[flutter_plugin_tools] Improve version check error handling (flutter#4376)

Catches invalid CHANGELOG formats and logs useful error messages for them,
rather than throwing FormatExceptions.

Author: stuartmorgan-g

script/tool/CHANGELOG.md

@@ -8,6 +8,7 @@
 - `publish-check` now validates that there is an `AUTHORS` file.
 - Added flags to `version-check` to allow overriding the platform interface
   major version change restriction.
+- Improved error handling and error messages in CHANGELOG version checks.
 
 ## 0.7.1
 

script/tool/lib/src/version_check_command.dart

@@ -374,8 +374,9 @@ ${indentation}HTTP response: ${pubVersionFinderResponse.httpResponse.body}
       print(
           '${indentation}Found NEXT; validating next version in the CHANGELOG.');
       // Ensure that the version in pubspec hasn't changed without updating
-      // CHANGELOG. That means the next version entry in the CHANGELOG pass the
-      // normal validation.
+      // CHANGELOG. That means the next version entry in the CHANGELOG should
+      // pass the normal validation.
+      versionString = null;
       while (iterator.moveNext()) {
         if (iterator.current.trim().startsWith('## ')) {
           versionString = iterator.current.trim().split(' ').last;
@@ -384,11 +385,19 @@ ${indentation}HTTP response: ${pubVersionFinderResponse.httpResponse.body}
       }
     }
 
-    final Version? fromChangeLog =
-        versionString == null ? null : Version.parse(versionString);
-    if (fromChangeLog == null) {
-      printError(
-          '${indentation}Cannot find version on the first line CHANGELOG.md');
+    if (versionString == null) {
+      printError('${indentation}Unable to find a version in CHANGELOG.md');
+      print('${indentation}The current version should be on a line starting '
+          'with "## ", either on the first non-empty line or after a "## NEXT" '
+          'section.');
+      return false;
+    }
+
+    final Version fromChangeLog;
+    try {
+      fromChangeLog = Version.parse(versionString);
+    } on FormatException {
+      printError('"$versionString" could not be parsed as a version.');
       return false;
     }
 

script/tool/test/version_check_command_test.dart

@@ -601,6 +601,73 @@ This is necessary because of X, Y, and Z
       );
     });
 
+    test(
+        'fails gracefully if the version headers are not found due to using the wrong style',
+        () async {
+      final Directory pluginDirectory =
+          createFakePlugin('plugin', packagesDir, version: '1.0.0');
+
+      const String changelog = '''
+## NEXT
+* Some changes for a later release.
+# 1.0.0
+* Some other changes.
+''';
+      createFakeCHANGELOG(pluginDirectory, changelog);
+      gitShowResponses = <String, String>{
+        'master:packages/plugin/pubspec.yaml': 'version: 1.0.0',
+      };
+
+      Error? commandError;
+      final List<String> output = await runCapturingPrint(runner, <String>[
+        'version-check',
+        '--base-sha=master',
+      ], errorHandler: (Error e) {
+        commandError = e;
+      });
+
+      expect(commandError, isA<ToolExit>());
+      expect(
+        output,
+        containsAllInOrder(<Matcher>[
+          contains('Unable to find a version in CHANGELOG.md'),
+          contains('The current version should be on a line starting with '
+              '"## ", either on the first non-empty line or after a "## NEXT" '
+              'section.'),
+        ]),
+      );
+    });
+
+    test('fails gracefully if the version is unparseable', () async {
+      final Directory pluginDirectory =
+          createFakePlugin('plugin', packagesDir, version: '1.0.0');
+
+      const String changelog = '''
+## Alpha
+* Some changes.
+''';
+      createFakeCHANGELOG(pluginDirectory, changelog);
+      gitShowResponses = <String, String>{
+        'master:packages/plugin/pubspec.yaml': 'version: 1.0.0',
+      };
+
+      Error? commandError;
+      final List<String> output = await runCapturingPrint(runner, <String>[
+        'version-check',
+        '--base-sha=master',
+      ], errorHandler: (Error e) {
+        commandError = e;
+      });
+
+      expect(commandError, isA<ToolExit>());
+      expect(
+        output,
+        containsAllInOrder(<Matcher>[
+          contains('"Alpha" could not be parsed as a version.'),
+        ]),
+      );
+    });
+
     test('allows valid against pub', () async {
       mockHttpResponse = <String, dynamic>{
         'name': 'some_package',