providers: setup network kernel arguments in initrd

This adds initial/experimental support for providing supplemental
network kernel arguments in the initrd.
It is meant to support environment where custom network kargs may
be provided via a back-channel.
Such arguments are only retrieved and applied on first boot, and
their format is defined by dracut [0].
The feature is currently reachable as a dedicated subcommand,
but this does not contain any platform-specific implementation.

[0] https://mirrors.edge.kernel.org/pub/linux/utils/boot/dracut/dracut.html#_network

dracut/30afterburn/afterburn-network-kargs.service

@@ -0,0 +1,20 @@
+[Unit]
+Description=Afterburn Initrd Setup Network Kernel Arguments
+
+# This service may produce additional kargs fragments,
+# which are then consumed by dracut-cmdline(8).
+DefaultDependencies=no
+Before=dracut-cmdline.service
+Before=ignition-fetch.service
+After=systemd-journald.socket
+
+OnFailure=emergency.target
+OnFailureJobMode=isolate
+
+[Service]
+Environment=AFTERBURN_OPT_PROVIDER=--cmdline
+# AFTERBURN_NETWORK_KARGS_DEFAULT must be set externally by distributions.
+# If unset, variable expansion results in a missing argument and service
+# hard-failure, on purpose.
+ExecStart=/usr/bin/afterburn exp rd-network-kargs ${AFTERBURN_OPT_PROVIDER} --default-value $AFTERBURN_NETWORK_KARGS_DEFAULT
+Type=oneshot

dracut/30afterburn/module-setup.sh

@@ -16,9 +16,12 @@ install() {
     inst_simple "$moddir/afterburn-hostname.service" \
         "$systemdutildir/system/afterburn-hostname.service"
 
-    # We want the afterburn-hostname to be firstboot only, so Ignition-provided
-    # hostname changes do not get overwritten on subsequent boots
+    inst_simple "$moddir/afterburn-network-kargs.service" \
+        "$systemdutildir/system/afterburn-network-kargs.service"
 
+    # These services are only run once on first-boot, so they piggyback
+    # on Ignition completion target.
     mkdir -p "$initdir/$systemdsystemunitdir/ignition-complete.target.requires"
     ln -s "../afterburn-hostname.service" "$initdir/$systemdsystemunitdir/ignition-complete.target.requires/afterburn-hostname.service"
+    ln -s "../afterburn-network-kargs.service" "$initdir/$systemdsystemunitdir/ignition-complete.target.requires/afterburn-network-kargs.service"
 }

src/cli/exp.rs

@@ -0,0 +1,74 @@
+//! `exp` CLI sub-command.
+
+use crate::errors::*;
+use crate::{initrd, util};
+use clap::ArgMatches;
+use error_chain::bail;
+
+/// Experimental subcommands.
+#[derive(Debug)]
+pub enum CliExp {
+    RdNetworkKargs(CliRdNetworkKargs),
+}
+
+impl CliExp {
+    /// Parse sub-command into configuration.
+    pub(crate) fn parse(app_matches: &ArgMatches) -> Result<super::CliConfig> {
+        if app_matches.subcommand_name().is_none() {
+            bail!("missing subcommand for 'exp'");
+        }
+
+        let cfg = match app_matches.subcommand() {
+            ("rd-network-kargs", Some(matches)) => CliRdNetworkKargs::parse(matches)?,
+            (x, _) => unreachable!("unrecognized subcommand for 'exp': '{}'", x),
+        };
+
+        Ok(super::CliConfig::Exp(cfg))
+    }
+
+    // Run sub-command.
+    pub(crate) fn run(&self) -> Result<()> {
+        match self {
+            CliExp::RdNetworkKargs(cmd) => cmd.run()?,
+        };
+        Ok(())
+    }
+}
+
+/// Sub-command for network kernel arguments.
+#[derive(Debug)]
+pub struct CliRdNetworkKargs {
+    platform: String,
+    default_kargs: String,
+}
+
+impl CliRdNetworkKargs {
+    /// Parse sub-command into configuration.
+    pub(crate) fn parse(matches: &ArgMatches) -> Result<CliExp> {
+        let platform = super::parse_provider(matches)?;
+        let default_kargs = matches
+            .value_of("default-value")
+            .ok_or_else(|| "missing network kargs default value")?
+            .to_string();
+
+        let cfg = Self {
+            platform,
+            default_kargs,
+        };
+        Ok(CliExp::RdNetworkKargs(cfg))
+    }
+
+    /// Run the sub-command.
+    pub(crate) fn run(&self) -> Result<()> {
+        if util::has_network_kargs(super::CMDLINE_PATH)? {
+            slog_scope::warn!("kernel cmdline already specifies network arguments, skipping");
+            return Ok(());
+        };
+
+        let provider_kargs = initrd::fetch_network_kargs(&self.platform)?;
+        let kargs = provider_kargs
+            .as_ref()
+            .unwrap_or_else(|| &self.default_kargs);
+        initrd::write_network_kargs(kargs)
+    }
+}