cli: add install print-configuration --sysroot

We would like to only inspect a container image with
{bootc,}image-builder and not actually run the container.

One reason is (too much?) paranoia, i.e. we want to eventually
support bootc containers coming from the users that get passed
into the service and not having to run anything on the container
initially to generate the osbuild manifest minimizes the risk.

So to still get the require parameters like preferred rootfs
or kargs we would like to run our own bootc and then pass
```
bootc install print-configuration --sysroot /path/to/mounted/cnt
```
to generate the manifest. The actual build will still run
the `boots install to-filesystem` from the container. But that
step happens in an isolated instance that is destroyed after
each build (we already do this for package based image builds
as users can also inject custom content/rpms here).

It also tweaks print_configuration to make it easier to
test. With that we could drop parts of the tests for
PR#1820 from the container.rs and move them in here.

Signed-off-by: Michael Vogt <michael.vogt@gmail.com>
Co-authored-by: Colin Walters <walters@verbum.org>

Author: mvo5

crates/lib/src/install.rs

@@ -18,7 +18,7 @@ use std::collections::HashMap;
 use std::io::Write;
 use std::os::fd::{AsFd, AsRawFd};
 use std::os::unix::process::CommandExt;
-use std::path::Path;
+use std::path::{Path, PathBuf};
 use std::process;
 use std::process::Command;
 use std::str::FromStr;
@@ -453,6 +453,13 @@ pub(crate) struct InstallPrintConfigurationOpts {
     /// Print configuration that is usually handled internally, like kargs.
     #[clap(long)]
     pub(crate) all: bool,
+
+    /// Set an alternative root filesystem.
+    ///
+    /// This can be used as part of a multi-stage build operation to detect
+    /// the desired configuration before executing code from a target container image.
+    #[clap(long, default_value = "/")]
+    pub(crate) sysroot: PathBuf,
 }
 
 /// Global state captured from the container.
@@ -742,12 +749,19 @@ impl SourceInfo {
 }
 
 pub(crate) fn print_configuration(opts: InstallPrintConfigurationOpts) -> Result<()> {
-    let mut install_config = config::load_config()?.unwrap_or_default();
+    let stdout = std::io::stdout().lock();
+    print_configuration_to_writer(opts, stdout)
+}
+
+fn print_configuration_to_writer<W: Write>(
+    opts: InstallPrintConfigurationOpts,
+    writer: W,
+) -> Result<()> {
+    let mut install_config = config::load_config_at(&opts.sysroot)?.unwrap_or_default();
     if !opts.all {
         install_config.filter_to_external();
     }
-    let stdout = std::io::stdout().lock();
-    anyhow::Ok(install_config.to_canon_json_writer(stdout)?)
+    anyhow::Ok(install_config.to_canon_json_writer(writer)?)
 }
 
 #[context("Creating ostree deployment")]
@@ -2517,6 +2531,8 @@ pub(crate) async fn install_finalize(target: &Utf8Path) -> Result<()> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::fs;
+    use tempfile::tempdir;
 
     #[test]
     fn install_opts_serializable() {
@@ -2721,4 +2737,46 @@ UUID=boot-uuid /boot ext4 defaults 0 0
 
         Ok(())
     }
+
+    #[test]
+    fn test_print_configuration_with_root_dir() -> Result<()> {
+        use crate::install::config::{
+            Filesystem, InstallConfiguration, InstallConfigurationToplevel,
+        };
+
+        let temp_dir = tempdir()?;
+        let root_path = temp_dir.path();
+
+        let config_dir = root_path.join("etc/bootc/install");
+        fs::create_dir_all(&config_dir)?;
+        let config_path = config_dir.join("10-install.toml");
+
+        let test_config = InstallConfigurationToplevel {
+            install: Some(InstallConfiguration {
+                root_fs_type: Some(Filesystem::Xfs),
+                kargs: Some(vec!["quiet".to_string(), "karg2=2".to_string()]),
+                ..Default::default()
+            }),
+        };
+        let toml_content = toml::to_string(&test_config)?;
+        fs::write(config_path, toml_content)?;
+
+        let opts = InstallPrintConfigurationOpts {
+            sysroot: root_path.to_owned(),
+            all: true,
+        };
+
+        let mut buffer = Vec::new();
+        print_configuration_to_writer(opts, &mut buffer)?;
+
+        let output_json = String::from_utf8(buffer)?;
+        let output_config: crate::install::config::InstallConfiguration =
+            serde_json::from_str(&output_json)?;
+
+        let install_config = test_config.install.unwrap();
+        assert_eq!(install_config.kargs, output_config.kargs);
+        assert_eq!(install_config.root_fs_type, output_config.root_fs_type);
+
+        Ok(())
+    }
 }

crates/lib/src/install/config.rs

@@ -6,6 +6,7 @@ use anyhow::{Context, Result};
 use clap::ValueEnum;
 use fn_error_context::context;
 use serde::{Deserialize, Serialize};
+use std::path::Path;
 
 #[cfg(feature = "install-to-disk")]
 use super::baseline::BlockSetup;
@@ -191,11 +192,20 @@ impl InstallConfiguration {
 #[context("Loading configuration")]
 /// Load the install configuration, merging all found configuration files.
 pub(crate) fn load_config() -> Result<Option<InstallConfiguration>> {
+    load_config_at("/")
+}
+
+pub(crate) fn load_config_at(root_dir: impl AsRef<Path>) -> Result<Option<InstallConfiguration>> {
     let env = EnvProperties {
         sys_arch: std::env::consts::ARCH.to_string(),
     };
-    const SYSTEMD_CONVENTIONAL_BASES: &[&str] = &["/usr/lib", "/usr/local/lib", "/etc", "/run"];
-    let fragments = liboverdrop::scan(SYSTEMD_CONVENTIONAL_BASES, "bootc/install", &["toml"], true);
+    let root_dir = root_dir.as_ref();
+    const SYSTEMD_CONVENTIONAL_BASES: &[&str] = &["usr/lib", "usr/local/lib", "etc", "run"];
+    let systemd_conventional_bases = SYSTEMD_CONVENTIONAL_BASES
+        .iter()
+        .map(|v| root_dir.join(v))
+        .collect::<Vec<_>>();
+    let fragments = liboverdrop::scan(systemd_conventional_bases, "bootc/install", &["toml"], true);
     let mut config: Option<InstallConfiguration> = None;
     for (_name, path) in fragments {
         let buf = std::fs::read_to_string(&path)?;

docs/src/man/bootc-install-print-configuration.8.md

@@ -26,6 +26,12 @@ string-valued filesystem name suitable for passing to `mkfs.\$type`.
 
     Print all configuration
 
+**--sysroot**=*SYSROOT*
+
+    Set an alternative root filesystem
+
+    Default: /
+
 <!-- END GENERATED OPTIONS -->
 
 # VERSION