refactor: split into packages and add tests

internal/errors/errors.go

@@ -0,0 +1,82 @@
+package errors
+
+import (
+	"fmt"
+)
+
+// NixBuildError indicates an error during `nix build`
+type NixBuildError struct {
+	Derivation string
+	Stderr     string
+	Err        error // underlying error from exec.Cmd or similar
+}
+
+func (e *NixBuildError) Error() string {
+	return fmt.Sprintf("nix build for %s failed: %v (stderr: %s)", e.Derivation, e.Err, e.Stderr)
+}
+func (e *NixBuildError) Unwrap() error { return e.Err }
+
+// NixNoOutputPathError indicates `nix build` succeeded but produced no output path
+type NixNoOutputPathError struct {
+	Derivation string
+	Stderr     string
+}
+
+func (e *NixNoOutputPathError) Error() string {
+	return fmt.Sprintf("nix build for %s produced no output path (stderr: %s)", e.Derivation, e.Stderr)
+}
+
+// FileReadError indicates an error reading a file, often a derivation output
+type FileReadError struct {
+	Path string
+	Err  error
+}
+
+func (e *FileReadError) Error() string {
+	return fmt.Sprintf("failed to read file %s: %v", e.Path, e.Err)
+}
+func (e *FileReadError) Unwrap() error { return e.Err }
+
+// JSONUnmarshalError indicates an error unmarshalling JSON data
+type JSONUnmarshalError struct {
+	Source string // e.g. file path or "derivation output"
+	Err    error
+}
+
+func (e *JSONUnmarshalError) Error() string {
+	return fmt.Sprintf("failed to unmarshal JSON from %s: %v", e.Source, e.Err)
+}
+func (e *JSONUnmarshalError) Unwrap() error { return e.Err }
+
+// ScriptExecutionError indicates an error starting or waiting for a script
+type ScriptExecutionError struct {
+	Path string // path to script that was attempted to run
+	Err  error
+}
+
+func (e *ScriptExecutionError) Error() string {
+	return fmt.Sprintf("script %s execution failed: %v", e.Path, e.Err)
+}
+func (e *ScriptExecutionError) Unwrap() error { return e.Err }
+
+// SnapshotCreateError indicates an error during snapshot creation
+type SnapshotCreateError struct {
+	FilePath string
+	Err      error
+}
+
+func (e *SnapshotCreateError) Error() string {
+	return fmt.Sprintf("failed to create/update snapshot %s: %v", e.FilePath, e.Err)
+}
+func (e *SnapshotCreateError) Unwrap() error { return e.Err }
+
+// SnapshotLoadError indicates an error loading a snapshot file
+type SnapshotLoadError struct {
+	FilePath string
+	Err      error
+}
+
+func (e *SnapshotLoadError) Error() string {
+	return fmt.Sprintf("failed to load/parse snapshot %s: %v", e.FilePath, e.Err)
+}
+func (e *SnapshotLoadError) Unwrap() error { return e.Err }