droiddeck-fex: use DroidDeck's own FEX and x86 root without Steam's

x86 Linux programs on the desktop (an x86_64 AppImage) needed Valve's
FEX (Steam app 3127680) and Steam Linux Runtime 3.0; without them they
exited with status 126. The x86-64 client work built our own FEX
(release fex-2609-r3: openat2 fallback for proot, GL/Vulkan/EGL/ALSA
thunks) installed under /usr/local, run against FEX's Ubuntu 24.04 x86
image.

Look for FEX in /usr/local as well as /usr, and when Steam's runtime is
absent accept a plain x86 root at /opt/droiddeck-fex/rootfs once its
unpack is marked complete. Steam's FEX and runtime still win when
installed.

tools/linuxfs/overlay/usr/local/bin/droiddeck-fex

@@ -30,6 +30,11 @@ FEX_APPID = "3127680"
 SNIPER_APPID = "1628350"
 INSTALL_DIRS = {FEX_APPID: "FEX-Emu", SNIPER_APPID: "SteamLinuxRuntime_sniper"}
 MARKER = ".droiddeck-fex"
+# Without Steam's FEX and runtime: DroidDeck's own FEX build in /usr/local (the x86-64 client's,
+# with GL/Vulkan thunks) and a plain x86 root - FEX's Ubuntu image unpacked - marked complete here.
+OWN_ROOTFS = "/opt/droiddeck-fex/rootfs"
+OWN_ROOTFS_DONE = ".droiddeck-complete"
+SYSTEM_PREFIXES = ("/usr/local", "/usr")
 LAYOUT = 1
 GUEST_LIBDIRS = (("x86_64", "lib/x86_64-linux-gnu"), ("i386", "lib/i386-linux-gnu"))
 PRELOAD_NAMES = ("libblsession.so", "libfakeinput.so")
@@ -142,7 +147,7 @@ def fex_install():
     tool = app_dir(FEX_APPID)
     if tool:
         prefixes += [(os.path.join(tool, sub), True) for sub in ("usr", "", "files/usr", "files")]
-    prefixes.append(("/usr", False))
+    prefixes += [(prefix, False) for prefix in SYSTEM_PREFIXES]
     for prefix, portable in prefixes:
         binary = next((os.path.join(prefix, "bin", name) for name in ("FEX", "FEXInterpreter")
                        if os.access(os.path.join(prefix, "bin", name), os.X_OK)), None)
@@ -179,6 +184,13 @@ def runtime_platform():
     return max(platforms, key=platform_version, default=None)
 
 
+def own_rootfs():
+    if os.path.isfile(os.path.join(OWN_ROOTFS, OWN_ROOTFS_DONE)) and \
+            os.path.isfile(os.path.join(OWN_ROOTFS, "lib/x86_64-linux-gnu/libc.so.6")):
+        return OWN_ROOTFS
+    return None
+
+
 def unescape(raw):
     return ESCAPE.sub(lambda match: bytes([int(match.group(1), 8)]), raw)
 
@@ -308,6 +320,7 @@ def request_install():
 
 
 def describe(fex, platform):
+    # platform: Steam's sniper platform folder, or the own root when Steam's runtime is absent.
     return {
         "fex": fex["bin"] if fex else None,
         "server": fex["server"] if fex else None,
@@ -322,7 +335,7 @@ def describe(fex, platform):
 def prepare():
     with locked():
         fex = fex_install()
-        platform = runtime_platform()
+        platform = runtime_platform() or own_rootfs()
         status = describe(fex, platform)
         if not status["ready"]:
             missing = []
@@ -341,9 +354,12 @@ def prepare():
                 "the Steam client was asked to install it" if requested else "open Steam once and it is installed for you"))
             write_if_changed(STATUS_FILE, json.dumps(status, indent=2) + "\n")
             return None
-        root = os.path.join(platform, "files")
-        if materialize(platform):
-            say("prepared %s as the x86 runtime" % os.path.basename(platform))
+        if platform == OWN_ROOTFS:
+            root = OWN_ROOTFS
+        else:
+            root = os.path.join(platform, "files")
+            if materialize(platform):
+                say("prepared %s as the x86 runtime" % os.path.basename(platform))
         install_preloads(root)
         write_config(fex, root)
         fex["rootfs"] = root

tools/tests/test_droiddeck_fex.py

@@ -245,6 +245,28 @@ class PrepareTest(FexTestCase):
         self.assertFalse((self.home / ".config/droiddeck/fex-wanted").exists())
         self.assertEqual(self.quiet(FEX["prepare"])[1], "")
 
+    def test_the_own_build_and_root_stand_in_for_steams(self):
+        local = self.tmp / "usr-local"
+        self.write(local / "bin/FEX", elf(183), 0o755)
+        self.write(local / "bin/FEXServer", elf(183), 0o755)
+        (local / "lib/fex-emu/HostThunks").mkdir(parents=True)
+        (local / "share/fex-emu/GuestThunks").mkdir(parents=True)
+        self.write(local / "share/fex-emu/ThunksDB.json", "{}")
+        root = self.tmp / "own-rootfs"
+        self.write(root / "lib/x86_64-linux-gnu/libc.so.6", elf(62))
+        with mock.patch.dict(G, {"SYSTEM_PREFIXES": (str(local),), "OWN_ROOTFS": str(root),
+                                 "steam_running": lambda: False}):
+            # An unpack that did not finish is not a root.
+            self.assertIsNone(self.quiet(FEX["prepare"])[0])
+            self.write(root / ".droiddeck-complete")
+            fex, said = self.quiet(FEX["prepare"])
+        self.assertEqual((fex["bin"], fex["rootfs"], fex["portable"]), (str(local / "bin/FEX"), str(root), False))
+        self.assertNotIn("prepared", said)
+        status = json.loads((self.home / ".local/share/droiddeck-fex/status.json").read_text())
+        self.assertEqual((status["ready"], status["runtime"], status["thunks"]), (True, str(root), True))
+        config = json.loads((self.home / ".local/share/droiddeck-fex/config/Config.json").read_text())
+        self.assertEqual(config["Config"]["RootFS"], str(root))
+
     def test_status_refreshes_what_the_app_reads_once_steam_installed_fex(self):
         with mock.patch.dict(G, {"steam_running": lambda: False}):
             self.quiet(FEX["prepare"])