bins.py: host-only Python/Lua in sandboxed bundles

_which() probes host dirs only in Flatpak (/run/host/usr/bin) and AppImage
(/usr/bin); apply_host_libs prepares env for host subprocesses (prepend host
libs in Flatpak, strip $APPDIR pollution + PYTHONHOME in AppImage); user
override resolved via _which() for bare names.

src/testium/interpreter/utils/bins.py

@@ -17,7 +17,7 @@ Public API
 ``reset()``             : clear the cache (mostly useful for tests)
 """
 
-import shutil
+import os
 import subprocess
 
 import api.testium as tm
@@ -30,10 +30,126 @@ from runtime.tum_except import ETUMRuntimeError
 _PYTHON_CANDIDATES = ["python3", "python"]
 _LUA_CANDIDATES = ["lua", "lua5.5", "lua5.4", "lua5.3", "lua5.2", "lua5.1"]
 
+# When running inside a Flatpak, --filesystem=host-os mounts the host at
+# /run/host (read-only). Binaries and libraries from the host are not on the
+# sandbox PATH/LD_LIBRARY_PATH, so we probe and inject them explicitly.
+_FLATPAK_HOST_DIRS = [
+    "/run/host/usr/local/bin",
+    "/run/host/usr/bin",
+    "/run/host/bin",
+]
+_FLATPAK_HOST_LIB_DIRS = [
+    "/run/host/usr/lib",
+    "/run/host/usr/lib64",
+    "/run/host/usr/local/lib",
+]
+
+# Inside an AppImage, AppRun prepends $APPDIR/usr/bin to PATH and exports a
+# bundle-local PYTHONHOME / PYTHONPATH / LD_LIBRARY_PATH. We want py_func and
+# lua_func to run under the *host* interpreter (not the bundled one), so we
+# probe standard host bin dirs directly and scrub APPDIR-prefixed entries from
+# the env passed to host subprocesses.
+_APPIMAGE_HOST_DIRS = [
+    "/usr/local/bin",
+    "/usr/bin",
+    "/bin",
+]
+
+
+def _in_flatpak():
+    return os.path.isfile("/.flatpak-info")
+
+
+def _in_appimage():
+    return "APPIMAGE" in os.environ
+
+
+def apply_host_lua_paths(env):
+    """Prepend host Lua module dirs to LUA_PATH / LUA_CPATH (Flatpak only).
+
+    Must be called after user-defined lua_env overrides are applied, so host
+    paths are always first regardless of user config. User-defined paths remain
+    in the variable but after the host ones.
+    """
+    if not _in_flatpak():
+        return
+    _LUA_VERSIONS = ["5.5", "5.4", "5.3", "5.2", "5.1"]
+    _HOST = "/run/host/usr"
+    cpath_dirs, lpath_dirs = [], []
+    for v in _LUA_VERSIONS:
+        for base in [f"{_HOST}/lib/lua/{v}",
+                     f"{_HOST}/lib64/lua/{v}",
+                     f"{_HOST}/lib/x86_64-linux-gnu/lua/{v}"]:
+            cpath_dirs.append(f"{base}/?.so")
+        lpath_dirs.append(f"{_HOST}/share/lua/{v}/?.lua")
+        lpath_dirs.append(f"{_HOST}/share/lua/{v}/?/init.lua")
+    sep = ";"
+    host_cpath = sep.join(cpath_dirs)
+    host_lpath = sep.join(lpath_dirs)
+    # ;; keeps Lua's compiled-in defaults at the end as last resort
+    env["LUA_CPATH"] = host_cpath + sep + env.get("LUA_CPATH", ";;")
+    env["LUA_PATH"]  = host_lpath + sep + env.get("LUA_PATH",  ";;")
+
+
+def apply_host_libs(env):
+    """Prepare *env* for launching a host binary from inside our bundle.
+
+    - Flatpak: prepend host library dirs to LD_LIBRARY_PATH so the dynamic
+      linker can find host .so files mounted under /run/host.
+    - AppImage: strip $APPDIR-prefixed entries from LD_LIBRARY_PATH and
+      PYTHONPATH and drop PYTHONHOME, so the host interpreter doesn't try
+      to load the bundled (incompatible) Python lib/site-packages.
+    - Otherwise: no-op.
+    """
+    if _in_flatpak():
+        dirs = ":".join(d for d in _FLATPAK_HOST_LIB_DIRS if os.path.isdir(d))
+        if dirs:
+            existing = env.get("LD_LIBRARY_PATH", "")
+            env["LD_LIBRARY_PATH"] = dirs + (":" + existing if existing else "")
+        return
+    if _in_appimage():
+        appdir = os.environ.get("APPDIR", "")
+        if appdir:
+            for var, sep in (("LD_LIBRARY_PATH", ":"),
+                             ("PYTHONPATH", os.pathsep),
+                             ("PATH", os.pathsep)):
+                cur = env.get(var, "")
+                if not cur:
+                    continue
+                cleaned = sep.join(
+                    p for p in cur.split(sep)
+                    if p and not p.startswith(appdir)
+                )
+                if cleaned:
+                    env[var] = cleaned
+                else:
+                    env.pop(var, None)
+        env.pop("PYTHONHOME", None)
+
 
 def _which(name):
-    func = sys_app_path_win if tm.OS() == "Windows" else sys_app_path_lin
-    return func(name)
+    if tm.OS() == "Windows":
+        return sys_app_path_win(name)
+    if _in_flatpak():
+        for d in _FLATPAK_HOST_DIRS:
+            p = os.path.join(d, name)
+            if os.path.isfile(p) and os.access(p, os.X_OK):
+                return p
+        return ""
+    if _in_appimage():
+        for d in _APPIMAGE_HOST_DIRS:
+            p = os.path.join(d, name)
+            if os.path.isfile(p) and os.access(p, os.X_OK):
+                return p
+        return ""
+    return sys_app_path_lin(name)
+
+
+def _probe_env():
+    """Subprocess env for probing host binaries (adds host libs in Flatpak)."""
+    env = os.environ.copy()
+    apply_host_libs(env)
+    return env
 
 
 def _python_version(path):
@@ -41,7 +157,7 @@ def _python_version(path):
     try:
         r = subprocess.run(
             cmd, capture_output=True, text=True,
-            encoding=tm.sys_encoding(), timeout=10,
+            encoding=tm.sys_encoding(), timeout=10, env=_probe_env(),
         )
     except (FileNotFoundError, PermissionError, subprocess.TimeoutExpired):
         return None
@@ -60,6 +176,7 @@ def _lua_version(path):
     try:
         r = subprocess.run(
             [path, "-v"], capture_output=True, text=True, timeout=10,
+            env=_probe_env(),
         )
     except (FileNotFoundError, PermissionError, subprocess.TimeoutExpired):
         return None
@@ -97,8 +214,16 @@ def _resolve(name):
 
     path = ""
     if override:
-        if shutil.which(override) and validator(override):
-            path = override
+        # Absolute path: accept as-is (user knows exactly what they want).
+        # Bare name: resolve via _which() so the override stays host-only in
+        # Flatpak/AppImage instead of silently picking the bundled interpreter.
+        if os.path.isabs(override):
+            resolved = override if (os.path.isfile(override)
+                                    and os.access(override, os.X_OK)) else ""
+        else:
+            resolved = _which(override)
+        if resolved and validator(resolved):
+            path = resolved
         else:
             tm.print_warn(
                 f"Configured {display} interpreter '{override}' is not usable; "

src/testium/interpreter/utils/lua_process.py

@@ -60,6 +60,7 @@ class LuaProcessBase:
 
         lua_env = tm.gd("lua_env", {})
         env = os.environ.copy()
+        bins.apply_host_libs(env)
         if not isinstance(lua_env, dict):
             raise ETUMRuntimeError(f"The 'lua_env' global value should be a dictionary. But it is '{lua_env}'.")
 
@@ -70,6 +71,7 @@ class LuaProcessBase:
                     env[k] = e
                 else:
                     env[k] = e + ";" + env.get(k, "")
+        bins.apply_host_lua_paths(env)
 
         sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
         sock.bind(("localhost", 0))

src/testium/interpreter/utils/py_process.py

@@ -42,6 +42,10 @@ class PyProcessBase:
             raise ETUMRuntimeError(f"The 'py_env' global value should be a dictionary. But it is '{py_env}'.")
 
         env = os.environ.copy()
+        bins.apply_host_libs(env)
+        # PYTHONUSERBASE is set by the Flatpak runtime to isolate sandbox
+        # user packages; remove it so the host Python finds ~/.local packages.
+        env.pop("PYTHONUSERBASE", None)
         for k, v in self.CUST_ENV.items():
             e = py_env.get(k, "")
             if e != "":