feat(recipes): run a recipe under its declared runtime

docs/recipe-spec-v2.md

@@ -39,6 +39,7 @@ requires = ["systemd"]                        # capabilities: systemd, apt, apk,
 # Execution
 stage = "provision"  # "install" | "provision" (see Stages below)
 script = "install.sh"  # relative path to the script
+runtime = "sh"  # "sh" | "python3", interpreter the script runs under. Default: "sh"
 
 # Optional: OS-specific script overrides
 [scripts]
@@ -58,6 +59,7 @@ fedora = "install-fedora.sh"
 | `stage` | string | no | When to run: `install` or `provision`. Default: `provision` |
 | `script` | string | yes | Default script path, relative to recipe dir |
 | `scripts` | table | no | Per-OS script overrides |
+| `runtime` | string | no | Interpreter to run the script under: `sh` or `python3`. Default: `sh` |
 
 ### Capabilities
 
@@ -80,7 +82,8 @@ A recipe declaring `requires = ["systemd"]` is not offered to Alpine. Stoat reso
 
 Runs after the VM is booted and reachable over SSH. This is the common case: install packages, configure services, etc.
 
-- **apkovl/ssh backend**: Pushed over SSH via `sh -s`, same as today.
+- **apkovl/ssh backend**: Pushed over SSH via the recipe's `runtime` (`sh -s` by default, `python3 -` for `runtime = "python3"`).
+  A non-`sh` runtime is bootstrapped first: stoat checks the guest for it and installs it with the guest's package manager if missing, over a separate SSH call, before piping the recipe body.
 - **cloudinit backend**: Wrapped into a cloud-config `runcmd` block at VM creation, runs at first boot.
 
 ### `install`

docs/writing-recipes.md

@@ -65,6 +65,7 @@ arch = "install-arch.sh"
 | `scripts` | table | no | | Per-OS script overrides; unlisted OSes fall back to `script` |
 | `run` | string | no | `"once"` | `"once"`, `"always"`, or `"manual"` |
 | `auto` | bool | no | `false` | Run automatically the first time the VM becomes reachable |
+| `runtime` | string | no | `"sh"` | Interpreter the script runs under: `"sh"` or `"python3"` |
 
 `name` and `script` are the only required fields; a manifest missing either
 fails to parse (`internal/recipes/manifest.go`).
@@ -100,11 +101,40 @@ this against `guest.OS` at list/check time, before ssh is even involved.
 Runs after the VM has booted and is reachable over ssh. This is the common
 case: install packages, enable services, write config.
 
-- **apkovl/ssh backends**: the script is piped into `sh -s` over ssh, same as
-  the current bundled recipes.
+- **apkovl/ssh backends**: the script is piped into the declared runtime over
+  ssh, `sh -s` by default.
 - **cloudinit backend**: the script is wrapped into the cloud-config
   `runcmd` block at VM creation and runs at first boot.
 
+## Runtime
+
+`runtime` picks the interpreter a `provision`-stage script runs under. It
+defaults to `"sh"`, always present in a POSIX guest. Setting it to
+`"python3"` lets a recipe write a real Python script instead of shell.
+
+Before running a `python3` recipe, stoat checks the guest for `python3` and
+installs it with the guest's package manager if missing (`apk`, `apt-get`,
+`pacman`, or `dnf`, matching the `requires` capability table above). The
+check and install happen once, over ssh, right before the recipe body itself
+runs, piped into `python3 -`.
+
+A minimal Python recipe, `recipe.toml`:
+
+```toml
+name = "hello-py"
+description = "prints the guest hostname with a Python script"
+os = ["alpine", "ubuntu"]
+runtime = "python3"
+script = "install.py"
+```
+
+and `install.py`:
+
+```python
+import socket
+print("hello from", socket.gethostname())
+```
+
 ### `install`
 
 Reserved for initial disk setup, before the first real boot. Alpine disk-mode

internal/recipes/manifest.go

@@ -21,8 +21,9 @@ type Manifest struct {
 	Script      string            `toml:"script"`
 	Scripts     map[string]string `toml:"scripts"` // OS-specific overrides
 	Auto        bool              `toml:"auto"`
-	Run         string            `toml:"run"`    // "once" | "always" | "manual"
-	Reboot      bool              `toml:"reboot"` // guest needs a reboot after this recipe to take effect
+	Run         string            `toml:"run"`     // "once" | "always" | "manual"
+	Reboot      bool              `toml:"reboot"`  // guest needs a reboot after this recipe to take effect
+	Runtime     string            `toml:"runtime"` // "sh" | "python3", the interpreter the script runs under
 
 	dir string // recipe directory, set by ParseManifest; scripts resolve against it
 }
@@ -31,6 +32,8 @@ var validStages = map[string]bool{"install": true, "provision": true}
 
 var validRuns = map[string]bool{"once": true, "always": true, "manual": true}
 
+var validRuntimes = map[string]bool{"sh": true, "python3": true}
+
 // ParseManifest reads and validates a recipe.toml at path. Defaults are
 // applied before validation: Stage defaults to "provision" (the common
 // case, docs/recipe-spec-v2.md's Stages section), Run defaults to "once".
@@ -47,6 +50,9 @@ func ParseManifest(path string) (Manifest, error) {
 	if m.Run == "" {
 		m.Run = "once"
 	}
+	if m.Runtime == "" {
+		m.Runtime = "sh"
+	}
 
 	if m.Name == "" {
 		return Manifest{}, fmt.Errorf("%s: missing required field %q", path, "name")
@@ -60,6 +66,9 @@ func ParseManifest(path string) (Manifest, error) {
 	if !validRuns[m.Run] {
 		return Manifest{}, fmt.Errorf("%s: invalid run %q, want %q, %q, or %q", path, m.Run, "once", "always", "manual")
 	}
+	if !validRuntimes[m.Runtime] {
+		return Manifest{}, fmt.Errorf("%s: invalid runtime %q, want %q or %q", path, m.Runtime, "sh", "python3")
+	}
 
 	return m, nil
 }

internal/recipes/manifest_test.go

@@ -96,6 +96,38 @@ script = "install.sh"
 	if m.Reboot {
 		t.Error("Reboot = true, want default false")
 	}
+	if m.Runtime != "sh" {
+		t.Errorf("Runtime = %q, want default sh", m.Runtime)
+	}
+}
+
+func TestParseManifestRuntimePython3(t *testing.T) {
+	dir := t.TempDir()
+	path := writeManifestFile(t, dir, `
+name = "pyrecipe"
+script = "install.py"
+runtime = "python3"
+`)
+	m, err := ParseManifest(path)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if m.Runtime != "python3" {
+		t.Errorf("Runtime = %q, want python3", m.Runtime)
+	}
+}
+
+func TestParseManifestBadRuntime(t *testing.T) {
+	dir := t.TempDir()
+	path := writeManifestFile(t, dir, `
+name = "docker"
+script = "install.sh"
+runtime = "perl"
+`)
+	_, err := ParseManifest(path)
+	if err == nil || !strings.Contains(err.Error(), `invalid runtime "perl"`) {
+		t.Errorf("err = %v, want an invalid-runtime error", err)
+	}
 }
 
 func TestParseManifestMissingName(t *testing.T) {

internal/recipes/recipes.go

@@ -339,6 +339,20 @@ func ScriptBody(name, osName string) (string, error) {
 	return m.ScriptContent(osName)
 }
 
+// RuntimeFor returns the interpreter that runs name's script: the manifest's
+// Runtime field, or "sh" for a v1 flat file, which has no manifest to
+// declare one.
+func RuntimeFor(name, osName string) (string, error) {
+	m, ok, err := ManifestFor(name)
+	if err != nil {
+		return "", err
+	}
+	if !ok {
+		return "sh", nil
+	}
+	return m.Runtime, nil
+}
+
 // ScriptHash returns the hex sha256 of ScriptBody(name, osName). A caller
 // compares it against a stored AppliedRecipe.Hash to tell whether a "once"
 // recipe's script changed since it last ran, even at the same manifest

internal/recipes/recipes_test.go

@@ -373,6 +373,42 @@ func TestScriptBodyReadsV1FlatFile(t *testing.T) {
 	}
 }
 
+func TestRuntimeForV1FlatFile(t *testing.T) {
+	t.Setenv("STOAT_HOME", t.TempDir())
+	if err := os.MkdirAll(dir(), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(dir(), "legacy.sh"), []byte("echo hi\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	got, err := RuntimeFor("legacy.sh", "alpine")
+	if err != nil {
+		t.Fatalf("RuntimeFor: %v", err)
+	}
+	if got != "sh" {
+		t.Errorf("RuntimeFor(legacy.sh) = %q, want sh", got)
+	}
+}
+
+func TestRuntimeForManifestPython3(t *testing.T) {
+	t.Setenv("STOAT_HOME", t.TempDir())
+	rd := filepath.Join(dir(), "pyrecipe")
+	if err := os.MkdirAll(rd, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	toml := "name = \"pyrecipe\"\nscript = \"install.py\"\nruntime = \"python3\"\n"
+	if err := os.WriteFile(filepath.Join(rd, "recipe.toml"), []byte(toml), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	got, err := RuntimeFor("pyrecipe", "alpine")
+	if err != nil {
+		t.Fatalf("RuntimeFor: %v", err)
+	}
+	if got != "python3" {
+		t.Errorf("RuntimeFor(pyrecipe) = %q, want python3", got)
+	}
+}
+
 // TestScriptHashMatchesScriptBody pins ScriptHash to the sha256 of the exact
 // bytes ScriptBody resolves, so a caller can compare it against a stored
 // AppliedRecipe.Hash without re-deriving the sum itself.

internal/recipes/runtime.go

@@ -0,0 +1,71 @@
+package recipes
+
+import "fmt"
+
+// installCommand maps a guest OS to the shell command that installs a
+// package by name, one word per argument the way apk/apt/pacman/dnf all
+// accept. BootstrapScript appends the package name itself.
+var installCommand = map[string][]string{
+	"alpine": {"apk", "--wait", "60", "add"},
+	"ubuntu": {"apt-get", "install", "-y"},
+	"debian": {"apt-get", "install", "-y"},
+	"arch":   {"pacman", "-S", "--noconfirm"},
+	"fedora": {"dnf", "install", "-y"},
+}
+
+// runtimePackage maps a non-sh runtime to the package name that provides it
+// per guest OS. Arch names its Python 2 successor package "python", not
+// "python3"; every other OS here uses "python3".
+var runtimePackage = map[string]map[string]string{
+	"python3": {
+		"alpine": "python3",
+		"ubuntu": "python3",
+		"debian": "python3",
+		"fedora": "python3",
+		"arch":   "python",
+	},
+}
+
+// interpreterCommand maps a runtime to the guest command that reads a script
+// from stdin. "sh -s" and "python3 -" both read a script body piped to
+// them, matching how sshx.Provision pipes ScriptBody as the recipe's stdin.
+var interpreterCommand = map[string][]string{
+	"sh":      {"sh", "-s"},
+	"python3": {"python3", "-"},
+}
+
+// InterpreterArgs returns the guest command that runs a recipe body under
+// runtime, for use as the ssh command's trailing argv.
+func InterpreterArgs(runtime string) []string {
+	if args, ok := interpreterCommand[runtime]; ok {
+		return args
+	}
+	return []string{runtime}
+}
+
+// BootstrapScript returns a sh snippet that installs runtime on osName if
+// missing, or "" when runtime needs no install step ("sh" is always present
+// in a POSIX guest). sshx.Provision pipes this to `sh -s` over ssh before
+// running a recipe under a non-sh runtime.
+//
+// The check-then-install shape, and apk's --wait 60, mirror the bundled
+// recipes' own idempotent-install pattern (internal/recipes/bundled), so a
+// recipe re-applied on a VM that already has the runtime does nothing.
+func BootstrapScript(runtime, osName string) string {
+	if runtime == "sh" {
+		return ""
+	}
+	pkg := runtimePackage[runtime][osName]
+	if pkg == "" {
+		return ""
+	}
+	install := installCommand[osName]
+	if install == nil {
+		return ""
+	}
+	cmd := ""
+	for _, w := range install {
+		cmd += w + " "
+	}
+	return fmt.Sprintf("set -e\nif ! command -v %s >/dev/null 2>&1; then\n%s%s\nfi\n", runtime, cmd, pkg)
+}

internal/recipes/runtime_test.go

@@ -0,0 +1,42 @@
+package recipes
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestBootstrapScriptShIsEmpty(t *testing.T) {
+	if got := BootstrapScript("sh", "alpine"); got != "" {
+		t.Errorf("BootstrapScript(sh, alpine) = %q, want empty", got)
+	}
+}
+
+func TestBootstrapScriptPython3Alpine(t *testing.T) {
+	got := BootstrapScript("python3", "alpine")
+	for _, want := range []string{"command -v python3", "apk --wait 60 add python3"} {
+		if !strings.Contains(got, want) {
+			t.Errorf("BootstrapScript(python3, alpine) missing %q, got:\n%s", want, got)
+		}
+	}
+}
+
+func TestBootstrapScriptPython3Arch(t *testing.T) {
+	got := BootstrapScript("python3", "arch")
+	for _, want := range []string{"command -v python3", "pacman -S --noconfirm python"} {
+		if !strings.Contains(got, want) {
+			t.Errorf("BootstrapScript(python3, arch) missing %q, got:\n%s", want, got)
+		}
+	}
+	if strings.Contains(got, "add python3") {
+		t.Errorf("BootstrapScript(python3, arch) installed python3, want the arch package name python")
+	}
+}
+
+func TestInterpreterArgs(t *testing.T) {
+	if got := InterpreterArgs("sh"); len(got) != 2 || got[0] != "sh" || got[1] != "-s" {
+		t.Errorf("InterpreterArgs(sh) = %v, want [sh -s]", got)
+	}
+	if got := InterpreterArgs("python3"); len(got) != 2 || got[0] != "python3" || got[1] != "-" {
+		t.Errorf("InterpreterArgs(python3) = %v, want [python3 -]", got)
+	}
+}

internal/sshx/sshx.go

@@ -247,9 +247,32 @@ func Provision(ctx context.Context, v *config.VM) (err error) {
 			fmt.Fprintf(log, "FAILED: recipe %s: %v\n", name, err)
 			return err
 		}
+		runtime, err := recipes.RuntimeFor(name, v.OS)
+		if err != nil {
+			fmt.Fprintf(log, "FAILED: recipe %s: %v\n", name, err)
+			return err
+		}
 		fmt.Fprintf(log, "\n%s\n", RecipeMarker(name))
 
-		cmd := exec.CommandContext(ctx, "ssh", Args(v, "sh", "-s")...)
+		if bootstrap := recipes.BootstrapScript(runtime, v.OS); bootstrap != "" {
+			fmt.Fprintf(log, "ensuring %s is installed...\n", runtime)
+			bs := exec.CommandContext(ctx, "ssh", Args(v, "sh", "-s")...)
+			bs.Cancel = func() error { return bs.Process.Signal(syscall.SIGTERM) }
+			bs.WaitDelay = recipeShutdownGrace
+			bs.Stdin = strings.NewReader(bootstrap)
+			bs.Stdout = log
+			bs.Stderr = log
+			if err := bs.Run(); err != nil {
+				if ctxErr := ctx.Err(); ctxErr != nil {
+					fmt.Fprintf(log, "CANCELLED: recipe %s: %v\n", name, ctxErr)
+					return ctxErr
+				}
+				fmt.Fprintf(log, "FAILED: recipe %s: installing %s: %v\n", name, runtime, err)
+				return fmt.Errorf("recipe %s: installing %s: %w", name, runtime, err)
+			}
+		}
+
+		cmd := exec.CommandContext(ctx, "ssh", Args(v, recipes.InterpreterArgs(runtime)...)...)
 		cmd.Cancel = func() error { return cmd.Process.Signal(syscall.SIGTERM) }
 		cmd.WaitDelay = recipeShutdownGrace
 		cmd.Stdin = strings.NewReader(body)