Bug 1680752 - Provide a python-3 opt-in for tests on Taskcluster, , bhearsum r=bc,bhearsum

This requires tests to specify `python-3: true` in order to be run
with Python 3. When nothing is specified things work just like today,
so it's a more conservative change than the one in bug 1672181.

Obviously in the long term we will remove this and switch to Python 3
only, but this unblocks moving harnesses to py3 today.

Differential Revision: https://phabricator.services.mozilla.com/D98766

taskcluster/scripts/tester/test-linux.sh

@@ -255,7 +255,7 @@ fi
 
 # Use |mach python| if a source checkout exists so in-tree packages are
 # available.
-[[ -x "${GECKO_PATH}/mach" ]] && python="python2.7 ${GECKO_PATH}/mach python" || python="python2.7"
+[[ -x "${GECKO_PATH}/mach" ]] && python="${PYTHON:-python2.7} ${GECKO_PATH}/mach python" || python="${PYTHON:-python2.7}"
 
 # Save the computed mozharness command to a binary which is useful for
 # interactive mode.

taskcluster/taskgraph/transforms/job/mozharness_test.py

@@ -141,6 +141,9 @@ def mozharness_test_on_docker(config, job, taskdesc):
         }
     )
 
+    if test.get("python-3"):
+        env["PYTHON"] = "python3"
+
     # Legacy linux64 tests rely on compiz.
     if test.get("docker-image", {}).get("in-tree") == "desktop1604-test":
         env.update({"NEED_COMPIZ": "true"})
@@ -352,31 +355,42 @@ def mozharness_test_on_generic_worker(config, job, taskdesc):
             "artifact-reference": "<decision/public/tests-by-manifest.json.gz>"
         }
 
+    py_3 = test.get("python-3", False)
+
     if is_windows:
+        py_binary = "c:\\mozilla-build\\{python}\\{python}.exe".format(
+            python="python3" if py_3 else "python"
+        )
         mh_command = [
-            "c:\\mozilla-build\\python\\python.exe",
+            py_binary,
             "-u",
             "mozharness\\scripts\\" + normpath(mozharness["script"]),
         ]
     elif is_bitbar:
+        py_binary = "python3" if py_3 else "python"
         mh_command = ["bash", "./{}".format(bitbar_script)]
     elif is_macosx and "macosx1014-64" in test["test-platform"]:
+        py_binary = "/usr/local/bin/{}".format("python3" if py_3 else "python2")
         mh_command = [
-            "/usr/local/bin/python2",
+            py_binary,
             "-u",
             "mozharness/scripts/" + mozharness["script"],
         ]
     else:
         # is_linux or is_macosx
+        py_binary = "/usr/bin/{}".format("python3" if py_3 else "python2")
         mh_command = [
             # Using /usr/bin/python2.7 rather than python2.7 because
             # /usr/local/bin/python2.7 is broken on the mac workers.
             # See bug #1547903.
-            "/usr/bin/python2.7",
+            py_binary,
             "-u",
             "mozharness/scripts/" + mozharness["script"],
         ]
 
+    if py_3:
+        env["PYTHON"] = py_binary
+
     for mh_config in mozharness["config"]:
         cfg_path = "mozharness/configs/" + mh_config
         if is_windows:

taskcluster/taskgraph/transforms/tests.py

@@ -572,6 +572,8 @@ test_description_schema = Schema(
         Optional("fetches"): {
             text_type: optionally_keyed_by("test-platform", [text_type])
         },
+        # Opt-in to Python 3 support
+        Optional("python-3"): bool,
     }
 )