avoid bad links in version dropdowns (#2501)

* add fallback case for no latest version

* add get-valid-versions function

* use get-valid-versions function

src/components/docs-version-switcher/index.tsx

@@ -73,6 +73,12 @@ const DocsVersionSwitcher = ({
 		selectedOption = options.find(
 			(option: VersionSelectItem) => option.isLatest === true
 		)
+		// In some edge cases, there may be no latest version, such as for
+		// versioned docs that no longer exist in the latest version. For these
+		// cases, fallback to selecting the first option.
+		if (!selectedOption) {
+			selectedOption = options[0]
+		}
 	}
 
 	const projectNameForLabel = setProjectForAriaLabel(

src/views/docs-view/server.ts

@@ -36,6 +36,7 @@ import {
 import tutorialMap from 'data/_tutorial-map.generated.json'
 
 // Local imports
+import { getValidVersions } from './utils/get-valid-versions'
 import { getProductUrlAdjuster } from './utils/product-url-adjusters'
 import { getBackToLink } from './utils/get-back-to-link'
 import { getDeployPreviewLoader } from './utils/get-deploy-preview-loader'
@@ -384,6 +385,21 @@ export function getStaticGenerationFunctions<
 				mainWidth: isDocsLanding ? 'wide' : 'narrow',
 			}
 
+			/**
+			 * Filter versions to include only those where this document exists
+			 */
+			// Construct a document path that the content API will recognize
+			const pathWithoutVersion = pathParts
+				.filter((part) => part !== versionPathPart)
+				.join('/')
+			const fullPath = `doc#${path.join(basePathForLoader, pathWithoutVersion)}`
+			// Filter for valid versions, fetching from the content API under the hood
+			const validVersions = await getValidVersions(
+				versions,
+				fullPath,
+				productSlugForLoader
+			)
+
 			/**
 			 * Determine whether to show the version selector
 			 *
@@ -392,8 +408,8 @@ export function getStaticGenerationFunctions<
 			 * (We use `v0.0.x` as a placeholder version for un-versioned documentation)
 			 */
 			const hasMeaningfulVersions =
-				versions.length > 0 &&
-				(versions.length > 1 || versions[0].version !== 'v0.0.x')
+				validVersions.length > 0 &&
+				(validVersions.length > 1 || validVersions[0].version !== 'v0.0.x')
 
 			/**
 			 * We want to show "Edit on GitHub" links for public content repos only.
@@ -440,7 +456,7 @@ export function getStaticGenerationFunctions<
 					!hideVersionSelector &&
 					!isReleaseNotesPage(currentPathUnderProduct) && // toggle version dropdown
 					hasMeaningfulVersions
-						? versions
+						? validVersions
 						: null,
 			}
 

src/views/docs-view/utils/get-valid-versions.ts

@@ -0,0 +1,64 @@
+/**
+ * Copyright (c) HashiCorp, Inc.
+ * SPDX-License-Identifier: MPL-2.0
+ */
+
+// Types
+import type { VersionSelectItem } from '../loaders/remote-content'
+
+const CONTENT_API_URL = process.env.MKTG_CONTENT_API
+const VERSIONS_ENDPOINT = '/api/content-versions'
+
+/**
+ * Given a list of all possible versions, as well as a document path and
+ * content repo identifier for our content API,
+ * Return a filter list of versions that includes only those versions
+ * where this document exists.
+ *
+ * To determine in which versions this document exists, we make a request
+ * to a content API that returns a list of strings representing known versions.
+ * We use this to filter out unknown versions from our incoming version list.
+ */
+export async function getValidVersions(
+	/**
+	 * An array of version select items representing all possible versions for
+	 * the content source repository in question (`productSlugForLoader`).
+	 * May be undefined or empty if versioned docs are not enabled, for example
+	 * during local preview.
+	 */
+	versions: VersionSelectItem[],
+	/**
+	 * A identifier for the document, consumable by our content API.
+	 * For markdown documents, this is `doc#` followed by the full path of the
+	 * document within the content source repository.
+	 */
+	fullPath: string,
+	/**
+	 * The product slug for the document, consumable by our content API.
+	 * The naming here is difficult, as the actual function here is to identify
+	 * specific content source repositories. These are often but not always
+	 * product slugs. For example Terraform has multiple content source repos
+	 * for different parts of the product.
+	 */
+	productSlugForLoader: string
+): Promise<VersionSelectItem[]> {
+	// If versions are falsy or empty, we can skip the API calls and return []
+	if (!versions || versions.length === 0) return []
+	try {
+		// Build the URL to fetch known versions of this document
+		const validVersionsUrl = new URL(VERSIONS_ENDPOINT, CONTENT_API_URL)
+		validVersionsUrl.searchParams.set('product', productSlugForLoader)
+		validVersionsUrl.searchParams.set('fullPath', fullPath)
+		// Fetch known versions of this document
+		const response = await fetch(validVersionsUrl.toString())
+		const { versions: knownVersions } = await response.json()
+		// Apply the filter, and return the valid versions
+		return versions.filter((option) => knownVersions.includes(option.version))
+	} catch (error) {
+		console.error(
+			`[docs-view/server] error fetching known versions for "${productSlugForLoader}" document "${fullPath}". Falling back to showing all versions.`,
+			error
+		)
+		return versions
+	}
+}