Detect missing pipewiresrc element before host startup

`gst-launch-1.0` and `pipewiresrc` ship in separate packages on most
distros (Arch: `gst-plugin-pipewire`, Debian: `gstreamer1.0-pipewire`,
Fedora/openSUSE: `pipewire-gstreamer`). Having the gst binary present
was no guarantee the Wayland capture pipeline would actually work —
without the plugin gst would bail at runtime with `no element
"pipewiresrc"`, which then cascaded into ffmpeg seeing EOF on its
stdin and exiting before its HTTP listener bound, then the host
hitting "Connection refused" against its own port. Confusing.

Now `deps::check_host_binaries` probes `gst-inspect-1.0 --exists
pipewiresrc` on Wayland and fails early with a per-distro install
hint pointing at the right package.

src/common/deps.rs

@@ -1,5 +1,6 @@
 use anyhow::{Result, bail};
 use std::path::PathBuf;
+use std::process::Command;
 
 use crate::common::display::DisplayServer;
 
@@ -7,6 +8,8 @@ pub fn check_host_binaries(display: DisplayServer) -> Result<()> {
     require("ffmpeg")?;
     if display == DisplayServer::Wayland {
         require("gst-launch-1.0")?;
+        require("gst-inspect-1.0")?;
+        require_gst_element("pipewiresrc")?;
     }
     Ok(())
 }
@@ -14,11 +17,23 @@ pub fn check_host_binaries(display: DisplayServer) -> Result<()> {
 fn require(bin: &str) -> Result<PathBuf> {
     match which(bin) {
         Some(p) => Ok(p),
-        None => {
-            let hint = install_hint(bin);
-            bail!("`{bin}` not found on PATH.\n{hint}");
+        None => bail!("`{bin}` not found on PATH.\n{}", install_hint_for_bin(bin)),
     }
+}
+
+fn require_gst_element(name: &str) -> Result<()> {
+    let ok = Command::new("gst-inspect-1.0")
+        .args(["--exists", name])
+        .status()
+        .map(|s| s.success())
+        .unwrap_or(false);
+    if !ok {
+        bail!(
+            "GStreamer element `{name}` not available.\n{}",
+            install_hint_for_gst_element(name)
+        );
     }
+    Ok(())
 }
 
 fn which(bin: &str) -> Option<PathBuf> {
@@ -32,19 +47,37 @@ fn which(bin: &str) -> Option<PathBuf> {
     None
 }
 
-fn install_hint(bin: &str) -> String {
+fn install_hint_for_bin(bin: &str) -> String {
     let distro = detect_distro();
     let pkg = match bin {
         "ffmpeg" => "ffmpeg",
-        "gst-launch-1.0" => match distro.as_deref() {
-            Some("arch" | "cachyos" | "manjaro" | "endeavouros") => "gst-plugins-base gst-plugins-good",
-            Some("debian" | "ubuntu" | "pop" | "linuxmint") => "gstreamer1.0-tools gstreamer1.0-plugins-good",
-            Some("fedora" | "nobara") => "gstreamer1-plugins-good",
-            _ => "gstreamer (and tools / good-plugins)",
+        "gst-launch-1.0" | "gst-inspect-1.0" => match distro.as_deref() {
+            Some("arch" | "cachyos" | "manjaro" | "endeavouros") => "gstreamer gst-plugins-base",
+            Some("debian" | "ubuntu" | "pop" | "linuxmint") => "gstreamer1.0-tools",
+            Some("fedora" | "nobara") => "gstreamer1 gstreamer1-plugins-base-tools",
+            _ => "gstreamer + tools",
         },
         _ => bin,
     };
+    install_command(&distro, pkg)
+}
 
+fn install_hint_for_gst_element(name: &str) -> String {
+    let distro = detect_distro();
+    let pkg = match name {
+        "pipewiresrc" => match distro.as_deref() {
+            Some("arch" | "cachyos" | "manjaro" | "endeavouros") => "gst-plugin-pipewire",
+            Some("debian" | "ubuntu" | "pop" | "linuxmint") => "gstreamer1.0-pipewire",
+            Some("fedora" | "nobara") => "pipewire-gstreamer",
+            Some("opensuse" | "opensuse-tumbleweed" | "opensuse-leap") => "pipewire-gstreamer",
+            _ => "the GStreamer PipeWire plugin",
+        },
+        _ => name,
+    };
+    install_command(&distro, pkg)
+}
+
+fn install_command(distro: &Option<String>, pkg: &str) -> String {
     let cmd = match distro.as_deref() {
         Some("arch" | "cachyos" | "manjaro" | "endeavouros") => format!("sudo pacman -S {pkg}"),
         Some("debian" | "ubuntu" | "pop" | "linuxmint") => format!("sudo apt install {pkg}"),
@@ -52,7 +85,6 @@ fn install_hint(bin: &str) -> String {
         Some("opensuse" | "opensuse-tumbleweed" | "opensuse-leap") => format!("sudo zypper install {pkg}"),
         _ => format!("install the `{pkg}` package via your distro's package manager"),
     };
-
     format!("Install hint: {cmd}")
 }