Add systemd path-activation file for starting Saraswati after USB devs

CHANGES.rst

@@ -7,6 +7,8 @@ in progress
 ===========
 
 - Don't automatically start tailing the log file after "saraswati setup"
+- Add systemd path-activation file in order to start the Saraswati service only
+  after all USB sound devices have been registered by watching ``/dev/bus/usb``.
 
 
 2021-06-23 0.5.0

saraswati/setup/systemd/__init__.py

@@ -37,17 +37,20 @@ def run():
         )
         os.system(f"cp {defaultfile} /etc/default/saraswati")
 
-    # Copy systemd unit file. Always.
+    # Copy systemd unit and path-activation files. Always.
     unitfile = pkg_resources.resource_filename(
         "saraswati.setup.systemd", "saraswati.service"
     )
-    os.system(f"cp {unitfile} /usr/lib/systemd/system/")
+    pathfile = pkg_resources.resource_filename(
+        "saraswati.setup.systemd", "saraswati.path"
+    )
+    os.system(f"cp {unitfile} {pathfile} /usr/lib/systemd/system/")
 
     os.system(
-        "systemctl daemon-reload && systemctl enable saraswati && systemctl restart saraswati"
+        "systemctl daemon-reload && systemctl enable saraswati"
     )
     print()
-    print("Saraswati service started successfully")
+    print("Saraswati unit enabled successfully. The service will start when /dev/bus/usb is not empty.")
     print()
 
     # print("Start watching logfile using 'journalctl --unit=saraswati --follow'")

saraswati/setup/systemd/saraswati.path

@@ -0,0 +1,9 @@
+# https://superuser.com/questions/1322777/in-systemd-service-file-how-do-i-say-after-usb-is-ready
+# https://www.freedesktop.org/software/systemd/man/systemd.path.html
+
+[Unit]
+Description=Saraswati multi-channel audio recording system
+Documentation=https://github.com/hiveeyes/saraswati
+
+[Path]
+DirectoryNotEmpty=/dev/bus/usb

saraswati/setup/systemd/saraswati.service

@@ -1,7 +1,6 @@
 [Unit]
 Description=Saraswati multi-channel audio recording system
 Documentation=https://github.com/hiveeyes/saraswati
-After=network.target
 
 [Service]
 User=saraswati