Env-var substitution

reinventing supervisord one step at a time.
(I kid)

README.md

@@ -42,6 +42,9 @@ why this is a great idea](https://www.bugsink.com/multi-process-docker-images/)
 * **Graceful Shutdown**: Ensure the parent process waits for all children to exit
   before shutting down.
 
+* **Variable Substition**: Substitute variables from the command line because [Docker
+  can't be bothered to do so itself](https://github.com/moby/moby/issues/5509)
+
 ### Installation
 
 To use this script, clone the repository or install via your preferred method:

monofy/scripts/monofy.py

@@ -4,6 +4,10 @@
 import sys
 import os
 from time import sleep
+import re
+
+# match both ${VAR} and $VAR:
+MATCH_ENV_VAR = re.compile(r'\$\{([A-Za-z_][A-Za-z0-9_]*)\}|\$([A-Za-z_][A-Za-z0-9_]*)')
 
 
 class ParentProcess:
@@ -82,7 +86,18 @@ def connect_childrens_fates(self):
                     self.terminate_children(except_child=child)
 
     @classmethod
-    def get_pre_start_command_args(self, argv):
+    def substitute_env_vars(cls, arg):
+        # I don't _want_ to do this, but Docker doesn't want to do it either, so here we are.
+        #
+        # One (the only) other way to do this is to have CMD invoke a shell; but that has drawbacks too (e.g.  signal
+        # forwarding). Since monofy is already the "extra layer" we add to the system, we might as well solve the
+        # problems here.
+        #
+        # https://github.com/moby/moby/issues/5509
+        return MATCH_ENV_VAR.sub(lambda m: os.environ.get(m.group(1) or m.group(2), ""), arg)
+
+    @classmethod
+    def get_pre_start_command_args(cls, argv):
         """Splits our own arguments into a list of args for each of the pre-start commands, we split on "&&"."""
 
         # We don't want to pass the first argument, as that is the script name
@@ -97,12 +112,12 @@ def get_pre_start_command_args(self, argv):
                 result.append(this)
                 this = []
             else:
-                this.append(arg)
+                this.append(cls.substitute_env_vars(arg))
 
         return result
 
     @classmethod
-    def get_parallel_command_args(self, argv):
+    def get_parallel_command_args(cls, argv):
         """Splits our own arguments into a list of args for each of the children each, we split on "|||"."""
 
         # We don't want to pass the first argument, as that is the script name
@@ -116,7 +131,7 @@ def get_parallel_command_args(self, argv):
             if arg == "|||":
                 result.append([])
             else:
-                result[-1].append(arg)
+                result[-1].append(cls.substitute_env_vars(arg))
 
         return result
 

monofy/tests.py

@@ -1,3 +1,4 @@
+import os
 import unittest
 
 from monofy.scripts.monofy import ParentProcess
@@ -26,12 +27,24 @@ def _check(argv, expected_pre_start, expected_parallel):
             [["a", "b"]],
         )
 
+        _check(
+            ["monofy.py", "a", "|||", "b", "|||", "c"],
+            [],
+            [["a"], ["b"], ["c"]],
+        )
+
         _check(
             ["monofy.py", "a", "b", "|||", "c", "d", "|||", "e", "f"],
             [],
             [["a", "b"], ["c", "d"], ["e", "f"]],
         )
 
+        _check(
+            ["monofy.py", "a", "&&", "b", "|||", "c"],
+            [["a"]],
+            [["b"], ["c"]],
+        )
+
         _check(
             ["monofy.py", "a", "b", "&&", "c", "d", "|||", "e", "f"],
             [["a", "b"]],
@@ -56,6 +69,24 @@ def _check(argv, expected_pre_start, expected_parallel):
             [["e", "f"], ["g", "h"], ["i", "j"]],
         )
 
+        _check(
+            ["monofy.py", "$USER", "&&", "$USER", "|||", "$USER"],
+            [[os.environ["USER"]]],
+            [[os.environ["USER"]], [os.environ["USER"]]],
+        )
+
+    def test_substitute_env_vars(self):
+        # test-the-test: we need a non-empty USER to be able to test against
+        self.assertTrue(os.environ.get("USER"))
+
+        self.assertEqual("donttouchme", ParentProcess.substitute_env_vars("donttouchme"))
+        self.assertEqual("", ParentProcess.substitute_env_vars(""))
+        self.assertEqual("foo %s" % os.environ["USER"], ParentProcess.substitute_env_vars("foo $USER"))
+        self.assertEqual("bar %s foo" % os.environ["USER"], ParentProcess.substitute_env_vars("bar ${USER} foo"))
+        self.assertEqual("", ParentProcess.substitute_env_vars("$THISWILLNOTEXIST"))
+        self.assertEqual(
+            "%s %s" % (os.environ["USER"], os.environ["USER"]), ParentProcess.substitute_env_vars("$USER $USER"))
+
 
 if __name__ == '__main__':
     unittest.main()