systemctl: fix edit and cat verbs with --global flag (#39606)

The --global flag has been broken since commit 
d77d42ed3a, which added a
blanket restriction on acquiring D-Bus connections when
arg_runtime_scope is RUNTIME_SCOPE_GLOBAL. This was done to prevent
crashes, but inadvertently broke legitimate use cases like 'systemctl
edit --global' and 'systemctl cat --global'.

The issue is that verb_edit() and verb_cat() were unconditionally
calling acquire_bus(), which triggers the restriction and fails with
"--global is not supported for this operation."

This commit fixes the issue by making bus acquisition conditional,
following the same pattern used in verb_enable():

- Only acquire the bus when install_client_side() returns NO (i.e., for
system and user scopes)
- For client-side operations (--global, --root, etc.), skip bus
acquisition and use mangle_names() instead of expand_unit_names()
- Update find_paths_to_edit() and verb_cat() to handle NULL bus by
forcing client-side path lookups
- Skip bus-dependent checks (unit_is_masked, need_daemon_reload) when
bus is NULL

This allows both 'systemctl edit --global' and 'systemctl cat --global'
to work correctly by performing all operations client-side without
requiring a connection to the system or user manager.

Fixes #31272

test/units/TEST-26-SYSTEMCTL.sh

@@ -573,4 +573,48 @@ systemctl daemon-reload
 systemctl enable --now test-WantedBy.service || :
 systemctl daemon-reload
 
+# Test systemctl edit --global and systemctl cat --global (issue #31272)
+GLOBAL_UNIT_NAME="systemctl-test-$RANDOM.service"
+GLOBAL_MASKED_UNIT="systemctl-test-masked-$RANDOM.service"
+
+# Test 1: Create a new global user unit with --force and --runtime
+systemctl edit --global --runtime --stdin --full --force "$GLOBAL_UNIT_NAME" <<EOF
+[Unit]
+Description=Test global unit
+
+[Service]
+ExecStart=/bin/true
+EOF
+
+# Verify the unit file was created in /run/systemd/user/
+test -f "/run/systemd/user/$GLOBAL_UNIT_NAME"
+
+# Test 2: Read the global unit with systemctl cat --global
+systemctl cat --global "$GLOBAL_UNIT_NAME" | grep -q "ExecStart=/bin/true"
+
+# Test 3: Edit existing global unit (add a drop-in)
+systemctl edit --global --runtime --stdin "$GLOBAL_UNIT_NAME" <<EOF
+[Service]
+Environment=TEST=value
+EOF
+
+# Verify drop-in was created
+test -f "/run/systemd/user/$GLOBAL_UNIT_NAME.d/override.conf"
+systemctl cat --global "$GLOBAL_UNIT_NAME" | grep -q "Environment=TEST=value"
+
+# Test 4: Create a masked global unit in /run/
+mkdir -p /run/systemd/user
+ln -sf /dev/null "/run/systemd/user/$GLOBAL_MASKED_UNIT"
+
+# Test 5: Verify cat shows it's masked
+systemctl cat --global "$GLOBAL_MASKED_UNIT" 2>&1 | grep -q "masked"
+
+# Test 6: Verify edit refuses to edit masked unit
+(! systemctl edit --global --runtime --stdin --full "$GLOBAL_MASKED_UNIT" </dev/null 2>&1) | grep -q "masked"
+
+# Cleanup global test units
+rm -f "/run/systemd/user/$GLOBAL_UNIT_NAME"
+rm -rf "/run/systemd/user/$GLOBAL_UNIT_NAME.d"
+rm -f "/run/systemd/user/$GLOBAL_MASKED_UNIT"
+
 touch /testok