sandboxlib/linux_user_chroot.py
+++ b/sandboxlib/linux_user_chroot.py
@@ -281,6 +281,11 @@ def create_mount_points_if_missing(filesystem_root, mount_info_list):
             os.makedirs(path)
 
 
+def linux_user_chroot_program():
+    # Raises sandboxlib.ProgramNotFound if not found.
+    return sandboxlib.utils.find_program('linux-user-chroot')
+
+
 def run_sandbox(command, cwd=None, env=None,
                 filesystem_root='/', filesystem_writable_paths='all',
                 mounts='undefined', extra_mounts=None,
@@ -289,7 +294,7 @@ def run_sandbox(command, cwd=None, env=None,
     if type(command) == str:
         command = [command]
 
-    linux_user_chroot_command = ['linux-user-chroot']
+    linux_user_chroot_command = [linux_user_chroot_program()]
 
     extra_mounts = sandboxlib.validate_extra_mounts(extra_mounts)
 
diff --git a/sandboxlib/utils.py b/sandboxlib/utils.py
new file mode 100644
index 0000000..af5fe3e
--- /dev/null
+++ b/sandboxlib/utils.py
@@ -0,0 +1,45 @@
+# Copyright (C) 2015  Codethink Limited
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; version 2 of the License.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License along
+# with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+
+import logging
+import os
+import shutil
+import subprocess
+import sys
+
+import sandboxlib
+
+
+def find_program(program_name):
+    search_path = os.environ.get('PATH')
+
+    # Python 3.3 and newer provide a 'find program in PATH' function. Otherwise
+    # we fall back to the `which` program.
+    if sys.version_info.major >= 3 and sys.version_info.minor >= 3:
+        program_path = shutil.which(program_name, path=search_path)
+    else:
+        try:
+            argv = ['which', program_name]
+            program_path = subprocess.check_output(argv).strip()
+        except subprocess.CalledProcessError as e:
+            logging.debug("Error searching for %s: %s", program_name, e)
+            program_path = None
+
+    if program_path is None:
+        raise sandboxlib.ProgramNotFound(
+            "Did not find '%s' in PATH. Searched '%s'" % (
+                program_name, search_path))
+
+    return program_path