WIP: engine.#Exec

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

stdlib/europa/dagger/engine/exec.cue

@@ -0,0 +1,81 @@
+package engine
+
+// Execute a command in a container
+#Exec: {
+	_type: "Exec"
+
+	// Container filesystem
+	input: #FS
+
+	// Transient filesystem mounts
+	//   Key is an arbitrary name, for example "app source code"
+	//   Value is mount configuration
+	mounts: [name=string]: #Mount
+
+	// Command to execute
+	// Example: ["echo", "hello, world!"]
+	args: [...string]
+
+	// Environment variables
+	env: [key=string]: string
+
+	// Working directory
+	workdir: string | *"/"
+
+	// User ID or name
+	user: string | *"root"
+
+	// If set, always execute even if the operation could be cached
+	always: true | *false
+
+	// Inject hostname resolution into the container
+	// key is hostname, value is IP
+	hosts: [hostname=string]: string
+
+	// Modified filesystem
+	output: #FS
+
+	// Command exit code
+	// Currently this field can only ever be zero.
+	// If the command fails, DAG execution is immediately terminated.
+	// FIXME: expand API to allow custom handling of failed commands
+	exit: int & 0
+}
+
+// A transient filesystem mount.
+#Mount: {
+	dest: string
+	type: string
+	{
+		type:     "cache"
+		contents: #CacheDir
+	} | {
+		type:     "tmp"
+		contents: #TempDir
+	} | {
+		type:     "service"
+		contents: #Service
+	} | {
+		type:     "fs"
+		contents: #FS
+		source?:  string
+		ro?:      true | *false
+	} | {
+		type:     "secret"
+		contents: #Secret
+		uid:      uint32 | *0
+		gid:      uint32 | *0
+		optional: true | *false
+	}
+}
+
+// A (best effort) persistent cache dir
+#CacheDir: {
+	id:          string
+	concurrency: *"shared" | "private" | "locked"
+}
+
+// A temporary directory for command execution
+#TempDir: {
+	size: int64 | *0
+}