pkg/podman: Add error parsing method

This new helper function parses the output of Podman in stderr into
a dedicated structure with defined methods for working with the error.

The most significant part is the method Is() that wraps around
errors.Is() and if that fails to find a match resorts to string
matching. This is needed because there are no defined errors to catch
errors coming from Podman.

The structure is only meant to be used internally.

containers#786

src/pkg/podman/podman.go

@@ -19,8 +19,10 @@ package podman
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
+	"strings"
 
 	"github.com/HarryMichal/go-version"
 	"github.com/containers/toolbox/pkg/shell"
@@ -332,3 +334,67 @@ func SystemMigrate(ociRuntimeRequired string) error {
 
 	return nil
 }
+
+type internalError struct {
+	Err error
+}
+
+func (e *internalError) Error() string {
+	if e.Err == nil {
+		return ""
+	}
+	return e.Err.Error()
+}
+
+func (e *internalError) Wrap(format string, a ...interface{}) {
+	errMsg := fmt.Sprintf(format, a...)
+	if e.Err == nil {
+		e.Err = errors.New(errMsg)
+	} else {
+		e.Err = fmt.Errorf("%v: %w", errMsg, e.Err)
+	}
+}
+
+// Is wraps around errors.Is() and add support for string comparison of errors
+func (e internalError) Is(err error) bool {
+	if errors.Is(e.Err, err) {
+		return true
+	}
+
+	errMsgParts := strings.Split(e.Err.Error(), ": ")
+	for _, part := range errMsgParts {
+		if part == err.Error() {
+			return true
+		}
+	}
+
+	return false
+}
+
+// parseErrorMsg serves for converting error output of Podman into an error
+// that can be further used in Go.
+//
+// The returned error can be examined using methods Unwrap, Is & As.
+func parseErrorMsg(stderr *bytes.Buffer) internalError {
+	var err internalError
+	var errMsg string
+	var errMsgParts []string
+
+	errMsg = stderr.String()
+	errMsg = strings.TrimSpace(errMsg)
+	// Podman error messages are usually prepended with the "Error: " string
+	errMsg = strings.TrimPrefix(errMsg, "Error: ")
+	// Sometimes an error contains a newline (e.g. responses from Docker
+	// registry). Normlaize them into further parseable error message.
+	errMsg = strings.ReplaceAll(errMsg, "\n", ": ")
+	// Wrapped error messages are usually separated by a colon followed by
+	// a single space character
+	errMsgParts = strings.Split(errMsg, ": ")
+
+	// Wrapping happens from the first to the last error
+	for i := len(errMsgParts) - 1; i >= 0; i-- {
+		err.Wrap(errMsgParts[i])
+	}
+
+	return err
+}