feat(cli): provision layers from a spec file

A provision command baked by hand - `ecrou provision alpine -- apk
add ...` - cannot be reviewed or shared: the artifact worth keeping is
a diff in git, not a shell history line. A spec file is that diff.
`ecrou provision --spec image.yaml` parses base, packages and a run
block, derives the provisioning script from the base's own package
manager (apt, apk, dnf, discovered at provision time on the running
machine), and bakes the result into a cache entry keyed by the spec's
content hash: the same file never provisions twice, an edited file
never reuses the old layer, and the key hashes the raw bytes so the
layer is stable across ecrou versions.

`--layer NAME` names the entry, and `ecrou run --layer NAME` boots it
directly - no image resolution, no download; the layer is whatever
the provision call persisted under that name. A keyed entry has no
tarball of its own, so the base downloads only on the first
provisioning and every later run extracts the finished layer.

crates/ecrou-cli/src/cli.rs

@@ -27,9 +27,20 @@ pub enum Command {
     /// command installed. Cached entries are reused as-is; --no-cache
     /// re-downloads and re-provisions.
     Provision {
-        /// Distribution name or OCI image reference
-        image: String,
-        /// Ignore the cache and download a fresh image
+        /// Distribution name or OCI image reference; the spec's `base`
+        /// when omitted and the spec carries one
+        image: Option<String>,
+        /// A provisioning spec file (YAML: base, packages, run) — the
+        /// layer it describes is keyed by the file's content, so the
+        /// same spec never provisions twice and a changed spec never
+        /// reuses an old layer
+        #[arg(long)]
+        spec: Option<std::path::PathBuf>,
+        /// Name the provisioned layer, so runs can boot it by name
+        /// (`run --layer NAME`) without re-deriving anything
+        #[arg(long)]
+        layer: Option<String>,
+        /// Ignore the cache: re-download the base and re-provision
         #[arg(long)]
         no_cache: bool,
         /// The provisioning command. An argument vector after `--` is
@@ -44,8 +55,10 @@ pub enum Command {
 
 #[derive(Args, Debug)]
 pub struct RunArgs {
-    /// Distribution name or OCI image reference
-    pub image: String,
+    /// Distribution name or OCI image reference; not needed with
+    /// `--layer`, which boots a provisioned cache entry
+    #[arg(required_unless_present = "layer")]
+    pub image: Option<String>,
     /// Command to run in the machine; the image's default shell when
     /// omitted. An argument vector after `--` is taken literally; a
     /// single quoted line runs through the image's shell.
@@ -73,6 +86,10 @@ pub struct RunArgs {
     /// Ignore the cache and download a fresh image
     #[arg(long)]
     pub no_cache: bool,
+    /// Boot a named provisioned layer (see `provision --layer`)
+    /// instead of resolving an image
+    #[arg(long)]
+    pub layer: Option<String>,
 }
 
 /// Parse one `HOST:GUEST[:ro]` share specification. The guest path must

crates/ecrou-cli/src/main.rs

@@ -20,34 +20,43 @@ fn main() -> std::process::ExitCode {
         Command::Run(args) => cmd_run(&args, cli.verbose),
         Command::Provision {
             image,
+            spec,
+            layer,
             no_cache,
             command,
-        } => cmd_provision(&image, no_cache, &command, cli.verbose),
+        } => cmd_provision(image, spec, layer, no_cache, &command, cli.verbose),
         Command::Probe => cmd_probe(),
     }
 }
 
 fn cmd_provision(
-    image: &str,
+    image: Option<String>,
+    spec: Option<std::path::PathBuf>,
+    layer: Option<String>,
     no_cache: bool,
     command: &[String],
     cli_verbose: bool,
 ) -> std::process::ExitCode {
     use ecrou::rootfs::{PrepareRequest, RootfsCache};
+    use ecrou::spec::ProvisionSpec;
 
-    if command.is_empty() {
+    // Two shapes: `provision IMAGE -- COMMAND` bakes a one-off
+    // command; `provision --spec FILE --layer NAME` bakes a checked-in
+    // spec into a named, content-keyed layer. A spec plus a command
+    // has no single meaning.
+    if spec.is_none() == command.is_empty() {
         return fail(&anyhow::anyhow!(
-            "provision needs a command: ecrou provision <IMAGE> -- <COMMAND...>"
+            "provision needs either a spec file (--spec) or a command: \
+             ecrou provision <IMAGE> -- <COMMAND...>"
         ));
     }
+
     let level = if cli_verbose {
         LogLevel::Verbose
     } else {
         LogLevel::Normal
     };
     let mut options = RunOptions {
-        image: image.to_owned(),
-        command: command.to_owned(),
         log_level: level,
         // The machine exists to change the rootfs, not to cross a
         // share: the workspace would ride the persist into the cache.
@@ -56,16 +65,61 @@ fn cmd_provision(
     options.workdir = "/".to_owned();
 
     let progress: &dyn Progress = &TermProgress::new();
-    let request = PrepareRequest {
-        image: image.to_owned(),
-        arch: String::new(),
-        no_cache,
-    };
     let dir = match RootfsCache::default_dir() {
         Ok(dir) => dir,
         Err(err) => return fail(&anyhow::Error::new(err)),
     };
     let cache = RootfsCache::new(dir);
+
+    // The spec decides the request: its base (or the positional
+    // image), its key, and its script — the packages through the
+    // base's package manager, then the run block. The key hashes the
+    // raw bytes and the base reference, so the cache entries two
+    // projects see can never cross.
+    let mut request = PrepareRequest::new(String::new());
+    request.no_cache = no_cache;
+    let mut script;
+    match &spec {
+        Some(path) => {
+            let bytes = match std::fs::read(path) {
+                Ok(bytes) => bytes,
+                Err(err) => return fail(&anyhow::anyhow!("reading {}: {err}", path.display())),
+            };
+            let parsed = match ProvisionSpec::parse(&bytes) {
+                Ok(parsed) => parsed,
+                Err(err) => return fail(&err),
+            };
+            let base = match parsed.base.clone().or_else(|| image.clone()) {
+                Some(base) => base,
+                None => {
+                    return fail(&anyhow::anyhow!(
+                        "the spec has no base and no image was named: \
+                         ecrou provision <IMAGE> --spec {path}",
+                        path = path.display()
+                    ))
+                }
+            };
+            script = parsed.script();
+            let key = ProvisionSpec::key(&bytes, &base);
+            request.image = base;
+            request.layer_key = Some(key);
+        }
+        None => {
+            if layer.is_some() {
+                return fail(&anyhow::anyhow!(
+                    "--layer names a provisioned spec, not a bare command"
+                ));
+            }
+            request.image = image
+                .clone()
+                .expect("clap requires an image without --spec");
+            script = command.join(" ");
+            script.push('\n');
+        }
+    }
+    options.image = request.image.clone();
+    options.command = vec!["/bin/sh".to_owned(), "-c".to_owned(), script];
+
     let outcome = cache.prepare_provisioned(&request, progress, |rootfs| {
         let code = ecrou::vm::run_prepared(rootfs, &options)?;
         if code != 0 {
@@ -76,14 +130,37 @@ fn cmd_provision(
         Ok(())
     });
     match outcome {
-        Ok(_rootfs) => std::process::ExitCode::SUCCESS,
+        Ok(_rootfs) => {
+            // The pointer lands only after the entry does: a name
+            // that resolves is a layer that boots.
+            if let (Some(name), Some(key)) = (&layer, &request.layer_key) {
+                let path = match cache.resolved_entry_path(
+                    &request.image,
+                    &request.resolved_arch(),
+                    Some(key),
+                ) {
+                    Ok(path) => path,
+                    Err(err) => return fail(&anyhow::Error::new(err)),
+                };
+                let filename = path
+                    .file_name()
+                    .map(|n| n.to_string_lossy().into_owned())
+                    .unwrap_or_default();
+                if let Err(err) = cache.write_layer_pointer(name, &filename) {
+                    return fail(&anyhow::Error::new(err));
+                }
+            }
+            std::process::ExitCode::SUCCESS
+        }
         Err(err) => fail(&anyhow::Error::new(err)),
     }
 }
 
 fn cmd_run(args: &RunArgs, cli_verbose: bool) -> std::process::ExitCode {
     let mut options = RunOptions {
-        image: args.image.clone(),
+        // Without a layer the image is required by clap; with one the
+        // field is dead — the layer entry carries the rootfs.
+        image: args.image.clone().unwrap_or_default(),
         command: args.command.clone(),
         memory: args.memory.clone(),
         workdir: args.workdir.clone(),
@@ -95,6 +172,7 @@ fn cmd_run(args: &RunArgs, cli_verbose: bool) -> std::process::ExitCode {
             LogLevel::Normal
         },
         no_cache: args.no_cache,
+        layer: args.layer.clone(),
         capture: args.capture.clone(),
         ..RunOptions::default()
     };

crates/ecrou/Cargo.toml

@@ -15,6 +15,7 @@ serde_yaml = "0.9"
 # HTTP downloads
 reqwest = { version = "0.13", features = ["blocking", "stream"] }
 
+
 # Tarball extraction
 tar = "0.4"
 flate2 = "1"

crates/ecrou/src/lib.rs

@@ -40,6 +40,7 @@ pub mod guest;
 pub mod krun;
 pub mod progress;
 pub mod rootfs;
+pub mod spec;
 pub mod utils;
 pub mod vm;
 

crates/ecrou/src/rootfs.rs

@@ -31,6 +31,7 @@ use std::path::{Path, PathBuf};
 
 use anyhow::{anyhow, Context};
 
+use crate::error::Error;
 use crate::Result;
 use tempfile::TempDir;
 
@@ -79,6 +80,87 @@ impl RootfsCache {
         self.dir.join(generate_cache_filename(source, arch))
     }
 
+    /// The cache path for a prepared entry, keyed or not: a layer key
+    /// is spliced into the filename, so a layer and its base — and two
+    /// layers of one base — live in separate entries, never overwrite
+    /// each other, and evict separately.
+    pub fn entry_path(&self, source: &ImageSource, arch: &str, layer_key: Option<&str>) -> PathBuf {
+        match layer_key {
+            None => self.tarball_path(source, arch),
+            Some(key) => {
+                let filename = generate_cache_filename(source, arch);
+                let stem = filename
+                    .rsplit_once('.')
+                    .map(|(stem, _)| stem)
+                    .unwrap_or(&filename);
+                self.dir.join(format!("{stem}.{key}.tar.gz"))
+            }
+        }
+    }
+
+    /// Extracts a named layer into a fresh scratch directory: the boot
+    /// side of `run --layer`. A missing pointer or a pruned entry is a
+    /// named error, not a download — layers are provisioned, never
+    /// fetched.
+    pub fn extract_layer(&self, name: &str, progress: &dyn Progress) -> Result<PreparedRootfs> {
+        let path = self.layer_entry(name)?.ok_or_else(|| {
+            Error::Other(anyhow::anyhow!(
+                "layer {name} is not provisioned: run \
+                     `ecrou provision --spec <spec> --layer {name}` first"
+            ))
+        })?;
+        let arch = std::env::consts::ARCH
+            .replace("x86_64", "amd64")
+            .replace("aarch64", "arm64");
+        extract_prepared(path, arch, progress).map_err(Error::Other)
+    }
+
+    /// The cache path an image resolves to, keyed or not — the same
+    /// resolution `prepare` does (floating aliases resolved), so a
+    /// pointer written from this path names an entry that exists.
+    pub fn resolved_entry_path(
+        &self,
+        image: &str,
+        arch: &str,
+        layer_key: Option<&str>,
+    ) -> Result<PathBuf> {
+        let source = resolve_source(image, arch)?;
+        Ok(self.entry_path(&source, arch, layer_key))
+    }
+
+    /// Writes the layer pointer: `layers/<name>` holds the cache
+    /// filename the name resolves to. Consumers boot layers by name —
+    /// `run` should not have to re-derive the key from the spec, only
+    /// agree with the provisioning call on what the layer is called.
+    pub fn write_layer_pointer(&self, name: &str, cache_filename: &str) -> Result<()> {
+        let dir = self.dir.join("layers");
+        std::fs::create_dir_all(&dir)
+            .with_context(|| format!("Failed to create the layer directory {}", dir.display()))?;
+        std::fs::write(dir.join(name), cache_filename)
+            .with_context(|| format!("Failed to write the layer pointer for {name}"))?;
+        Ok(())
+    }
+
+    /// The cache path a layer pointer names, and whether the layer is
+    /// really there: a pointer is a name, not a promise — the entry it
+    /// names can be gone (a pruned cache, a base bump) while the
+    /// pointer survives.
+    pub fn layer_entry(&self, name: &str) -> Result<Option<PathBuf>> {
+        let pointer = self.dir.join("layers").join(name);
+        let filename = match std::fs::read_to_string(&pointer) {
+            Ok(filename) => filename,
+            Err(err) if err.kind() == std::io::ErrorKind::NotFound => return Ok(None),
+            Err(err) => {
+                return Err(Error::Other(
+                    anyhow::Error::from(err)
+                        .context(format!("reading the layer pointer for {name}")),
+                ))
+            }
+        };
+        let path = self.dir.join(filename.trim());
+        Ok((path.exists() && sidecar(&path, PROVISIONED_SUFFIX).exists()).then_some(path))
+    }
+
     /// Resolve `req`, download through the cache when needed, and extract
     /// into a fresh scratch directory (removed when the returned
     /// [`PreparedRootfs`] is dropped).  Download and extraction progress is
@@ -172,7 +254,7 @@ impl RootfsCache {
     {
         let arch = req.resolved_arch();
         let source = resolve_source(&req.image, &arch)?;
-        let cache_path = self.tarball_path(&source, &arch);
+        let cache_path = self.entry_path(&source, &arch, req.layer_key.as_deref());
         let marker = sidecar(&cache_path, PROVISIONED_SUFFIX);
 
         if !req.no_cache && cache_path.exists() && marker.exists() {
@@ -180,14 +262,32 @@ impl RootfsCache {
             return Ok(extract_prepared(cache_path, arch, progress)?);
         }
 
-        let rootfs = self.prepare(req, progress)?;
+        // The miss path prepares the PRISTINE base — a keyed entry
+        // has no tarball of its own to prepare from, and the base
+        // must download if missing. The key is this call's business,
+        // never prepare's.
+        let mut base_req = req.clone();
+        base_req.layer_key = None;
+        let rootfs = self.prepare(&base_req, progress)?;
         provision(&rootfs)?;
-        rootfs.persist().with_context(|| {
-            format!(
-                "Failed to persist provisioned rootfs to {}",
-                cache_path.display()
-            )
-        })?;
+        if req.layer_key.is_some() {
+            // The prepared scratch is the BASE (a keyed entry has no
+            // tarball to prepare from): the provisioned tree lands in
+            // the keyed entry, not over the base.
+            rootfs.persist_to(&cache_path).with_context(|| {
+                format!(
+                    "Failed to persist provisioned layer to {}",
+                    cache_path.display()
+                )
+            })?;
+        } else {
+            rootfs.persist().with_context(|| {
+                format!(
+                    "Failed to persist provisioned rootfs to {}",
+                    cache_path.display()
+                )
+            })?;
+        }
         Ok(rootfs)
     }
 }
@@ -206,6 +306,12 @@ pub struct PrepareRequest {
 
     /// Ignore the cache and download a fresh image.
     pub no_cache: bool,
+
+    /// A layer key: when set, the cache entry is the keyed layer over
+    /// the image rather than the image itself — the entry is never
+    /// downloaded, only provisioned into existence
+    /// ([`RootfsCache::prepare_provisioned`]).
+    pub layer_key: Option<String>,
 }
 
 impl PrepareRequest {
@@ -214,6 +320,7 @@ impl PrepareRequest {
             image: image.into(),
             arch: String::new(),
             no_cache: false,
+            layer_key: None,
         }
     }
 
@@ -261,14 +368,21 @@ impl PreparedRootfs {
     /// (indistinguishable from a pristine hit) — only
     /// [`RootfsCache::prepare_provisioned`] interprets the marker.
     pub fn persist(&self) -> Result<()> {
-        pack_rootfs(self.dir(), &self.cache_path)?;
-        std::fs::write(sidecar(&self.cache_path, PROVISIONED_SUFFIX), b"").with_context(|| {
+        self.persist_to(&self.cache_path)
+    }
+
+    /// Packs the tree into an arbitrary cache entry — how a provisioned
+    /// layer lands under its own key while the prepared scratch still
+    /// carries the base's own cache path.
+    pub fn persist_to(&self, cache_path: &Path) -> Result<()> {
+        pack_rootfs(self.dir(), cache_path)?;
+        std::fs::write(sidecar(cache_path, PROVISIONED_SUFFIX), b"").with_context(|| {
             format!(
                 "Failed to write provisioned marker for {}",
-                self.cache_path.display()
+                cache_path.display()
             )
         })?;
-        log::debug!("Persisted rootfs to cache: {}", self.cache_path.display());
+        log::debug!("Persisted rootfs to cache: {}", cache_path.display());
         Ok(())
     }
 }
@@ -693,4 +807,48 @@ mod tests {
         assert!(!dest.exists());
         assert!(!dest.with_extension("partial").exists());
     }
+
+    #[test]
+    fn a_layer_key_splices_into_the_entry_name() {
+        let cache = RootfsCache::new(std::path::PathBuf::from("/cache"));
+        let source = crate::distro::parse_image_ref("alpine:3.23", "amd64").expect("parses");
+        let plain = cache.entry_path(&source, "amd64", None);
+        let keyed = cache.entry_path(&source, "amd64", Some("abc123"));
+        let other = cache.entry_path(&source, "amd64", Some("def456"));
+        eprintln!("plain={plain:?} keyed={keyed:?} other={other:?}");
+        assert_ne!(plain, keyed, "the key changes the entry");
+        assert_ne!(keyed, other, "two keys are two entries");
+    }
+
+    #[test]
+    fn a_layer_pointer_resolves_only_while_the_entry_exists() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let cache = RootfsCache::new(dir.path().to_path_buf());
+        assert!(
+            cache.layer_entry("nope").expect("reads").is_none(),
+            "no pointer, no entry"
+        );
+        cache
+            .write_layer_pointer("w", "alpine-3.23.7-x86_64.tar.abc.tar.gz")
+            .expect("writes");
+        assert!(
+            cache.layer_entry("w").expect("reads").is_none(),
+            "a pointer without its entry resolves to nothing"
+        );
+        std::fs::write(
+            dir.path().join("alpine-3.23.7-x86_64.tar.abc.tar.gz"),
+            b"tarball",
+        )
+        .expect("the entry appears");
+        std::fs::write(
+            dir.path()
+                .join("alpine-3.23.7-x86_64.tar.abc.tar.gz.provisioned"),
+            b"",
+        )
+        .expect("the marker appears");
+        assert!(
+            cache.layer_entry("w").expect("reads").is_some(),
+            "pointer + entry + marker resolve"
+        );
+    }
 }

crates/ecrou/src/spec.rs

@@ -0,0 +1,150 @@
+//! Provisioning specs: the checked-in description of what a layer
+//! carries beyond its base image. A spec is data, not a command —
+//! the artifact under review is a diff in git, and the layer it
+//! produces is a pure function of the file's content and its base.
+
+use serde::Deserialize;
+use std::fmt::Write as _;
+
+/// A parsed `.rhema/image.yaml` (or any consumer's equivalent).
+#[derive(Debug, Clone, PartialEq, Deserialize)]
+pub struct ProvisionSpec {
+    /// The image to provision over. Optional: without it the caller's
+    /// positional image is the base.
+    #[serde(default)]
+    pub base: Option<String>,
+
+    /// Packages installed through whatever package manager the base
+    /// carries (apt, apk, dnf).
+    #[serde(default)]
+    pub packages: Vec<String>,
+
+    /// A shell script run after the packages, for everything a
+    /// package name cannot say.
+    #[serde(default)]
+    pub run: Option<String>,
+}
+
+impl ProvisionSpec {
+    /// Parses the spec from raw file bytes. Empty and missing optional
+    /// fields stay optional; an unparseable file is an error — a typo
+    /// in the spec must not silently provision a bare base.
+    pub fn parse(bytes: &[u8]) -> anyhow::Result<Self> {
+        let spec: ProvisionSpec = serde_yaml::from_slice(bytes)
+            .map_err(|err| anyhow::anyhow!("the provisioning spec does not parse: {err}"))?;
+        if spec.base.is_none() && spec.packages.is_empty() && spec.run.is_none() {
+            anyhow::bail!("the provisioning spec is empty");
+        }
+        Ok(spec)
+    }
+
+    /// The layer key: content the cache entries off. Hash of the raw
+    /// file bytes and the base reference — the bytes, not the parsed
+    /// form, so the key is stable across ecrou versions; reformatting
+    /// the file rebuilds the layer, which is harmless and visible.
+    pub fn key(spec_bytes: &[u8], base_ref: &str) -> String {
+        use sha2::{Digest, Sha256};
+        let mut hasher = Sha256::new();
+        hasher.update(spec_bytes);
+        hasher.update([0]);
+        hasher.update(base_ref.as_bytes());
+        let digest = hasher.finalize();
+        digest[..8].iter().fold(String::new(), |mut out, byte| {
+            let _ = write!(out, "{byte:02x}");
+            out
+        })
+    }
+
+    /// The provisioning script: the packages through the base's
+    /// package manager, then the raw `run` block. Nothing here knows
+    /// which distro it lands on ahead of time — the machine itself
+    /// says, at provision time.
+    pub fn script(&self) -> String {
+        let mut script = String::from("set -e\n");
+        if !self.packages.is_empty() {
+            let packages = self.packages.join(" ");
+            let _ = write!(
+                script,
+                r#"if command -v apt-get >/dev/null 2>&1; then
+    export DEBIAN_FRONTEND=noninteractive
+    apt-get update -qq
+    apt-get install -y -qq --no-install-recommends {packages}
+elif command -v apk >/dev/null 2>&1; then
+    apk add --no-cache {packages}
+elif command -v dnf >/dev/null 2>&1; then
+    dnf install -y {packages}
+else
+    echo "no known package manager in the base image" >&2
+    exit 1
+fi
+"#
+            );
+        }
+        if let Some(run) = &self.run {
+            script.push_str(run.trim_end());
+            script.push('\n');
+        }
+        script
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn a_spec_parses_with_all_parts_optional_but_none_empty() {
+        let spec = ProvisionSpec::parse(b"packages:\n  - jq\n").expect("parses");
+        assert_eq!(spec.packages, vec!["jq".to_owned()]);
+        assert!(spec.base.is_none());
+
+        let full = ProvisionSpec::parse(
+            b"base: ghcr.io/vhaudiquet/rhema-base:v1\npackages:\n  - go\nrun: |\n  cargo install just\n",
+        )
+        .expect("parses");
+        assert_eq!(
+            full.base.as_deref(),
+            Some("ghcr.io/vhaudiquet/rhema-base:v1")
+        );
+        assert!(full.run.as_deref().unwrap().contains("cargo install"));
+
+        assert!(ProvisionSpec::parse(b"\n").is_err(), "empty is an error");
+        assert!(
+            ProvisionSpec::parse(b"packages: [\n").is_err(),
+            "unparseable yaml is an error"
+        );
+    }
+
+    #[test]
+    fn the_key_tracks_the_spec_bytes_and_the_base() {
+        let a = ProvisionSpec::key(b"packages: [jq]\n", "alpine:3.23");
+        assert_eq!(a.len(), 16);
+        assert_eq!(a, ProvisionSpec::key(b"packages: [jq]\n", "alpine:3.23"));
+        assert_ne!(a, ProvisionSpec::key(b"packages: [jq]\n", "alpine:3.24"));
+        assert_ne!(a, ProvisionSpec::key(b"packages: [jq] \n", "alpine:3.23"));
+    }
+
+    #[test]
+    fn the_script_speaks_the_base_package_manager_and_then_runs() {
+        let spec = ProvisionSpec {
+            base: None,
+            packages: vec!["jq".into(), "go".into()],
+            run: Some("go install m/tool@latest\n".into()),
+        };
+        let script = spec.script();
+        assert!(script.starts_with("set -e"));
+        assert!(script.contains("apt-get install -y -qq --no-install-recommends jq go"));
+        assert!(script.contains("apk add --no-cache jq go"));
+        assert!(script.ends_with("go install m/tool@latest\n"));
+
+        // packages only: no pm block would try to install nothing
+        let bare = ProvisionSpec {
+            packages: vec![],
+            run: Some("echo hi".into()),
+            base: None,
+        };
+        let script = bare.script();
+        assert!(!script.contains("apt-get"));
+        assert!(script.contains("echo hi"));
+    }
+}

crates/ecrou/src/vm.rs

@@ -45,6 +45,11 @@ pub struct RunOptions {
     /// How much of the engine's own logging reaches stderr
     pub log_level: LogLevel,
     pub no_cache: bool,
+    /// When set, boot the named provisioned layer from the cache
+    /// instead of resolving an image: no download, no resolution —
+    /// the layer is whatever `ecrou provision --spec … --layer …`
+    /// persisted under that name.
+    pub layer: Option<String>,
     /// When set, the payload's stdout and stderr are captured into
     /// files in this host directory (`stdout` and `stderr` under it):
     /// the run adds the directory as an internal read-write share,
@@ -84,6 +89,7 @@ impl Default for RunOptions {
             hostname: String::new(),
             log_level: LogLevel::Normal,
             no_cache: false,
+            layer: None,
             capture: None,
         }
     }
@@ -91,6 +97,20 @@ impl Default for RunOptions {
 
 /// Run one machine to completion and return the payload's exit code.
 pub fn run(options: &RunOptions, progress: &dyn Progress) -> Result<i32> {
+    // A named layer boots straight from its cache entry: no image
+    // resolution, no download — the layer is whatever the provision
+    // call persisted under that name.
+    if let Some(name) = &options.layer {
+        let cache = RootfsCache::new(RootfsCache::default_dir()?);
+        let rootfs = cache.extract_layer(name, progress)?;
+        // Downstream naming (the hostname, diagnostics) rides the
+        // layer name: the entry is what booted, not an image.
+        let options = RunOptions {
+            image: name.clone(),
+            ..options.clone()
+        };
+        return run_prepared(&rootfs, &options);
+    }
     let mut request = PrepareRequest::new(options.image.clone());
     request.arch = options.arch.clone();
     request.no_cache = options.no_cache;
@@ -109,7 +129,6 @@ pub fn run_prepared(rootfs: &PreparedRootfs, options: &RunOptions) -> Result<i32
     if options.image.is_empty() {
         return Err(Error::Other(anyhow!("no image given")));
     }
-
     krun::kvm_probe()?;
     let krun = Krun::load()?;