meta: friendlier message for incorrect app command (#2272)

Provide a nicer error for when the command entires under
an specific application cannot be found.

Also, for future proofing, ensure a warning is raised when these
commands are not relative to the prime directory.

LP: #1791175
Fixes SNAPCRAFT-26
Fixes SNAPCRAFT-3M
Fixes SNAPCRAFT-3T
Fixes SNAPCRAFT-3W
Fixes SNAPCRAFT-43

Signed-off-by: Sergio Schvezov <sergio.schvezov@canonical.com>

snapcraft/internal/errors.py

@@ -167,10 +167,8 @@ class InvalidAppCommandError(SnapcraftError):
     fmt = (
         "Failed to generate snap metadata: "
         "The specified command {command!r} defined in the app {app!r} does "
-        "not exist or is not executable"
-        # FIXME split the errors to include in the message how to fix them.
-        # https://bugs.launchpad.net/snapcraft/+bug/1727425
-        # --elopio - 2017-10-25
+        "not exist or is not executable.\n"
+        "Ensure that {command!r} is relative to the prime directory."
     )
 
     def __init__(self, command, app):

snapcraft/internal/meta/_snap_packaging.py

@@ -581,6 +581,8 @@ def _wrap_app(self, name, app):
         for k in cmds:
             try:
                 app[k] = self._wrap_exe(app[k], "{}-{}".format(k, name))
+            except FileNotFoundError:
+                raise errors.InvalidAppCommandError(command=app[k], app=app)
             except meta_errors.CommandError as e:
                 raise errors.InvalidAppCommandError(str(e), name)
 

tests/unit/test_errors.py

@@ -271,7 +271,8 @@ class ErrorFormattingTestCase(unit.TestCase):
                 "expected_message": (
                     "Failed to generate snap metadata: "
                     "The specified command 'test-command' defined in the app "
-                    "'test-app' does not exist or is not executable"
+                    "'test-app' does not exist or is not executable.\n"
+                    "Ensure that 'test-command' is relative to the prime directory."
                 ),
             },
         ),

tests/unit/test_meta.py

@@ -1216,8 +1216,7 @@ def test_create_meta_with_grade(self):
             )
 
 
-# TODO this needs more tests.
-class WrapExeTestCase(unit.TestCase):
+class BaseWrapTest(unit.TestCase):
     def setUp(self):
         super().setUp()
 
@@ -1239,6 +1238,18 @@ def setUp(self):
         self.packager = _snap_packaging._SnapPackaging(config)
         self.packager._is_host_compatible_with_base = True
 
+
+class WrapAppTest(BaseWrapTest):
+    def test_app_not_found(self):
+        self.assertRaises(
+            errors.InvalidAppCommandError,
+            self.packager._wrap_apps,
+            apps=dict(app=dict(command="test-command")),
+        )
+
+
+# TODO this needs more tests.
+class WrapExeTest(BaseWrapTest):
     @patch("snapcraft.internal.common.assemble_env")
     def test_wrap_exe_must_write_wrapper(self, mock_assemble_env):
         mock_assemble_env.return_value = "PATH={0}/part1/install/usr/bin:{0}/part1/install/bin".format(