Make generate_grub_config return strings instead of writing files

Separate config generation from file I/O by having generate_grub_config()
and its helpers return strings. The base class make_bootable() now handles
writing grub.cfg.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

live-build/isobuilder/boot/amd64.py

@@ -87,102 +87,95 @@ class AMD64BootConfigurator(UEFIBootConfigurator):
                     ["*.mod", "*.lst", "*.o"],
                 )
 
-    def generate_grub_config(self) -> None:
-        """Generate grub.cfg and loopback.cfg for the boot tree."""
-        boot_grub_dir = self.iso_root.joinpath("boot", "grub")
-        boot_grub_dir.mkdir(parents=True, exist_ok=True)
-
-        grub_cfg = boot_grub_dir.joinpath("grub.cfg")
+    def generate_grub_config(self) -> str:
+        """Generate grub.cfg content for AMD64."""
+        result = self.grub_header()
 
         if self.project == "ubuntu-mini-iso":
-            self.write_grub_header(grub_cfg)
-            with grub_cfg.open("a") as f:
-                f.write(
-                    """menuentry "Choose an Ubuntu version to install" {
+            result += """\
+menuentry "Choose an Ubuntu version to install" {
     set gfxpayload=keep
     linux    /casper/vmlinuz iso-chooser-menu ip=dhcp ---
     initrd    /casper/initrd
 }
 """
-                )
-            return
+            return result
 
-        # Generate grub.cfg
         kernel_params = default_kernel_params(self.project)
 
-        # Write common GRUB header
-        self.write_grub_header(grub_cfg)
-
         # Main menu entry
-        with grub_cfg.open("a") as f:
-            f.write(
-                f"""menuentry "Try or Install {self.humanproject}" {{
+        result += f"""\
+menuentry "Try or Install {self.humanproject}" {{
     set gfxpayload=keep
     linux    /casper/vmlinuz {kernel_params}
     initrd    /casper/initrd
 }}
 """
-            )
 
         # All but server get safe-graphics mode
         if self.project != "ubuntu-server":
-            with grub_cfg.open("a") as f:
-                f.write(
-                    f"""menuentry "{self.humanproject} (safe graphics)" {{
+            result += f"""\
+menuentry "{self.humanproject} (safe graphics)" {{
     set gfxpayload=keep
     linux    /casper/vmlinuz nomodeset {kernel_params}
     initrd    /casper/initrd
 }}
 """
-                )
 
         # ubiquity based projects get OEM mode
         if "maybe-ubiquity" in kernel_params:
             oem_kernel_params = kernel_params.replace(
                 "maybe-ubiquity", "only-ubiquity oem-config/enable=true"
             )
-            with grub_cfg.open("a") as f:
-                f.write(
-                    f"""menuentry "OEM install (for manufacturers)" {{
+            result += f"""\
+menuentry "OEM install (for manufacturers)" {{
     set gfxpayload=keep
     linux    /casper/vmlinuz {oem_kernel_params}
     initrd    /casper/initrd
 }}
 """
-                )
 
         # Calamares-based projects get OEM mode
         if self.project in CALAMARES_PROJECTS:
-            with grub_cfg.open("a") as f:
-                f.write(
-                    f"""menuentry "OEM install (for manufacturers)" {{
+            result += f"""\
+menuentry "OEM install (for manufacturers)" {{
     set gfxpayload=keep
-    linux    /casper/vmlinuz {kernel_params} oem-config/enable=true
-    initrd    /casper/initrd
+    linux  /casper/vmlinuz {kernel_params} oem-config/enable=true
+    initrd /casper/initrd
 }}
 """
-                )
 
         # Currently only server is built with HWE, hence no safe-graphics/OEM
         if self.hwe:
-            with grub_cfg.open("a") as f:
-                f.write(
-                    f"""menuentry "{self.humanproject} with the HWE kernel" {{
+            result += f"""\
+menuentry "{self.humanproject} with the HWE kernel" {{
     set gfxpayload=keep
-    linux    /casper/hwe-vmlinuz {kernel_params}
-    initrd    /casper/hwe-initrd
+    linux  /casper/hwe-vmlinuz {kernel_params}
+    initrd /casper/hwe-initrd
 }}
 """
-                )
 
-        # Create the loopback config, based on the main config
-        with grub_cfg.open("r") as f:
-            content = f.read()
+        # UEFI Entries (wrapped in grub_platform check for dual BIOS/UEFI support)
+        uefi_menu_entries = self.uefi_menu_entries()
 
-        # sed: delete from line 1 to menu_color_highlight, delete from
-        # grub_platform to end and replace '---' with
-        # 'iso-scan/filename=${iso_path} ---' in lines with 'linux'
-        lines = content.split("\n")
+        result += f"""\
+grub_platform
+if [ "$grub_platform" = "efi" ]; then
+{uefi_menu_entries}
+fi
+"""
+
+        return result
+
+    @staticmethod
+    def generate_loopback_config(grub_content: str) -> str:
+        """Derive loopback.cfg from grub.cfg content.
+
+        Strips the header (up to menu_color_highlight) and the UEFI
+        trailer (from grub_platform to end), and adds iso-scan/filename
+        to linux lines.
+        """
+        lines = grub_content.split("\n")
         start_idx = 0
         for i, line in enumerate(lines):
             if "menu_color_highlight" in line:
@@ -205,16 +198,20 @@ class AMD64BootConfigurator(UEFIBootConfigurator):
             for line in loopback_lines
         ]
 
-        loopback_cfg = boot_grub_dir.joinpath("loopback.cfg")
-        with loopback_cfg.open("w") as f:
-            f.write("\n".join(loopback_lines))
+        return "\n".join(loopback_lines)
 
-        # UEFI Entries (wrapped in grub_platform check for dual BIOS/UEFI support)
-        with grub_cfg.open("a") as f:
-            f.write("grub_platform\n")
-            f.write('if [ "$grub_platform" = "efi" ]; then\n')
-
-        self.write_uefi_menu_entries(grub_cfg)
-
-        with grub_cfg.open("a") as f:
-            f.write("fi\n")
+    def make_bootable(
+        self,
+        workdir: pathlib.Path,
+        project: str,
+        capproject: str,
+        subarch: str,
+        hwe: bool,
+    ) -> None:
+        """Make the ISO bootable, including generating loopback.cfg."""
+        super().make_bootable(workdir, project, capproject, subarch, hwe)
+        grub_cfg = self.iso_root.joinpath("boot", "grub", "grub.cfg")
+        grub_content = grub_cfg.read_text()
+        self.iso_root.joinpath("boot", "grub", "loopback.cfg").write_text(
+            self.generate_loopback_config(grub_content)
+        )

live-build/isobuilder/boot/arm64.py

@@ -33,19 +33,15 @@ class ARM64BootConfigurator(UEFIBootConfigurator):
         with self.logger.logged("extracting ARM64 boot files"):
             self.extract_uefi_files()
 
-    def generate_grub_config(self) -> None:
+    def generate_grub_config(self) -> str:
         """Generate grub.cfg for ARM64."""
         kernel_params = default_kernel_params(self.project)
 
-        grub_cfg = self.iso_root.joinpath("boot", "grub", "grub.cfg")
-
-        # Write common GRUB header
-        self.write_grub_header(grub_cfg)
+        result = self.grub_header()
 
         # ARM64-specific: Snapdragon workarounds
-        with grub_cfg.open("a") as f:
-            f.write(
-                """set cmdline=
+        result += f"""\
+set cmdline=
 smbios --type 4 --get-string 5 --set proc_version
 regexp "Snapdragon.*" "$proc_version"
 if [ $? = 0 ]; then
@@ -63,11 +59,9 @@ menuentry "Try or Install {self.humanproject}" {{
 \tinitrd\t/casper/initrd
 }}
 """
-            )
 
         # HWE kernel option if available
-        self.write_hwe_menu_entry(
-            grub_cfg,
+        result += self.hwe_menu_entry(
             "vmlinuz",
             f"{kernel_params} console=tty0",
             extra_params="$cmdline ",
@@ -77,4 +71,6 @@ menuentry "Try or Install {self.humanproject}" {{
         # but it's not actually set anywhere in the grub.cfg, so we omit it here
 
         # UEFI Entries (ARM64 is UEFI-only, no grub_platform check needed)
-        self.write_uefi_menu_entries(grub_cfg)
+        result += self.uefi_menu_entries()
+
+        return result

live-build/isobuilder/boot/grub.py

@@ -39,59 +39,52 @@ class GrubBootConfigurator(BaseBootConfigurator):
     Subclasses must implement generate_grub_config().
     """
 
-    def write_grub_header(
-        self, grub_cfg: pathlib.Path, include_loadfont: bool = True
-    ) -> None:
-        """Write common GRUB config header (timeout, colors).
+    def grub_header(self, include_loadfont: bool = True) -> str:
+        """Return common GRUB config header (timeout, colors).
 
         Args:
-            grub_cfg: Path to grub.cfg file
             include_loadfont: Whether to include 'loadfont unicode'
                               (not needed for RISC-V)
         """
-        with grub_cfg.open("a") as f:
-            f.write("set timeout=30\n\n")
-            if include_loadfont:
-                f.write("loadfont unicode\n\n")
-            f.write(
-                """set menu_color_normal=white/black
+        result = "set timeout=30\n\n"
+        if include_loadfont:
+            result += "loadfont unicode\n\n"
+        result += """\
+set menu_color_normal=white/black
 set menu_color_highlight=black/light-gray
 
 """
-            )
+        return result
 
-    def write_hwe_menu_entry(
+    def hwe_menu_entry(
         self,
-        grub_cfg: pathlib.Path,
         kernel_name: str,
         kernel_params: str,
         extra_params: str = "",
-    ) -> None:
-        """Write HWE kernel menu entry if HWE is enabled.
+    ) -> str:
+        """Return HWE kernel menu entry if HWE is enabled.
 
         Args:
-            grub_cfg: Path to grub.cfg file
             kernel_name: Kernel binary name (vmlinuz or vmlinux)
             kernel_params: Kernel parameters to append
             extra_params: Additional parameters (e.g., console=tty0, $cmdline)
         """
-        if self.hwe:
-            with grub_cfg.open("a") as f:
-                f.write(
-                    f"""menuentry "{self.humanproject} with the HWE kernel" {{
-\tset gfxpayload=keep
-\tlinux\t/casper/hwe-{kernel_name} {extra_params}{kernel_params}
-\tinitrd\t/casper/hwe-initrd
+        if not self.hwe:
+            return ""
+        return f"""\
+menuentry "{self.humanproject} with the HWE kernel" {{
+    set gfxpayload=keep
+    linux /casper/hwe-{kernel_name} {extra_params}{kernel_params}
+    initrd /casper/hwe-initrd
 }}
 """
-                )
 
     @abstractmethod
-    def generate_grub_config(self) -> None:
-        """Generate grub.cfg configuration file.
+    def generate_grub_config(self) -> str:
+        """Generate grub.cfg content.
 
-        Each GRUB-based architecture must implement this to create its
-        specific GRUB configuration.
+        Each GRUB-based architecture must implement this to return the
+        GRUB configuration.
         """
         ...
 
@@ -106,4 +99,7 @@ set menu_color_highlight=black/light-gray
         """Make the ISO bootable by extracting files and generating GRUB config."""
         super().make_bootable(workdir, project, capproject, subarch, hwe)
         with self.logger.logged("generating grub config"):
-            self.generate_grub_config()
+            content = self.generate_grub_config()
+            grub_dir = self.iso_root.joinpath("boot", "grub")
+            grub_dir.mkdir(parents=True, exist_ok=True)
+            grub_dir.joinpath("grub.cfg").write_text(content)

live-build/isobuilder/boot/ppc64el.py

@@ -53,27 +53,22 @@ class PPC64ELBootConfigurator(GrubBootConfigurator):
             grub_pkg_dir, self.iso_root, "powerpc-ieee1275", ["*.mod", "*.lst"]
         )
 
-    def generate_grub_config(self) -> None:
+    def generate_grub_config(self) -> str:
         """Generate grub.cfg for PPC64EL."""
         kernel_params = default_kernel_params(self.project)
 
-        grub_cfg = self.iso_root.joinpath("boot", "grub", "grub.cfg")
-
-        # Write common GRUB header
-        self.write_grub_header(grub_cfg)
+        result = self.grub_header()
 
         # Main menu entry
-        with grub_cfg.open("a") as f:
-            f.write(
-                f"""menuentry "Try or Install {self.humanproject}" {{
-\tset gfxpayload=keep
-\tlinux\t/casper/vmlinux quiet {kernel_params}
-\tinitrd\t/casper/initrd
+        result += f"""\
+menuentry "Try or Install {self.humanproject}" {{
+    set gfxpayload=keep
+    linux /casper/vmlinux quiet {kernel_params}
+    initrd /casper/initrd
 }}
 """
-            )
 
         # HWE kernel option if available
-        self.write_hwe_menu_entry(
-            grub_cfg, "vmlinux", kernel_params, extra_params="quiet "
-        )
+        result += self.hwe_menu_entry("vmlinux", kernel_params, extra_params="quiet ")
+
+        return result

live-build/isobuilder/boot/riscv64.py

@@ -126,35 +126,28 @@ class RISCV64BootConfigurator(GrubBootConfigurator):
         # Add DTBs to ESP
         self.logger.run(["mcopy", "-s", "-i", efi_img, dtb_dir, "::/."], check=True)
 
-    def generate_grub_config(self) -> None:
+    def generate_grub_config(self) -> str:
         """Generate grub.cfg for RISC-V64."""
-        grub_dir = self.iso_root.joinpath("boot", "grub")
-        grub_dir.mkdir(parents=True, exist_ok=True)
-
-        grub_cfg = grub_dir.joinpath("grub.cfg")
-
-        # Write GRUB header (without loadfont for RISC-V)
-        self.write_grub_header(grub_cfg, include_loadfont=False)
+        result = self.grub_header(include_loadfont=False)
 
         # Main menu entry
-        with grub_cfg.open("a") as f:
-            f.write(
-                f"""menuentry "Try or Install {self.humanproject}" {{
-\tset gfxpayload=keep
-\tlinux\t/casper/vmlinux efi=debug sysctl.kernel.watchdog_thresh=60 ---
-\tinitrd\t/casper/initrd
+        result += f"""\
+menuentry "Try or Install {self.humanproject}" {{
+    set gfxpayload=keep
+    linux /casper/vmlinux efi=debug sysctl.kernel.watchdog_thresh=60 ---
+    initrd /casper/initrd
 }}
 """
-            )
 
         # HWE kernel option if available
-        self.write_hwe_menu_entry(
-            grub_cfg,
+        result += self.hwe_menu_entry(
             "vmlinux",
             "---",
             extra_params="efi=debug sysctl.kernel.watchdog_thresh=60 ",
         )
 
+        return result
+
     def post_process_iso(self, iso_path: pathlib.Path) -> None:
         """Add GPT partitions with U-Boot for SiFive Unmatched board.
 

live-build/isobuilder/boot/uefi.py

@@ -122,18 +122,16 @@ class UEFIBootConfigurator(GrubBootConfigurator):
             self.logger, self.iso_root, self.scratch.joinpath("cd-boot-efi.img")
         )
 
-    def write_uefi_menu_entries(self, grub_cfg: pathlib.Path) -> None:
-        """Write UEFI firmware menu entries."""
-        with grub_cfg.open("a") as f:
-            f.write(
-                """menuentry 'Boot from next volume' {
+    def uefi_menu_entries(self) -> str:
+        """Return UEFI firmware menu entries."""
+        return """\
+menuentry 'Boot from next volume' {
 \texit 1
 }
 menuentry 'UEFI Firmware Settings' {
 \tfwsetup
 }
 """
-            )
 
     def get_uefi_mkisofs_opts(self) -> list[str | pathlib.Path]:
         """Return common UEFI mkisofs options."""