engineering: Add AZL4 distro detection and extend GRUB path to AZL4

crates/osutils/src/mkinitrd.rs

@@ -118,6 +118,8 @@ mod functional_test {
     fn test_regenerate_initrd() {
         let pattern = if osrelease::is_azl3().unwrap() {
             "/boot/initramfs-*.azl3.img"
+        } else if osrelease::is_azl4().unwrap() {
+            "/boot/initramfs-*.azl4.img"
         } else {
             "/boot/initrd.img-*"
         };

crates/osutils/src/osrelease.rs

@@ -31,6 +31,11 @@ pub fn is_azl3() -> Result<bool, Error> {
     Ok(OsRelease::read()?.get_distro().is_azl3())
 }
 
+/// Returns whether the host is running Azure Linux 4.
+pub fn is_azl4() -> Result<bool, Error> {
+    Ok(OsRelease::read()?.get_distro().is_azl4())
+}
+
 /// Represents the contents of the /etc/os-release file.
 ///
 /// See <https://www.freedesktop.org/software/systemd/man/latest/os-release.html>
@@ -146,6 +151,8 @@ impl OsRelease {
                             AzureLinuxRelease::AzL2
                         } else if v.starts_with("3.") {
                             AzureLinuxRelease::AzL3
+                        } else if v.starts_with("4.") {
+                            AzureLinuxRelease::AzL4
                         } else {
                             trace!("Unknown Azure Linux release: {v}");
                             AzureLinuxRelease::Other
@@ -342,6 +349,10 @@ impl Distro {
         self == &Distro::AzureLinux(AzureLinuxRelease::AzL3)
     }
 
+    pub fn is_azl4(&self) -> bool {
+        self == &Distro::AzureLinux(AzureLinuxRelease::AzL4)
+    }
+
     pub fn is_acl(&self) -> bool {
         self == &Distro::AzureContainerLinux
     }
@@ -354,6 +365,7 @@ pub enum AzureLinuxRelease {
     Other,
     AzL2,
     AzL3,
+    AzL4,
 }
 
 #[cfg(test)]
@@ -429,6 +441,41 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_parse_azl4() {
+        let data = indoc::indoc! {
+            r#"
+            NAME="Azure Linux"
+            VERSION="4.0 (Four Alpha2)"
+            RELEASE_TYPE=development
+            ID=azurelinux
+            ID_LIKE=fedora
+            VERSION_ID="4.0"
+            VERSION_CODENAME=""
+            PRETTY_NAME="Azure Linux 4.0 (Four Alpha2)"
+            ANSI_COLOR="0;38;2;60;110;180"
+            LOGO=azurelinux-logo-icon
+            CPE_NAME="cpe:/o:azurelinuxproject:azurelinux:4.0"
+            DEFAULT_HOSTNAME="azurelinux"
+            HOME_URL="https://aka.ms/azurelinux"
+            DOCUMENTATION_URL="https://aka.ms/azurelinux"
+            SUPPORT_URL="https://aka.ms/azurelinux"
+            BUG_REPORT_URL="https://aka.ms/azurelinux"
+            SUPPORT_END=2026-05-15
+            "#,
+        };
+
+        let os_release = OsRelease::parse(data);
+        assert_eq!(os_release.id, Some("azurelinux".to_string()));
+        assert_eq!(os_release.version_id, Some("4.0".to_string()));
+        assert_eq!(os_release.id_like, Some("fedora".to_string()));
+        assert_eq!(os_release.release_type, Some("development".to_string()));
+        assert_eq!(
+            os_release.get_distro(),
+            Distro::AzureLinux(AzureLinuxRelease::AzL4)
+        );
+    }
+
     #[test]
     fn test_parse_extension_release() {
         let data = indoc::indoc! {

crates/osutils/src/testutils/osrelease.rs

@@ -38,11 +38,36 @@ const AZURE_LINUX_3_OS_RELEASE: &str = indoc::indoc! {
     "#,
 };
 
+/// Azure Linux 4.0 sample os-release file.
+const AZURE_LINUX_4_OS_RELEASE: &str = indoc::indoc! {
+    r#"
+    NAME="Azure Linux"
+    VERSION="4.0 (Cloud Variant Beta)"
+    RELEASE_TYPE=development
+    ID=azurelinux
+    ID_LIKE=fedora
+    VERSION_ID="4.0"
+    VERSION_CODENAME=""
+    PRETTY_NAME="Azure Linux 4.0 (Cloud Variant Beta)"
+    ANSI_COLOR="0;38;2;60;110;180"
+    LOGO=azurelinux-logo-icon
+    CPE_NAME="cpe:/o:azurelinuxproject:azurelinux:4.0"
+    DEFAULT_HOSTNAME="azurelinux"
+    HOME_URL="https://aka.ms/azurelinux"
+    DOCUMENTATION_URL="https://aka.ms/azurelinux"
+    SUPPORT_URL="https://aka.ms/azurelinux"
+    BUG_REPORT_URL="https://aka.ms/azurelinux"
+    VARIANT="Cloud Variant"
+    VARIANT_ID=cloud
+    "#,
+};
+
 /// Creates a mock /etc/os-release file with the given Azure Linux release.
 pub fn make_mock_os_release(root_path: &Path, azl_release: AzureLinuxRelease) -> Result<(), Error> {
     let os_release_content = match azl_release {
         AzureLinuxRelease::AzL2 => AZURE_LINUX_2_OS_RELEASE,
         AzureLinuxRelease::AzL3 => AZURE_LINUX_3_OS_RELEASE,
+        AzureLinuxRelease::AzL4 => AZURE_LINUX_4_OS_RELEASE,
         AzureLinuxRelease::Other => bail!("Unsupported Azure Linux release 'other'"),
     };
 

crates/trident/src/engine/boot/grub.rs

@@ -63,9 +63,10 @@ pub(super) fn update_configs(ctx: &EngineContext) -> Result<(), Error> {
     let boot_grub_config_path = Path::new(ROOT_MOUNT_POINT_PATH).join(GRUB2_CONFIG_RELATIVE_PATH);
 
     // Update GRUB config on the boot device (volume holding /boot)
-    match ctx.host_os_release.get_distro() {
-        Distro::AzureLinux(AzureLinuxRelease::AzL3) => {
-            update_grub_config_azl3(ctx, &root_device_path, &boot_grub_config_path)?;
+    // Use the *image* distro (the OS being installed), not the host (MOS ISO).
+    match ctx.image_distro() {
+        Distro::AzureLinux(AzureLinuxRelease::AzL3 | AzureLinuxRelease::AzL4) => {
+            update_grub_config(ctx, &root_device_path, &boot_grub_config_path)?;
         }
 
         d => bail!("Unsupported distro for GRUB config update: {d:?}"),
@@ -85,13 +86,13 @@ pub(super) fn update_configs(ctx: &EngineContext) -> Result<(), Error> {
     ))
 }
 
-/// Updates the GRUB config for Azure Linux 3.0 using OS modifier.
-fn update_grub_config_azl3(
+/// Updates the GRUB config for Azure Linux using OS modifier.
+fn update_grub_config(
     ctx: &EngineContext,
     root_device_path: &Path,
     boot_grub_config_path: &Path,
 ) -> Result<(), Error> {
-    // For azl 3.0, we need to disable cloud-init's network configuration when Trident is
+    // For azl 3.0+, we need to disable cloud-init's network configuration when Trident is
     // configuring the network. This is done by setting the 'network-config' kernel parameter
     // to 'disabled'.
     if ctx.spec.os.netplan.is_some() {
@@ -103,7 +104,7 @@ fn update_grub_config_azl3(
             .context("Failed to disable default cloud-init network config")?;
     }
 
-    debug!("Updating GRUB config for Azure Linux 3.0 with OS modifier");
+    debug!("Updating GRUB config with OS modifier");
 
     // OS modifier will read values of verity, selinux, root device, and overlay from original GRUB config
     // stamp them into /etc/default/grub and regenerate the GRUB config using grub-mkconfig.
@@ -222,7 +223,7 @@ fn update_grub_config_azl3(
     osmodifier::modify_boot(&osmod_ctx, &config)
         .context("Failed to apply boot configuration modifications")?;
 
-    debug!("Finished updating GRUB config for Azure Linux 3.0 with OS modifier");
+    debug!("Finished updating GRUB config with OS modifier");
 
     Ok(())
 }

crates/trident/src/engine/context/mod.rs

@@ -441,8 +441,20 @@ impl EngineContext {
     }
 
     /// Retrieves the distribution of the OS image.
+    ///
+    /// Prefers the image's own os-release (e.g., from the COSI being installed).
+    /// Falls back to the host os-release only when no image is mounted
+    /// (functional tests, runtime operations outside an install flow).
+    ///
+    /// If an image IS present but its distro is unrecognized, the image's
+    /// distro is returned as-is (Distro::Other) so callers can bail
+    /// explicitly rather than silently using the host's distro.
     pub(crate) fn image_distro(&self) -> Distro {
-        self.image_os_release().get_distro()
+        if self.image.is_some() {
+            self.image_os_release().get_distro()
+        } else {
+            self.host_os_release.get_distro()
+        }
     }
 }