install: Add to-existing-root

This is just mild sugar on top of the existing `to-filesystem`
right now.  But, I think we can do more with this later.

Partially addresses #380

.github/workflows/ci.yml

@@ -143,6 +143,5 @@ jobs:
           sudo chattr -i /ostree/deploy/default/deploy/*
           sudo rm /ostree/deploy/default -rf
           sudo podman run --rm -ti --privileged --env BOOTC_SKIP_SELINUX_HOST_CHECK=1 --env RUST_LOG=debug -v /:/target -v /var/lib/containers:/var/lib/containers -v ./usr/bin/bootc:/usr/bin/bootc --pid=host --security-opt label=disable \
-            quay.io/centos-bootc/centos-bootc-dev:stream9 bootc install to-filesystem \
-            --replace=alongside /target
+            quay.io/centos-bootc/centos-bootc-dev:stream9 bootc install to-existing-root
           sudo ls -ldZ / /ostree/deploy/default/deploy/* /ostree/deploy/default/deploy/*/etc

docs/src/bootc-install.md

@@ -225,7 +225,7 @@ podman run --rm --privileged --pid=host --security-opt label=type:unconfined_t
 
 Notice that we use `--generic-image` for this use case.
 
-### Using `bootc install to-filesystem --replace=alongside`
+### Using `bootc install to-existing-root`
 
 This is a variant of `install to-filesystem`, which maximizes convenience for using
 an existing Linux system, converting it into the target container image.  Note that
@@ -241,13 +241,23 @@ The core command should look like this (root/elevated permission required):
 podman run --rm --privileged -v /var/lib/containers:/var/lib/containers -v /:/target \
              --pid=host --security-opt label=type:unconfined_t \
              <image> \
-             bootc install to-filesystem --replace=alongside /target
+             bootc install to-existing-root
 ```
 
-At the current time, leftover data in `/` is **NOT** automatically cleaned up.  This can
+It is assumed in this command that the target rootfs is pased via `-v /:/target` at this time.
+
+As noted above, the data in `/boot` will be wiped, but everything else in the existing
+operating `/` is **NOT** automatically cleaned up.  This can
 be useful, because it allows the new image to automatically import data from the previous
-host system!  For example, things like SSH keys or container images can be copied
-and then deleted from the original.
+host system!  For example, container images, database, user home directory data, config
+files in `/etc` are all available after the subsequent reboot in `/sysroot` (which
+is the "physical root").
+
+A special case of this trick is using the `--root-ssh-authorized-keys` flag to inherit
+root's SSH keys (which may have been injected from e.g. cloud instance userdata
+via a tool like `cloud-init`).  To do this, just add
+`--root-ssh-authorized-keys /target/root/.ssh/authorized_keys`
+to the above.
 
 ### Using `bootc install to-filesystem --source-imgref <imgref>`
 

lib/src/cli.rs

@@ -118,6 +118,7 @@ pub(crate) enum InstallOpts {
     ToDisk(crate::install::InstallToDiskOpts),
     /// Install to the target filesystem
     ToFilesystem(crate::install::InstallToFilesystemOpts),
+    ToExistingRoot(crate::install::InstallToExistingRootOpts),
     /// Output JSON to stdout that contains the merged installation configuration
     /// as it may be relevant to calling processes using `install to-filesystem`
     /// that want to honor e.g. `root-fs-type`.
@@ -533,6 +534,9 @@ async fn run_from_opt(opt: Opt) -> Result<()> {
         Opt::Install(opts) => match opts {
             InstallOpts::ToDisk(opts) => crate::install::install_to_disk(opts).await,
             InstallOpts::ToFilesystem(opts) => crate::install::install_to_filesystem(opts).await,
+            InstallOpts::ToExistingRoot(opts) => {
+                crate::install::install_to_existing_root(opts).await
+            }
             InstallOpts::PrintConfiguration => crate::install::print_configuration(),
         },
         #[cfg(feature = "install")]

lib/src/install.rs

@@ -249,6 +249,28 @@ pub(crate) struct InstallToFilesystemOpts {
     pub(crate) config_opts: InstallConfigOpts,
 }
 
+/// Perform an installation to the host root filesystem.
+#[derive(Debug, Clone, clap::Parser)]
+pub(crate) struct InstallToExistingRootOpts {
+    /// Configure how existing data is treated.
+    #[clap(long, default_value = "alongside")]
+    pub(crate) replace: Option<ReplaceMode>,
+
+    #[clap(flatten)]
+    pub(crate) source_opts: InstallSourceOpts,
+
+    #[clap(flatten)]
+    pub(crate) target_opts: InstallTargetOpts,
+
+    #[clap(flatten)]
+    pub(crate) config_opts: InstallConfigOpts,
+
+    /// Path to the mounted root; it's expected to invoke podman with
+    /// `-v /:/target`, then supplying this argument is unnecessary.
+    #[clap(default_value = "/target")]
+    pub(crate) root_path: Utf8PathBuf,
+}
+
 /// Global state captured from the container.
 #[derive(Debug, Clone)]
 pub(crate) struct SourceInfo {
@@ -1355,6 +1377,22 @@ pub(crate) async fn install_to_filesystem(opts: InstallToFilesystemOpts) -> Resu
     Ok(())
 }
 
+pub(crate) async fn install_to_existing_root(opts: InstallToExistingRootOpts) -> Result<()> {
+    let opts = InstallToFilesystemOpts {
+        filesystem_opts: InstallTargetFilesystemOpts {
+            root_path: opts.root_path,
+            root_mount_spec: None,
+            boot_mount_spec: None,
+            replace: opts.replace,
+        },
+        source_opts: opts.source_opts,
+        target_opts: opts.target_opts,
+        config_opts: opts.config_opts,
+    };
+
+    install_to_filesystem(opts).await
+}
+
 #[test]
 fn install_opts_serializable() {
     let c: InstallToDiskOpts = serde_json::from_value(serde_json::json!({

tests/integration/playbooks/install.yaml

@@ -72,7 +72,7 @@
          -v /var/lib/containers:/var/lib/containers \
          --security-opt label=type:unconfined_t \
          {{ test_image_url }} \
-         bootc install to-filesystem --replace=alongside /target"
+         bootc install to-existing-root"
       become: true
 
     - name: Reboot to deploy new system