Give less scary error messages when a containerised command fails

This affects errors encountered at build time and at system-integration time.

New errors look like this:

    ERROR: Command failed: baserock/system-integration/02-install-gerrit-gerrit-installation-binaries-misc-0000:
    Containerisation settings: {'mounts': (('dev/shm', 'tmpfs', 'none'), ('tmp', 'tmpfs', 'none')), 'mount_proc': True, 'root': '/var/tmp/staging/tmp1YQ2yN/minimal-system-x86_64-generic.inst'}
    Error output:
    + install -D /usr/share/gerrit/gerrit-2.9.war /home/gerrit2/gerrit/gerrit-2.9.war -o gerrit2 -g gerrit2 -m 644
    install: can't change ownership of /home/gerrit2/gerrit/gerrit-2.9.war: Operation not permitted

Previously the error message would have been this:

    Command failed: unshare --mount -- sh -ec.
    mount --make-rprivate /
    root="$1"
    shift
    while true; do
        case "$1" in
        --)
            shift
            break
            ;;
        *)
            mount_point="$1"
            mount_type="$2"
            mount_source="$3"
            shift 3
            path="$root/$mount_point"
            mount -t "$mount_type" "$mount_source" "$path"
            ;;
        esac
    done

    exec "$@"
     - /var/tmp/staging/tmppeA1Iw/gerrit-x86_64.inst/ dev/shm tmpfs none tmp tmpfs none -- linux-user-chroot --chdir . --mount-proc proc
    /var/tmp/staging/tmppeA1Iw/gerrit-x86_64.inst/ baserock/system-integration/02-install-gerrit-gerrit-installation-binaries-misc-0000
    + install -D /usr/share/gerrit/gerrit-2.9.war /home/gerrit2/gerrit/gerrit-2.9.war -o gerrit2 -g gerrit2 -m 644
    install: can't change ownership of /home/gerrit2/gerrit/gerrit-2.9.war: Operation not permitted

4 files changed, 61 insertions, 22 deletions

diff --git a/morphlib/app.py b/morphlib/app.py
index 930e023d..b5bcb601 100644
--- a/morphlib/app.py
+++ b/morphlib/app.py
@@ -348,7 +348,7 @@ class Morph(cliapp.Application):
             self.output.write('%s %s\n' % (timestamp, text))
             self.output.flush()
 
-    def runcmd(self, argv, *args, **kwargs):
+    def _prepare_for_runcmd(self, argv, args, kwargs):
         if 'env' not in kwargs:
             kwargs['env'] = dict(os.environ)
 
@@ -377,9 +377,14 @@ class Morph(cliapp.Application):
         morphlib.util.log_environment_changes(self, kwargs['env'], prev)
         self.prev_env = kwargs['env']
 
-        # run the command line
+    def runcmd(self, argv, *args, **kwargs):
+        self._prepare_for_runcmd(argv, args, kwargs)
         return cliapp.Application.runcmd(self, argv, *args, **kwargs)
 
+    def runcmd_unchecked(self, argv, *args, **kwargs):
+        self._prepare_for_runcmd(argv, args, kwargs)
+        return cliapp.Application.runcmd_unchecked(self, argv, *args, **kwargs)
+
     def parse_args(self, args, configs_only=False):
         return self.settings.parse_args(args,
                          configs_only=configs_only,
diff --git a/morphlib/builder2.py b/morphlib/builder2.py
index f71f21db..9e158ea1 100644
--- a/morphlib/builder2.py
+++ b/morphlib/builder2.py
@@ -665,11 +665,18 @@ class SystemBuilder(BuilderBase):  # pragma: no cover
         )
         try:
             for bin in sorted(os.listdir(sys_integration_dir)):
-                self.app.runcmd(
-                    morphlib.util.containerised_cmdline(
-                        [os.path.join(SYSTEM_INTEGRATION_PATH, bin)],
-                        root=rootdir, mounts=to_mount, mount_proc=True),
-                    env=env)
+                argv = [os.path.join(SYSTEM_INTEGRATION_PATH, bin)]
+                container_config = dict(
+                    root=rootdir, mounts=to_mount, mount_proc=True)
+                cmdline = morphlib.util.containerised_cmdline(
+                    argv, **container_config)
+                exit, out, err = self.app.runcmd_unchecked(
+                    cmdline, env=env)
+                if exit != 0:
+                    logging.debug('Command returned code %i', exit)
+                    msg = error_message_for_containerised_commandline(
+                        argv, err, container_config)
+                    raise cliapp.AppException(msg)
         except BaseException, e:
             self.app.status(
                     msg='Error while running system integration commands',
diff --git a/morphlib/stagingarea.py b/morphlib/stagingarea.py
index b676d4db..7060382a 100644
--- a/morphlib/stagingarea.py
+++ b/morphlib/stagingarea.py
@@ -274,22 +274,33 @@ class StagingArea(object):
         else:
             binds = ()
 
+        container_config=dict(
+            cwd=kwargs.pop('cwd', '/'),
+            root=chroot_dir,
+            mounts=self.to_mount,
+            binds=binds,
+            mount_proc=mount_proc,
+            writable_paths=do_not_mount_dirs)
+
         cmdline = morphlib.util.containerised_cmdline(
-                    argv, cwd=kwargs.pop('cwd', '/'),
-                    root=chroot_dir, mounts=self.to_mount,
-                    binds=binds, mount_proc=mount_proc,
-                    writable_paths=do_not_mount_dirs)
-        try:
-            if kwargs.get('logfile') != None:
-                logfile = kwargs.pop('logfile')
-                teecmd = ['tee', '-a', logfile]
-                return self._app.runcmd(cmdline, teecmd, **kwargs)
-            else:
-                return self._app.runcmd(cmdline, **kwargs)
-        except cliapp.AppException as e:
-            raise cliapp.AppException('In staging area %s: running '
-                                      'command \'%s\' failed.' % 
-                                      (self.dirname, ' '.join(argv)))
+            argv, **container_config)
+
+        if kwargs.get('logfile') != None:
+            logfile = kwargs.pop('logfile')
+            teecmd = ['tee', '-a', logfile]
+            exit, out, err = self._app.runcmd_unchecked(
+                cmdline, teecmd, **kwargs)
+        else:
+            exit, out, err = self._app.runcmd_unchecked(cmdline, **kwargs)
+
+        if exit == 0:
+            return out
+        else:
+            logging.debug('Command returned code %i', exit)
+            msg = morphlib.util.error_message_for_containerised_commandline(
+                argv, err, container_config)
+            raise cliapp.AppException(
+                'In staging area %s: %s' % (self.dirname, msg))
 
     def abort(self): # pragma: no cover
         '''Handle what to do with a staging area in the case of failure.
diff --git a/morphlib/util.py b/morphlib/util.py
index 6f735387..736af92e 100644
--- a/morphlib/util.py
+++ b/morphlib/util.py
@@ -15,11 +15,13 @@
 
 import contextlib
 import itertools
+import logging
 import os
 import re
 import subprocess
 import textwrap
 
+import cliapp
 import fs.osfs
 
 import morphlib
@@ -626,3 +628,17 @@ def containerised_cmdline(args, cwd='.', root='/', binds=(),
     cmdargs.append(root)
     cmdargs.extend(args)
     return unshared_cmdline(cmdargs, root=root, **kwargs)
+
+
+def error_message_for_containerised_commandline(
+        argv, err, container_kwargs):  # pragma: no cover
+    '''Return a semi-readable error message for a containerised command.'''
+
+    # This function should do some formatting of the container_kwargs dict,
+    # rather than just dumping it in the error message, but that is better than
+    # nothing.
+
+    return 'Command failed: %s:\n' \
+           'Containerisation settings: %s\n' \
+           'Error output:\n%s' \
+           % (' '.join(argv), container_kwargs, err)