units: split out BindsTo= line to a generated dropin

In subsequent commits, systemd-zram-setup@.service will be used
for non-swap purposes, so the BindsTo= line needs to become optional.

Unit descriptions are changed to use titlecase. This is the general
recommended systemd style, because those strings are used in messages
like "Starting ..." and "Stopping ...", and a non-capitalized string
looks strange.

src/generator.rs

@@ -90,6 +90,22 @@ pub fn run_generator(devices: &[Device], output_directory: &Path, fake_mode: boo
     Ok(())
 }
 
+fn write_contents(output_directory: &Path, filename: &str, contents: &str) -> Result<()> {
+    let path = output_directory.join(filename);
+    make_parent(&path)?;
+
+    let contents = format!(
+        "\
+# Automatically generated by {exe_name}
+
+{contents}",
+        exe_name = std::env::current_exe().unwrap().display(),
+        contents = contents
+    );
+
+    fs::write(&path, contents).with_context(|| format!("Failed to write {:?}", path))
+}
+
 fn handle_device(output_directory: &Path, device: &Device) -> Result<u64> {
     let swap_name = format!("dev-{}.swap", device.name);
     info!(
@@ -99,14 +115,30 @@ fn handle_device(output_directory: &Path, device: &Device) -> Result<u64> {
         device.disksize / 1024 / 1024
     );
 
-    let swap_path = output_directory.join(&swap_name);
+    /* systemd-zram-setup@.service.
+     * We use the packaged unit, and only need to provide a small drop-in. */
 
-    let contents = format!(
+    write_contents(
+        output_directory,
+        &format!(
+            "systemd-zram-setup@{}.service.d/bindsto-swap.conf",
+            device.name
+        ),
         "\
-# Automatically generated by {exe_name}
+[Unit]
+BindsTo=dev-%i.swap
+",
+    )?;
+
+    /* dev-zramX.swap */
 
+    write_contents(
+        output_directory,
+        &swap_name,
+        &format!(
+            "\
 [Unit]
-Description=Compressed swap on /dev/{zram_device}
+Description=Compressed Swap on /dev/{zram_device}
 Documentation=man:zram-generator(8) man:zram-generator.conf(5)
 Requires=systemd-zram-setup@{zram_device}.service
 After=systemd-zram-setup@{zram_device}.service
@@ -115,20 +147,18 @@ After=systemd-zram-setup@{zram_device}.service
 What=/dev/{zram_device}
 Priority=100
 ",
-        exe_name = std::env::current_exe().unwrap().display(),
-        zram_device = device.name
-    );
-    fs::write(&swap_path, contents).with_context(|| {
-        format!(
-            "Failed to write a swap service into {}",
-            swap_path.display()
-        )
-    })?;
+            zram_device = device.name
+        ),
+    )?;
+
+    /* enablement symlink */
 
     let symlink_path = output_directory.join("swap.target.wants").join(&swap_name);
     let target_path = format!("../{}", swap_name);
     make_symlink(&target_path, &symlink_path)?;
 
+    /* Return the device number */
+
     device.name[4..]
         .parse()
         .with_context(|| format!("zram device \"{}\" number", device.name))

tests/01-basic/run.expected/units/dev-zram0.swap

@@ -1,7 +1,7 @@
 # Automatically generated by zram-generator
 
 [Unit]
-Description=Compressed swap on /dev/zram0
+Description=Compressed Swap on /dev/zram0
 Documentation=man:zram-generator(8) man:zram-generator.conf(5)
 Requires=systemd-zram-setup@zram0.service
 After=systemd-zram-setup@zram0.service

tests/01-basic/run.expected/units/systemd-zram-setup@zram0.service.d/bindsto-swap.conf

@@ -0,0 +1,4 @@
+# Automatically generated by zram-generator
+
+[Unit]
+BindsTo=dev-%i.swap

tests/02-zstd/run.expected/units/dev-zram0.swap

@@ -1,7 +1,7 @@
 # Automatically generated by zram-generator
 
 [Unit]
-Description=Compressed swap on /dev/zram0
+Description=Compressed Swap on /dev/zram0
 Documentation=man:zram-generator(8) man:zram-generator.conf(5)
 Requires=systemd-zram-setup@zram0.service
 After=systemd-zram-setup@zram0.service

tests/02-zstd/run.expected/units/systemd-zram-setup@zram0.service.d/bindsto-swap.conf

@@ -0,0 +1,4 @@
+# Automatically generated by zram-generator
+
+[Unit]
+BindsTo=dev-%i.swap

tests/04-dropins/run.expected/units/dev-zram0.swap

@@ -1,7 +1,7 @@
 # Automatically generated by zram-generator
 
 [Unit]
-Description=Compressed swap on /dev/zram0
+Description=Compressed Swap on /dev/zram0
 Documentation=man:zram-generator(8) man:zram-generator.conf(5)
 Requires=systemd-zram-setup@zram0.service
 After=systemd-zram-setup@zram0.service

tests/04-dropins/run.expected/units/dev-zram2.swap

@@ -1,7 +1,7 @@
 # Automatically generated by zram-generator
 
 [Unit]
-Description=Compressed swap on /dev/zram2
+Description=Compressed Swap on /dev/zram2
 Documentation=man:zram-generator(8) man:zram-generator.conf(5)
 Requires=systemd-zram-setup@zram2.service
 After=systemd-zram-setup@zram2.service

tests/04-dropins/run.expected/units/systemd-zram-setup@zram0.service.d/bindsto-swap.conf

@@ -0,0 +1,4 @@
+# Automatically generated by zram-generator
+
+[Unit]
+BindsTo=dev-%i.swap

tests/04-dropins/run.expected/units/systemd-zram-setup@zram2.service.d/bindsto-swap.conf

@@ -0,0 +1,4 @@
+# Automatically generated by zram-generator
+
+[Unit]
+BindsTo=dev-%i.swap

tests/05-kernel-disabled/run.expected/units/systemd-zram-setup@zram0.service.d/bindsto-swap.conf

@@ -0,0 +1,3 @@
+# Automatically generated by zram-generator
+[Unit]
+BindsTo=dev-%i.swap

tests/06-kernel-enabled/run.expected/units/dev-zram0.swap

@@ -1,7 +1,7 @@
 # Automatically generated by zram-generator
 
 [Unit]
-Description=Compressed swap on /dev/zram0
+Description=Compressed Swap on /dev/zram0
 Documentation=man:zram-generator(8) man:zram-generator.conf(5)
 Requires=systemd-zram-setup@zram0.service
 After=systemd-zram-setup@zram0.service

tests/06-kernel-enabled/run.expected/units/systemd-zram-setup@zram0.service.d/bindsto-swap.conf

@@ -0,0 +1,4 @@
+# Automatically generated by zram-generator
+
+[Unit]
+BindsTo=dev-%i.swap

units/systemd-zram-setup@.service.in

@@ -6,7 +6,6 @@
 Description=Create swap on /dev/%i
 Documentation=man:zram-generator(8) man:zram-generator.conf(5)
 After=dev-%i.device
-BindsTo=dev-%i.swap
 DefaultDependencies=false
 
 [Service]