Display better 404 messages when generated dartdoc page is missing. (#…

…8038)

app/lib/dartdoc/models.dart

@@ -7,7 +7,7 @@ import 'package:json_annotation/json_annotation.dart';
 part 'models.g.dart';
 
 /// Describes the resolved version and the URL redirect info.
-@JsonSerializable()
+@JsonSerializable(includeIfNull: false)
 class ResolvedDocUrlVersion {
   /// The version to use to display the documentation.
   final String version;
@@ -20,13 +20,18 @@ class ResolvedDocUrlVersion {
   /// * `""` (indicating empty response)
   final String urlSegment;
 
+  /// When the resolution is empty, the 404 response will have this message.
+  final String? message;
+
   ResolvedDocUrlVersion({
     required this.version,
     required this.urlSegment,
+    this.message,
   });
 
-  ResolvedDocUrlVersion.empty()
-      : version = '',
+  ResolvedDocUrlVersion.empty({
+    required this.message,
+  })  : version = '',
         urlSegment = '';
 
   factory ResolvedDocUrlVersion.fromJson(Map<String, dynamic> json) =>
@@ -35,4 +40,5 @@ class ResolvedDocUrlVersion {
   Map<String, dynamic> toJson() => _$ResolvedDocUrlVersionToJson(this);
 
   bool get isEmpty => version.isEmpty || urlSegment.isEmpty;
+  bool get isLatestStable => urlSegment == 'latest';
 }

app/lib/frontend/handlers/documentation.dart

@@ -50,7 +50,10 @@ Future<shelf.Response> documentationHandler(shelf.Request request) async {
   final version = docFilePath.version!;
   final resolved = await _resolveDocUrlVersion(package, version);
   if (resolved.isEmpty) {
-    return notFoundHandler(request);
+    return notFoundHandler(
+      request,
+      body: resolved.message ?? default404NotFound,
+    );
   }
   if (version != resolved.urlSegment) {
     return redirectResponse(pkgDocUrl(
@@ -147,7 +150,8 @@ Future<ResolvedDocUrlVersion> _resolveDocUrlVersion(
     if (version == 'latest') {
       final latestFinished = await taskBackend.latestFinishedVersion(package);
       if (latestFinished == null) {
-        return ResolvedDocUrlVersion.empty();
+        return ResolvedDocUrlVersion.empty(
+            message: 'Analysis has not started yet.');
       }
       final latestVersion = await packageBackend.getLatestVersion(package);
       return ResolvedDocUrlVersion(
@@ -159,7 +163,7 @@ Future<ResolvedDocUrlVersion> _resolveDocUrlVersion(
     // Do not resolve if package version does not exists.
     final pv = await packageBackend.lookupPackageVersion(package, version);
     if (pv == null) {
-      return ResolvedDocUrlVersion.empty();
+      return ResolvedDocUrlVersion.empty(message: 'Not found.');
     }
 
     // Select the closest version (may be the same as version) that has a finished analysis.

app/lib/task/handlers.dart

@@ -9,6 +9,7 @@ import 'package:pub_dev/shared/handlers.dart';
 import 'package:pub_dev/shared/redis_cache.dart';
 import 'package:pub_dev/shared/urls.dart';
 import 'package:pub_dev/task/backend.dart';
+import 'package:pub_dev/task/models.dart';
 import 'package:shelf/shelf.dart' as shelf;
 
 const _safeMimeTypes = {
@@ -111,7 +112,31 @@ Future<shelf.Response> handleDartDoc(
     });
     // We use empty string to indicate missing file or bug in the file
     if (htmlBytes == null || htmlBytes.isEmpty) {
-      return notFoundHandler(request);
+      final status = await taskBackend.packageStatus(package);
+      final vs = status.versions[version];
+      if (vs == null) {
+        return notFoundHandler(
+          request,
+          body: resolvedDocUrlVersion.isLatestStable
+              ? 'Analysis has not started yet.'
+              : 'Version not selected for analysis.',
+        );
+      }
+      String? message;
+      switch (vs.status) {
+        case PackageVersionStatus.pending:
+        case PackageVersionStatus.running:
+          message = 'Analysis has not finished yet.';
+          break;
+        case PackageVersionStatus.failed:
+          message =
+              'Analysis has failed, no `dartdoc` output has been generated.';
+          break;
+        case PackageVersionStatus.completed:
+          message = '`dartdoc` did not generate this page.';
+          break;
+      }
+      return notFoundHandler(request, body: message);
     }
     return htmlBytesResponse(htmlBytes);
   }