Support PEX_SCRIPT=pex3 ./pex ... for scie.

Add a `--scie-busybox-pex-entrypoint-env-passthrough` option to trade a
small amount of perf for a Pex scie busybox accepting
PEX_{INTERPRETER,MODULE,SCRIPT} entry point control like its Pex PEX
sibling.

pex/scie/__init__.py

@@ -144,6 +144,19 @@ def register_options(parser):
             "review the `install` command help."
         ),
     )
+    parser.add_argument(
+        "--scie-busybox-pex-entrypoint-env-passthrough",
+        "--no-scie-busybox-pex-entrypoint-env-passthrough",
+        dest="scie_busybox_pex_entrypoint_env_passthrough",
+        default=False,
+        type=bool,
+        action=HandleBoolAction,
+        help=(
+            "When creating a busybox, allow entrypoint control via PEX_INTERPRETER, PEX_SCRIPT and "
+            "PEX_MODULE. Note that when using --venv this adds modest startup overhead on the "
+            "order of 10ms."
+        ),
+    )
     parser.add_argument(
         "--scie-platform",
         dest="scie_platforms",
@@ -269,6 +282,8 @@ def render_options(options):
         entrypoints = list(options.busybox_entrypoints.console_scripts_manifest.iter_specs())
         entrypoints.extend(map(str, options.busybox_entrypoints.ad_hoc_entry_points))
         args.append(",".join(entrypoints))
+    if options.busybox_pex_entrypoint_env_passthrough:
+        args.append("--scie-busybox-pex-entrypoint-env-passthrough")
     for platform in options.platforms:
         args.append("--scie-platform")
         args.append(str(platform))
@@ -368,6 +383,7 @@ def extract_options(options):
         naming_style=options.naming_style,
         scie_only=options.scie_only,
         busybox_entrypoints=entry_points,
+        busybox_pex_entrypoint_env_passthrough=options.scie_busybox_pex_entrypoint_env_passthrough,
         platforms=tuple(OrderedSet(options.scie_platforms)),
         pbs_release=options.scie_pbs_release,
         pypy_release=options.scie_pypy_release,

pex/scie/model.py

@@ -359,6 +359,7 @@ class ScieOptions(object):
     naming_style = attr.ib(default=None)  # type: Optional[PlatformNamingStyle.Value]
     scie_only = attr.ib(default=False)  # type: bool
     busybox_entrypoints = attr.ib(default=None)  # type: Optional[BusyBoxEntryPoints]
+    busybox_pex_entrypoint_env_passthrough = attr.ib(default=False)  # type: bool
     platforms = attr.ib(default=())  # type: Tuple[SciePlatform.Value, ...]
     pbs_release = attr.ib(default=None)  # type: Optional[str]
     pypy_release = attr.ib(default=None)  # type: Optional[str]

pex/scie/science.py

@@ -159,18 +159,26 @@ def args(
 
         def create_cmd(named_entry_point):
             # type: (NamedEntryPoint) -> Dict[str, Any]
-            return {
-                "name": named_entry_point.name,
-                "env": {
+
+            if (
+                configuration.options.busybox_pex_entrypoint_env_passthrough
+                and named_entry_point.entry_point == pex_entry_point
+            ):
+                env = {"default": default_env(named_entry_point), "remove_exact": ["PEX_VENV"]}
+            else:
+                env = {
                     "default": default_env(named_entry_point),
                     "replace": {"PEX_MODULE": str(named_entry_point.entry_point)},
                     "remove_exact": ["PEX_INTERPRETER", "PEX_SCRIPT", "PEX_VENV"],
-                },
+                }
+            return {
+                "name": named_entry_point.name,
+                "env": env,
                 "exe": "{scie.bindings.configure:PYTHON}",
                 "args": args(named_entry_point, "{scie.bindings.configure:PEX}"),
             }
 
-        if pex_info.venv:
+        if pex_info.venv and not configuration.options.busybox_pex_entrypoint_env_passthrough:
             # N.B.: Executing the console script directly instead of bouncing through the PEX
             # __main__.py using PEX_SCRIPT saves ~10ms of re-exec overhead in informal testing; so
             # it's worth specializing here.

scripts/create-packages.py

@@ -126,6 +126,7 @@ def build_pex_scies(
             "sha256",
             "--scie-busybox",
             "@pex",
+            "--scie-busybox-pex-entrypoint-env-passthrough",
             "-o",
             output_file,
             "-c",