Include version and checksum validation when updating packages (#27)

This PR closes #24. Here, we now check against two conditions when
updating the package
- whether the version is up to date plus valid, and
- whether the SHA-256 checksum matches the one present in the recipe
file.

Therefore, updating the package has three scenarios, and here is a
summary of what happens:
- local version updated manually to the newest available PyPI version ➡️
checksums will be updated, too
- both version and checksum are out of date ➡️ both of them will be
updated (as it was before these changes)
- the checksum is correct and the version is out of date ➡️ this is
enough information for us to proceed with updating the version

Additionally, there's now a case where one manually updates the version
(in error, for example), to a version that is not released or available
on PyPI (yet), which means that the metadata for the version won't exist
either. We raise an exception early here, asking the user so that they
can check the version while updating. This is a rare situation, so it's
more about raising a helpful error that aids the user in debugging the
problem. I have skipped adding a test for such a case, but I can add one
if needed. Please let me know your thoughts!

CHANGELOG.md

@@ -9,6 +9,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- The `pyodide skeleton pypi --update` command and the `--update-patched` variant now
+  validate the version and the source checksum when updating a package's recipe.
+  [#27](https://github.com/pyodide/pyodide-build/pull/27)
+
 - `pyo3_config_file` is no longer available in `pyodide config` command.
   Pyodide now sets `PYO3_CROSS_PYTHON_VERSION`, `PYO3_CROSS_LIB_DIR` to specify the cross compilation environment
   for PyO3.

pyodide_build/mkpkg.py

@@ -245,18 +245,46 @@ def update_package(
         old_fmt = "sdist"
 
     pypi_metadata = _get_metadata(package, version)
+
+    # Grab versions from metadata
     pypi_ver = Version(pypi_metadata["info"]["version"])
     local_ver = Version(yaml_content["package"]["version"])
-    already_up_to_date = pypi_ver <= local_ver and (
+
+    # and grab checksums from metadata
+    source_fmt = source_fmt or old_fmt
+    dist_metadata = _find_dist(pypi_metadata, [source_fmt])
+    sha256 = dist_metadata["digests"]["sha256"]
+    sha256_local = yaml_content["source"].get("sha256")
+
+    # fail if local version is newer than PyPI version
+    # since updating isn't possible in that case
+    if pypi_ver < local_ver:
+        raise MkpkgFailedException(
+            f"Local version {local_ver} is newer than PyPI version {pypi_ver}, "
+            f"cannot update {package}. Please verify in case the version was "
+            "updated manually and is correct."
+        )
+
+    # conditions to check if the package is up to date
+    is_sha256_up_to_date = sha256 == sha256_local
+    is_version_up_to_date = pypi_ver == local_ver
+
+    already_up_to_date = (is_sha256_up_to_date and is_version_up_to_date) and (
         source_fmt is None or source_fmt == old_fmt
     )
     if already_up_to_date:
         logger.success(
-            f"{package} already up to date. Local: {local_ver} PyPI: {pypi_ver}"
+            f"{package} already up to date."
+            f" Local: {local_ver} and PyPI: {pypi_ver}"
+            f" and checksum received: {sha256} matches local: {sha256_local} ✅"
         )
         return
 
-    logger.info(f"{package} is out of date: {local_ver} <= {pypi_ver}.")
+    logger.info(
+        f"{package} is out of date:"
+        f" either {local_ver} < {pypi_ver}"
+        f" or checksums might have mismatched: received {sha256} against local {sha256_local} 🚨"
+    )
 
     if yaml_content["source"].get("patches"):
         if update_patched:
@@ -267,6 +295,7 @@ def update_package(
         else:
             raise MkpkgFailedException(
                 f"Pyodide applies patches to {package}. Skipping update."
+                f" Use --update-patched to force updating {package}."
             )
 
     if source_fmt: