Split internal/daemon vm.go and guest_sessions.go by concern

vm.go (1529 LOC) splits into vm_create, vm_lifecycle, vm_set, vm_stats,
vm_disk, vm_authsync; firecracker/DNS/helpers stay in vm.go.

guest_sessions.go (1266 LOC) splits into session_controller,
session_lifecycle, session_attach, session_stream; scripts and helpers
stay in guest_sessions.go.

Mechanical move only. No behavior change. Adds doc.go and
ARCHITECTURE.md capturing subsystem map and current lock ordering as
the baseline for the upcoming subsystem extraction.

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

internal/daemon/vm_disk.go

@@ -0,0 +1,159 @@
+package daemon
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+
+	"banger/internal/guestconfig"
+	"banger/internal/guestnet"
+	"banger/internal/model"
+	"banger/internal/system"
+)
+
+type workDiskPreparation struct {
+	ClonedFromSeed bool
+}
+
+func (d *Daemon) ensureSystemOverlay(ctx context.Context, vm *model.VMRecord) error {
+	if exists(vm.Runtime.SystemOverlay) {
+		return nil
+	}
+	_, err := d.runner.Run(ctx, "truncate", "-s", strconv.FormatInt(vm.Spec.SystemOverlaySizeByte, 10), vm.Runtime.SystemOverlay)
+	return err
+}
+
+func (d *Daemon) patchRootOverlay(ctx context.Context, vm model.VMRecord, image model.Image) error {
+	resolv := []byte(fmt.Sprintf("nameserver %s\n", d.config.DefaultDNS))
+	hostname := []byte(vm.Name + "\n")
+	hosts := []byte(fmt.Sprintf("127.0.0.1 localhost\n127.0.1.1 %s\n", vm.Name))
+	sshdConfig := []byte(strings.Join([]string{
+		"LogLevel DEBUG3",
+		"PermitRootLogin yes",
+		"PubkeyAuthentication yes",
+		"AuthorizedKeysFile /root/.ssh/authorized_keys",
+		"StrictModes no",
+		"",
+	}, "\n"))
+	fstab, err := system.ReadDebugFSText(ctx, d.runner, vm.Runtime.DMDev, "/etc/fstab")
+	if err != nil {
+		fstab = ""
+	}
+	builder := guestconfig.NewBuilder()
+	builder.WriteFile("/etc/resolv.conf", resolv)
+	builder.WriteFile("/etc/hostname", hostname)
+	builder.WriteFile("/etc/hosts", hosts)
+	builder.WriteFile(guestnet.ConfigPath, guestnet.ConfigFile(vm.Runtime.GuestIP, d.config.BridgeIP, d.config.DefaultDNS))
+	builder.WriteFile(guestnet.GuestScriptPath, []byte(guestnet.BootstrapScript()))
+	builder.WriteFile("/etc/ssh/sshd_config.d/99-banger.conf", sshdConfig)
+	builder.DropMountTarget("/home")
+	builder.DropMountTarget("/var")
+	builder.AddMount(guestconfig.MountSpec{
+		Source:  "tmpfs",
+		Target:  "/run",
+		FSType:  "tmpfs",
+		Options: []string{"defaults", "nodev", "nosuid", "mode=0755"},
+		Dump:    0,
+		Pass:    0,
+	})
+	builder.AddMount(guestconfig.MountSpec{
+		Source:  "tmpfs",
+		Target:  "/tmp",
+		FSType:  "tmpfs",
+		Options: []string{"defaults", "nodev", "nosuid", "mode=1777"},
+		Dump:    0,
+		Pass:    0,
+	})
+	d.contributeGuestConfig(builder, vm, image)
+	builder.WriteFile("/etc/fstab", []byte(builder.RenderFSTab(fstab)))
+	files := builder.Files()
+	for _, guestPath := range builder.FilePaths() {
+		data := files[guestPath]
+		if guestPath == guestnet.GuestScriptPath {
+			if err := system.WriteExt4FileMode(ctx, d.runner, vm.Runtime.DMDev, guestPath, 0o755, data); err != nil {
+				return err
+			}
+			continue
+		}
+		if err := system.WriteExt4File(ctx, d.runner, vm.Runtime.DMDev, guestPath, data); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (d *Daemon) ensureWorkDisk(ctx context.Context, vm *model.VMRecord, image model.Image) (workDiskPreparation, error) {
+	if exists(vm.Runtime.WorkDiskPath) {
+		return workDiskPreparation{}, nil
+	}
+	if exists(image.WorkSeedPath) {
+		vmCreateStage(ctx, "prepare_work_disk", "cloning work seed")
+		if err := system.CopyFilePreferClone(image.WorkSeedPath, vm.Runtime.WorkDiskPath); err != nil {
+			return workDiskPreparation{}, err
+		}
+		seedInfo, err := os.Stat(image.WorkSeedPath)
+		if err != nil {
+			return workDiskPreparation{}, err
+		}
+		if vm.Spec.WorkDiskSizeBytes < seedInfo.Size() {
+			return workDiskPreparation{}, fmt.Errorf("requested work disk size %d is smaller than seed image %d", vm.Spec.WorkDiskSizeBytes, seedInfo.Size())
+		}
+		if vm.Spec.WorkDiskSizeBytes > seedInfo.Size() {
+			vmCreateStage(ctx, "prepare_work_disk", "resizing work disk")
+			if err := system.ResizeExt4Image(ctx, d.runner, vm.Runtime.WorkDiskPath, vm.Spec.WorkDiskSizeBytes); err != nil {
+				return workDiskPreparation{}, err
+			}
+		}
+		return workDiskPreparation{ClonedFromSeed: true}, nil
+	}
+	vmCreateStage(ctx, "prepare_work_disk", "creating empty work disk")
+	if _, err := d.runner.Run(ctx, "truncate", "-s", strconv.FormatInt(vm.Spec.WorkDiskSizeBytes, 10), vm.Runtime.WorkDiskPath); err != nil {
+		return workDiskPreparation{}, err
+	}
+	if _, err := d.runner.Run(ctx, "mkfs.ext4", "-F", vm.Runtime.WorkDiskPath); err != nil {
+		return workDiskPreparation{}, err
+	}
+	rootMount, cleanupRoot, err := system.MountTempDir(ctx, d.runner, vm.Runtime.DMDev, true)
+	if err != nil {
+		return workDiskPreparation{}, err
+	}
+	defer cleanupRoot()
+	workMount, cleanupWork, err := system.MountTempDir(ctx, d.runner, vm.Runtime.WorkDiskPath, false)
+	if err != nil {
+		return workDiskPreparation{}, err
+	}
+	defer cleanupWork()
+	vmCreateStage(ctx, "prepare_work_disk", "copying /root into work disk")
+	if err := system.CopyDirContents(ctx, d.runner, filepath.Join(rootMount, "root"), workMount, true); err != nil {
+		return workDiskPreparation{}, err
+	}
+	if err := d.flattenNestedWorkHome(ctx, workMount); err != nil {
+		return workDiskPreparation{}, err
+	}
+	return workDiskPreparation{}, nil
+}
+
+func (d *Daemon) flattenNestedWorkHome(ctx context.Context, workMount string) error {
+	nestedHome := filepath.Join(workMount, "root")
+	if !exists(nestedHome) {
+		return nil
+	}
+	if _, err := d.runner.RunSudo(ctx, "chmod", "755", nestedHome); err != nil {
+		return err
+	}
+	entries, err := os.ReadDir(nestedHome)
+	if err != nil {
+		return err
+	}
+	for _, entry := range entries {
+		sourcePath := filepath.Join(nestedHome, entry.Name())
+		if _, err := d.runner.RunSudo(ctx, "cp", "-a", sourcePath, workMount+"/"); err != nil {
+			return err
+		}
+	}
+	_, err = d.runner.RunSudo(ctx, "rm", "-rf", nestedHome)
+	return err
+}