feat(debian): auto-add an Exec arg (field code) in the .desktop file

.changes/pr256.md

@@ -0,0 +1,13 @@
+---
+"cargo-packager": "patch"
+"@crabnebula/packager": "patch"
+---
+
+Automatically add an Exec arg (field code) in the `.desktop` file.
+
+This adds an `{exec_arg}` field to the default `main.desktop` template.
+This field is populated with a sane default value, based on the
+`deep_link_protocols` or `file_associations` in the `Config` struct.
+
+This allows an installed Debian package to be invoked by other
+applications with URLs or files as arguments, as expected.

crates/packager/src/config/mod.rs

@@ -194,7 +194,7 @@ pub struct DebianConfig {
     /// {{#if comment}}
     /// Comment={{comment}}
     /// {{/if}}
-    /// Exec={{exec}}
+    /// Exec={{exec}} {{exec_arg}}
     /// Icon={{icon}}
     /// Name={{name}}
     /// Terminal=false
@@ -203,6 +203,21 @@ pub struct DebianConfig {
     /// MimeType={{mime_type}}
     /// {{/if}}
     /// ```
+    ///
+    /// The `exec_arg` will be set to:
+    /// * "%U", if at least one [Config::deep_link_protocols] was specified.
+    ///   * This means that your application can accommodate being invoked with multiple URLs.
+    /// * "%F", if at least one [Config::file_associations] was specified but no deep link protocols were given, or
+    ///   * This means that your application can accommodate being invoked with multiple file paths.
+    /// * An empty string "" (nothing) otherwise.
+    ///   * This means that your application will never be invoked with any URLs or file paths.
+    ///
+    /// To specify a custom `exec_arg`, just use plaintext directly instead of `{{exec_arg}}`:
+    /// ```text
+    /// Exec={{exec}} %u
+    /// ```
+    ///
+    /// See more here: <https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#exec-variables>.
     #[serde(alias = "desktop-template", alias = "desktop_template")]
     pub desktop_template: Option<PathBuf>,
     /// Define the section in Debian Control file. See : <https://www.debian.org/doc/debian-policy/ch-archive.html#s-subsections>
@@ -243,7 +258,7 @@ impl DebianConfig {
     /// {{#if comment}}
     /// Comment={{comment}}
     /// {{/if}}
-    /// Exec={{exec}}
+    /// Exec={{exec}} {{exec_arg}}
     /// Icon={{icon}}
     /// Name={{name}}
     /// Terminal=false

crates/packager/src/package/deb/main.desktop

@@ -3,7 +3,7 @@ Categories={{categories}}
 {{#if comment}}
 Comment={{comment}}
 {{/if}}
-Exec={{exec}}
+Exec={{exec}} {{exec_arg}}
 Icon={{icon}}
 Name={{name}}
 Terminal=false

crates/packager/src/package/deb/mod.rs

@@ -96,8 +96,8 @@ fn generate_desktop_file(config: &Config, data_dir: &Path) -> crate::Result<()>
         .join("usr/share/applications")
         .join(desktop_file_name);
 
-    // For more information about the format of this file, see
-    // https://developer.gnome.org/integration-guide/stable/desktop-files.html.en
+    // For more information about the format of this file, see:
+    // <https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html>
     let file = &mut util::create_file(&desktop_file_path)?;
 
     let mut handlebars = Handlebars::new();
@@ -117,14 +117,22 @@ fn generate_desktop_file(config: &Config, data_dir: &Path) -> crate::Result<()>
         categories: &'a str,
         comment: Option<&'a str>,
         exec: &'a str,
+        exec_arg: Option<&'a str>,
         icon: &'a str,
         name: &'a str,
         mime_type: Option<String>,
     }
 
+    // Set the argument code at the end of the `Exec` key.
+    // See the docs for `DebianConfig::desktop_template` for more details.
+    let mut exec_arg = None;
+
     let mut mime_type: Vec<String> = Vec::new();
 
     if let Some(associations) = &config.file_associations {
+        if !associations.is_empty() {
+            exec_arg = Some("%F");
+        }
         mime_type.extend(
             associations
                 .iter()
@@ -133,6 +141,11 @@ fn generate_desktop_file(config: &Config, data_dir: &Path) -> crate::Result<()>
     }
 
     if let Some(protocols) = &config.deep_link_protocols {
+        if !protocols.is_empty() {
+            // Use "%U" even if file associations were already provided,
+            // as it can also accommodate file names in addition to URLs.
+            exec_arg = Some("%U");
+        }
         mime_type.extend(
             protocols
                 .iter()
@@ -152,6 +165,7 @@ fn generate_desktop_file(config: &Config, data_dir: &Path) -> crate::Result<()>
                 .unwrap_or(""),
             comment: config.description.as_deref(),
             exec: &bin_name,
+            exec_arg,
             icon: &bin_name,
             name: config.product_name.as_str(),
             mime_type,