Fallback to the default section if one is not passed in the resolution context.

.changeset/odd-humans-chew.md

@@ -0,0 +1,5 @@
+---
+"gitbook-v2": patch
+---
+
+Fallback to the default section if one is not passed in the resolution context.

packages/gitbook-v2/src/lib/context.ts

@@ -239,12 +239,12 @@ export async function fetchSiteContextByIds(
         ...(customizations.site?.title ? { title: customizations.site.title } : {}),
     };
 
-    const sections = ids.siteSection
-        ? parseSiteSectionsAndGroups(siteStructure, ids.siteSection)
-        : null;
-
-    // Parse the current siteSpace and siteSpaces based on the site structure type.
-    const { siteSpaces, siteSpace }: { siteSpaces: SiteSpace[]; siteSpace: SiteSpace } = (() => {
+    // Parse the current siteSpace, siteSpaces, and sections based on the site structure type.
+    const {
+        siteSpaces,
+        siteSpace,
+        sections,
+    }: { siteSpaces: SiteSpace[]; siteSpace: SiteSpace; sections: SiteSections | null } = (() => {
         if (siteStructure.type === 'siteSpaces') {
             const siteSpaces = siteStructure.structure;
             const siteSpace = siteSpaces.find((siteSpace) => siteSpace.id === ids.siteSpace);
@@ -255,15 +255,54 @@ export async function fetchSiteContextByIds(
                 );
             }
 
-            return { siteSpaces, siteSpace };
+            return { siteSpaces, siteSpace, sections: null };
         }
 
         if (siteStructure.type === 'sections') {
-            assert(
-                sections,
-                `cannot find site space "${ids.siteSpace}" because parsed sections are missing siteStructure.type="sections" siteSection="${ids.siteSection}"`
-            );
-
+            /**
+             * Workaround for #inc-56-sites-are-crashing-with-application-error-a-client-side-exception-ha
+             *
+             * If a site structure is set up as sections, we should always be given a siteSection in the ids. However,
+             * in the incident we noticed it is sometimes undefined - perhaps due to caching.
+             *
+             * This workaround will pick the default section in the site structure if no explicit is provided. It should
+             * be eventually removed and replaced with an assert.
+             */
+            const currentSectionId = (() => {
+                if (ids.siteSection) {
+                    return ids.siteSection;
+                }
+
+                let defaultSectionId: string | undefined;
+
+                for (const sectionOrGroup of siteStructure.structure) {
+                    if (sectionOrGroup.object === 'site-section') {
+                        defaultSectionId = sectionOrGroup.id;
+                        break;
+                    }
+
+                    if (sectionOrGroup.object === 'site-section-group') {
+                        const defaultSection = sectionOrGroup.sections.find(
+                            (group) => group.default
+                        );
+
+                        if (defaultSection) {
+                            defaultSectionId = defaultSection.id;
+                            break;
+                        }
+                    }
+                }
+
+                if (!defaultSectionId) {
+                    throw new Error(
+                        `No default section found in structure type="sections" for site "${ids.site}"`
+                    );
+                }
+
+                return defaultSectionId;
+            })();
+
+            const sections = parseSiteSectionsAndGroups(siteStructure, currentSectionId);
             const currentSection = sections.current;
             const siteSpaces = currentSection.siteSpaces;
             const siteSpace = currentSection.siteSpaces.find(
@@ -276,11 +315,14 @@ export async function fetchSiteContextByIds(
                 );
             }
 
-            return { siteSpaces, siteSpace };
+            return { siteSpaces, siteSpace, sections };
         }
 
-        // @ts-expect-error
-        assertNever(siteStructure, `cannot handle site structure of type ${siteStructure.type}`);
+        assertNever(
+            siteStructure,
+            // @ts-expect-error
+            `cannot handle site structure of type ${siteStructure.type}`
+        );
     })();
 
     const customization = (() => {
@@ -404,7 +446,7 @@ export function checkIsRootSiteContext(context: GitBookSiteContext): boolean {
     }
 }
 
-function parseSiteSectionsAndGroups(structure: SiteStructure, siteSectionId: string) {
+function parseSiteSectionsAndGroups(structure: SiteStructure, siteSectionId: string): SiteSections {
     const sectionsAndGroups = getSiteStructureSections(structure, { ignoreGroups: false });
     const section = parseCurrentSection(structure, siteSectionId);
     assert(section, 'A section must be defined when there are multiple sections');