fix: prepare for removal of transient session vars from activation environment

In future releases the following variables will be omitted:
XDG_VTNR, XDG_SESSION_ID, XDG_SESSION_PATH, XDG_SEAT, XDG_SEAT_PATH.

These are transient variables that make sense only for a specific session
among others (graphical or otherwise) and should not be placed in activation
environment.

This is in contrast to vars like XDG_CURERNT_DESKTOP which are relevant to one
and only graphical session and may inform other sessions and environments on
what's going on there.

The problem though is that those vars are useful to have at hand. This commit
creates alternative way of propagaitng them to compositor and further:

 * compositor's service `EnvironmentFile=` directive and a file for it.
 * `uwsm app` now transmits them via `--setenv` arg to service units it
   launches.

Currently overall behavior is unchanged by default, the new mechanism is
redundant. In future releases these vars will be moved from `always_export` set
to `never_export` and `always_unset` sets. This behavior can be tested by adding
`UWSM_NO_SESSION_SPECIFIC_VARS=true` directly to activation environment
before `uwsm start`.

Reference for var exclusion in GNOME and MATE session managers:
https://gitlab.gnome.org/GNOME/gnome-session/-/issues/86
mate-desktop/mate-session-manager#282

Author: Vladimir-csp

man/uwsm.1.scd

@@ -92,8 +92,18 @@ be used for distro level defaults.
 |  *UWSM_SILENT_START*
 :  (int or boolean value)
 | 
-:  _True_ or *1* to inhibit stdout messages from *uwsm start*. *2* to also
+:  _True_ or *1* to inhibit stdout messages from "*uwsm start*". *2* to also
    inhibit warnings.
+|  *UWSM_NO_SESSION_SPECIFIC_VARS*
+:  (boolean value)
+| 
+:  Set _True_ to test future behavior: omit transient session vars:
+   *XDG_SEAT*, *XDG_SEAT_PATH*, *XDG_SESSION_ID*, *XDG_SESSION_PATH*, *XDG_VTNR*
+   from activation environment and load them via env file.
+   (!) Should be set in systemd activation environment before starting uwsm
+   to be seen by environment preloader
+   ("*systemctl --user set-environment UWSM_NO_SESSION_SPECIFIC_VARS=true*"
+   or add to user's *environment.d*).
 |  *DEBUG*
 :  (int or boolean value)
 | 

systemd/user/wayland-wm-app-daemon.service.in

@@ -11,5 +11,6 @@ Type=exec
 ExecStart=@BIN_PATH@ aux app-daemon
 Restart=on-failure
 RestartMode=direct
+EnvironmentFile=-%t/@BIN_NAME@/session_vars.env
 SyslogIdentifier=@BIN_NAME@_app-daemon
 Slice=session.slice

systemd/user/wayland-wm-env@.service.in

@@ -20,5 +20,6 @@ RemainAfterExit=yes
 ExecStart=@BIN_PATH@ aux prepare-env -- "%I"
 ExecStopPost=@BIN_PATH@ aux cleanup-env
 Restart=no
+EnvironmentFile=-%t/@BIN_NAME@/session_vars.env
 SyslogIdentifier=@BIN_NAME@_env-preloader
 Slice=session.slice

systemd/user/wayland-wm@.service.in

@@ -20,6 +20,7 @@ Type=notify
 NotifyAccess=all
 ExecStart=@BIN_PATH@ aux exec -- %I
 Restart=no
+EnvironmentFile=-%t/@BIN_NAME@/session_vars.env
 TimeoutStartSec=30
 TimeoutStopSec=10
 SyslogIdentifier=@BIN_NAME@_%I

uwsm/main.py

@@ -127,20 +127,38 @@ def _split_lines(self, text, width):
 class Varnames:
     "Sets of varnames"
 
-    always_export = {
-        "PATH",
-        "XDG_CURRENT_DESKTOP",
-        "XDG_MENU_PREFIX",
+    # TODO: remove this in future release
+    session_separate = str2bool_plus(
+        os.environ.get("UWSM_NO_SESSION_SPECIFIC_VARS", "0")
+    )
+
+    session_specific = {
         "XDG_SEAT",
         "XDG_SEAT_PATH",
-        "XDG_SESSION_DESKTOP",
         "XDG_SESSION_ID",
         "XDG_SESSION_PATH",
-        "XDG_SESSION_TYPE",
         "XDG_VTNR",
     }
-    never_export = {"PWD", "LS_COLORS", "INVOCATION_ID", "SHLVL", "SHELL", "TERM"}
-    always_unset = {"DISPLAY", "WAYLAND_DISPLAY"}
+    always_export = {
+        "PATH",
+        "XDG_CURRENT_DESKTOP",
+        "XDG_MENU_PREFIX",
+        "XDG_SESSION_DESKTOP",
+        "XDG_SESSION_CLASS",
+        "XDG_SESSION_TYPE",
+    } | (set() if session_separate else session_specific)
+    never_export = {
+        "PWD",
+        "LS_COLORS",
+        "INVOCATION_ID",
+        "SHLVL",
+        "SHELL",
+        "TERM",
+        "NOTIFY_SOCKET",
+    } | (set() if not session_separate else session_specific)
+    always_unset = {"DISPLAY", "WAYLAND_DISPLAY"} | (
+        set() if not session_separate else session_specific
+    )
     always_cleanup = {
         "DISPLAY",
         "LANG",
@@ -150,14 +168,11 @@ class Varnames:
         "XCURSOR_THEME",
         "XDG_CURRENT_DESKTOP",
         "XDG_MENU_PREFIX",
-        "XDG_SEAT",
-        "XDG_SEAT_PATH",
         "XDG_SESSION_DESKTOP",
-        "XDG_SESSION_ID",
-        "XDG_SESSION_PATH",
+        "XDG_SESSION_CLASS",
         "XDG_SESSION_TYPE",
-        "XDG_VTNR",
-    }
+        "NOTIFY_SOCKET",
+    } | session_specific
     never_cleanup = {"SSH_AGENT_LAUNCHER", "SSH_AUTH_SOCK", "SSH_AGENT_PID"}
 
 
@@ -1483,6 +1498,7 @@ def generate_units(rung: str = "run"):
             ExecStart={BIN_PATH} aux prepare-env -- "%I"
             ExecStopPost={BIN_PATH} aux cleanup-env
             Restart=no
+            EnvironmentFile=-%t/{BIN_NAME}/session_vars.env
             SyslogIdentifier={BIN_NAME}_env-preloader
             Slice=session.slice
             """
@@ -1517,6 +1533,7 @@ def generate_units(rung: str = "run"):
             NotifyAccess=all
             ExecStart={BIN_PATH} aux exec -- %I
             Restart=no
+            EnvironmentFile=-%t/{BIN_NAME}/session_vars.env
             TimeoutStartSec=30
             TimeoutStopSec=10
             SyslogIdentifier={BIN_NAME}_%I
@@ -1573,6 +1590,7 @@ def generate_units(rung: str = "run"):
             ExecStart={BIN_PATH} aux app-daemon
             Restart=on-failure
             RestartMode=direct
+            EnvironmentFile=-%t/{BIN_NAME}/session_vars.env
             SyslogIdentifier={BIN_NAME}_app-daemon
             Slice=session.slice
             """
@@ -1845,6 +1863,7 @@ def generate_tweaks(rung: str = "run"):
             [Service]
             # also put them in special graphical app slice
             Slice=app-graphical.slice
+            EnvironmentFile=-%t/{BIN_NAME}/session_vars.env
             """
         ),
         rung=rung,
@@ -2775,8 +2794,8 @@ def finalize(additional_vars=None):
     sys.exit(1)
 
 
-def save_env(filename: str, env: dict = None):
-    "Saves environment to a null-separated runtime file"
+def save_env(filename: str, env: dict = None, separator: str = "\0"):
+    "Saves environment to a null-separated (or value of 'separator') runtime file"
     if env is None:
         env = dict(os.environ)
     env = filter_varnames(env)
@@ -2786,7 +2805,14 @@ def save_env(filename: str, env: dict = None):
     env_file = os.path.join(BaseDirectory.get_runtime_dir(), BIN_NAME, filename)
     os.makedirs(os.path.dirname(env_file), exist_ok=True)
     with open(env_file, "w") as env_file_data:
-        env_file_data.write("\0".join((f"{key}={value}" for key, value in env.items())))
+        # No processing or escaping is done here.
+        # The intended use of alternative separator is for writing systemd
+        # EnvironmentFile= with session XDG_ vars which should not
+        # contain anything requiring escaping
+        env_file_data.write(
+            separator.join((f"{key}={value}" for key, value in env.items()))
+            + ("\n" if separator == "\n" else "")
+        )
     print_debug(f"written {env_file}", env)
 
 
@@ -3372,7 +3398,12 @@ def cleanup_env():
         print_normal("Restoring initial systemd vars.")
         set_systemd_vars(env_pre, verbose=False, bus_session=bus_session)
 
-    for drop_file in [cleanup_file, env_pre_file]:
+    # also remove session-specific env file
+    session_env_file = os.path.join(
+        BaseDirectory.get_runtime_dir(strict=True), BIN_NAME, "session_vars.env"
+    )
+
+    for drop_file in [cleanup_file, env_pre_file, session_env_file]:
         if not os.path.exists(drop_file):
             continue
         os.remove(drop_file)
@@ -4086,7 +4117,14 @@ def is_int(checkvar):
     if app_unit_type == "scope":
         final_args.append("--scope")
     else:
-        final_args.extend(["--property=Type=exec", "--property=ExitType=cgroup"])
+        final_args.extend(
+            ["--property=Type=exec", "--property=ExitType=cgroup"]
+            + [
+                f"--setenv={var}={os.environ.get(var, "")}"
+                for var in sorted(Varnames.session_specific)
+                if os.environ.get(var, "")
+            ]
+        )
         # silence service via unit properties
         if silent:
             if silent == "out":
@@ -5035,8 +5073,20 @@ def main():
             )
             print_debug(sprc)
 
+            # save login environment
             save_env("env_login")
 
+            # save session vars for unit EnvironmentFile=
+            save_env(
+                "session_vars.env",
+                env={
+                    var: os.environ.get(var, "")
+                    for var in sorted(Varnames.session_specific)
+                    if os.environ.get(var, "")
+                },
+                separator="\n",
+            )
+
             # fork out a process that will hold session scope open
             # until compositor unit is stopped
             mainpid = os.getpid()