pkg/podman: Add error parsing method

This new helper type serves for parsing the output of Podman in stderr
into a form that can be further analyzed.

The most significant part is the method Is() that lexically compares
error strings. This is needed because there are no defined errors to
catch errors coming from Podman.

#786

src/pkg/podman/podman.go

@@ -21,6 +21,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"strings"
 
 	"github.com/HarryMichal/go-version"
 	"github.com/containers/toolbox/pkg/shell"
@@ -332,3 +333,83 @@ func SystemMigrate(ociRuntimeRequired string) error {
 
 	return nil
 }
+
+// internalError serves for representing errors printed by Podman to stderr
+type internalError struct {
+	errors []string
+}
+
+func (e *internalError) Error() string {
+	if e.errors == nil || len(e.errors) == 0 {
+		return ""
+	}
+
+	var builder strings.Builder
+	for i, part := range e.errors {
+		if i != 0 {
+			builder.WriteString(": ")
+		}
+		builder.WriteString(part)
+	}
+	return builder.String()
+}
+
+// Is lexically compares errors
+//
+// The comparison is done for every part in the error chain not across.
+func (e *internalError) Is(target error) bool {
+	if target == nil {
+		return false
+	}
+
+	if e.errors == nil || len(e.errors) == 0 {
+		return false
+	}
+
+	for {
+		if e.errors[0] == target.Error() {
+			return true
+		}
+
+		if e = e.Unwrap(); e == nil {
+			return false
+		}
+	}
+}
+
+func (e internalError) Unwrap() *internalError {
+	if e.errors == nil || len(e.errors) <= 1 {
+		return nil
+	}
+
+	return &internalError{e.errors[1:]}
+}
+
+// parseErrorMsg serves for converting error output of Podman into an error
+// that can be further used in Go
+func parseErrorMsg(stderr *bytes.Buffer) error {
+	errMsg := stderr.String()
+	errMsg = strings.TrimSpace(errMsg)
+	// Sometimes an error contains a newline (e.g. responses from Docker
+	// registry). Normalize 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
+	errMsgPartsRaw := strings.Split(errMsg, ": ")
+	// The parts of the err message still can have a whitespace at the
+	// beginning ar at the end. Trim them.
+	var errMsgParts []string
+	for _, part := range errMsgPartsRaw {
+		part = strings.TrimSpace(part)
+		// Podman error messages are usually prepended with the "Error: " string
+		// Sometimes the error contains errors in a bullet list. This list is
+		// usually prepended with a message equal to "errors:"
+		if part == "Error" || part == "errors:" {
+			continue
+		}
+
+		errMsgParts = append(errMsgParts, part)
+	}
+
+	return &internalError{errMsgParts}
+}