AB#2046 : Add option to create SSH users for the first coordinator upon initialization (#133)

* Move `file`, `ssh` and `user` packages to internal
* Rename `SSHKey` to `(ssh.)UserKey`
* Rename KeyValue / Publickey to PublicKey
* Rename SSH key file from "debugd" to "ssh-keys"
* Add CreateSSHUsers function to Core
* Call CreateSSHUsers users on first control-plane node, when defined in config

Tests:
* Make StubUserCreator add entries to /etc/passwd
* Add NewLinuxUserManagerFake for unit tests
* Add unit tests & adjust existing ones to changes

internal/file/file.go

@@ -0,0 +1,131 @@
+/*
+Package file provides functions that combine file handling, JSON marshaling
+and file system abstraction.
+*/
+package file
+
+import (
+	"encoding/json"
+	"errors"
+	"io"
+	"io/fs"
+	"os"
+	"path"
+
+	"github.com/spf13/afero"
+	"gopkg.in/yaml.v3"
+)
+
+// Option is a bitmask of options for file operations.
+type Option uint
+
+// Has determines if a set of options contains the given options.
+func (o Option) Has(op Option) bool {
+	return o&op == op
+}
+
+const (
+	// OptNone is a no-op.
+	OptNone Option = 1 << iota / 2
+	// OptOverwrite overwrites an existing file.
+	OptOverwrite
+	// OptMkdirAll creates the path to the file.
+	OptMkdirAll
+)
+
+// Handler handles file interaction.
+type Handler struct {
+	fs *afero.Afero
+}
+
+// NewHandler returns a new file handler.
+func NewHandler(fs afero.Fs) Handler {
+	afs := &afero.Afero{Fs: fs}
+	return Handler{fs: afs}
+}
+
+// Read reads the file given name and returns the bytes read.
+func (h *Handler) Read(name string) ([]byte, error) {
+	file, err := h.fs.OpenFile(name, os.O_RDONLY, 0o644)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+	return io.ReadAll(file)
+}
+
+// Write writes the data bytes into the file with the given name.
+func (h *Handler) Write(name string, data []byte, options Option) error {
+	if options.Has(OptMkdirAll) {
+		if err := h.fs.MkdirAll(path.Dir(name), os.ModePerm); err != nil {
+			return err
+		}
+	}
+	flags := os.O_WRONLY | os.O_CREATE | os.O_EXCL
+	if options.Has(OptOverwrite) {
+		flags = os.O_WRONLY | os.O_CREATE | os.O_TRUNC
+	}
+	file, err := h.fs.OpenFile(name, flags, 0o644)
+	if err != nil {
+		return err
+	}
+	_, err = file.Write(data)
+	if errTmp := file.Close(); errTmp != nil && err == nil {
+		err = errTmp
+	}
+	return err
+}
+
+// ReadJSON reads a JSON file from name and unmarshals it into the content interface.
+// The interface content must be a pointer to a JSON marchalable object.
+func (h *Handler) ReadJSON(name string, content any) error {
+	data, err := h.Read(name)
+	if err != nil {
+		return err
+	}
+	return json.Unmarshal(data, content)
+}
+
+// WriteJSON marshals the content interface to JSON and writes it to the path with the given name.
+func (h *Handler) WriteJSON(name string, content any, options Option) error {
+	jsonData, err := json.MarshalIndent(content, "", "\t")
+	if err != nil {
+		return err
+	}
+	return h.Write(name, jsonData, options)
+}
+
+// ReadYAML reads a YAML file from name and unmarshals it into the content interface.
+// The interface content must be a pointer to a YAML marchalable object.
+func (h *Handler) ReadYAML(name string, content any) error {
+	data, err := h.Read(name)
+	if err != nil {
+		return err
+	}
+	return yaml.Unmarshal(data, content)
+}
+
+// WriteYAML marshals the content interface to YAML and writes it to the path with the given name.
+func (h *Handler) WriteYAML(name string, content any, options Option) (err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			err = errors.New("recovered from panic")
+		}
+	}()
+	data, err := yaml.Marshal(content)
+	if err != nil {
+		return err
+	}
+	return h.Write(name, data, options)
+}
+
+// Remove deletes the file with the given name.
+func (h *Handler) Remove(name string) error {
+	return h.fs.Remove(name)
+}
+
+// Stat returns a FileInfo describing the named file, or an error, if any
+// happens.
+func (h *Handler) Stat(name string) (fs.FileInfo, error) {
+	return h.fs.Stat(name)
+}