Panic on missing metadata.

[ogoffart]

Metadata::parse panics if some metadata header is not present because it unwraps the result of get()

polib/src/metadata.rs

Lines 69 to 81 in a1b9b57

	project_id_version: key_values.get("Project-Id-Version").unwrap().to_string(),
	pot_creation_date: key_values.get("POT-Creation-Date").unwrap().to_string(),
	po_revision_date: key_values.get("PO-Revision-Date").unwrap().to_string(),
	last_translator: key_values.get("Last-Translator").unwrap_or(&"").to_string(),
	language_team: key_values.get("Language-Team").unwrap().to_string(),
	mime_version: key_values.get("MIME-Version").unwrap().to_string(),
	content_type: key_values.get("Content-Type").unwrap().to_string(),
	content_transfer_encoding: key_values
	.get("Content-Transfer-Encoding")
	.unwrap()
	.to_string(),
	language: key_values.get("Language").unwrap().to_string(),
	plural_rules: CatalogPluralRules::parse(key_values.get("Plural-Forms").unwrap())?,

Example: parsing this basic file generated with xgettext with default options
Notice that it doesn't contains Plural-Forms

# SOME DESCRIPTIVE TITLE.
# Copyright (C) YEAR THE PACKAGE'S COPYRIGHT HOLDER
# This file is distributed under the same license as the PACKAGE package.
# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.
#
#, fuzzy
msgid ""
msgstr ""
"Project-Id-Version: PACKAGE VERSION\n"
"Report-Msgid-Bugs-To: \n"
"POT-Creation-Date: 2023-06-09 17:34+0200\n"
"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
"Language-Team: LANGUAGE <LL@li.org>\n"
"Language: \n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=CHARSET\n"
"Content-Transfer-Encoding: 8bit\n"

#: main.cpp:28
msgid "item"
msgstr ""

And parsing this files with polib::po_file::parse will panics with

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/polib-0.2.0/src/metadata.rs:81:84

It should use unwrap_or_default or something like that instead

Ideally, the MetaData should also keep the metadata as is so i can add messages to a .pot file without changing most metadata.

[BrettDong]

Thank you for reporting the issue. When I wrote the library I was assuming input data is a .po file of translation data for a particular language, so Plural-Forms is always present in the metadata. Some code changes in polib is necessary to accommodate .pot files.