Redirect /subdir to /subdir/ when the page is given by docs/subdir/index.md (#274)

* Redirect /subdir to /subdir/ when the page is given by docs/subdir/index.md.

Otherwise links seen in preview under /subdir can look correct, but will fail in the build.

To test, run:
> yarn dev --root test/input/build/multi/
and open http://127.0.0.1:3000/subsection

* more precise redirection:

Given a /dir/index(.html) path, we redirect the browser to /dir/ if there is a dir/index.md file, otherwise to /dir.

This accomodates /javascript in the documentation as well as test/input/build/multi/subsection/

* remove redundant check

Author: Fil

src/preview.ts

@@ -123,17 +123,31 @@ export class PreviewServer {
 
         // If this path is for /index, redirect to the parent directory for a
         // tidy path. (This must be done before implicitly adding /index below!)
+        // Respect precedence of dir/index.md over dir.md in choosing between
+        // dir/ and dir!
         if (basename(path, ".html") === "index") {
-          res.writeHead(302, {Location: dirname(pathname) + url.search});
-          res.end();
-          return;
+          try {
+            await stat(join(dirname(path), "index.md"));
+            res.writeHead(302, {Location: dirname(pathname) + "/" + url.search});
+            res.end();
+            return;
+          } catch (error) {
+            if (!isEnoent(error)) throw error;
+            res.writeHead(302, {Location: dirname(pathname) + url.search});
+            res.end();
+            return;
+          }
         }
 
         // If this path resolves to a directory, then add an implicit /index to
         // the end of the path, assuming that the corresponding index.md exists.
         try {
-          if ((await stat(path)).isDirectory() && (await stat(join(path, "index") + ".md")).isFile()) {
-            await access(join(path, "index") + ".md", constants.R_OK);
+          if ((await stat(path)).isDirectory() && (await stat(join(path, "index.md"))).isFile()) {
+            if (!pathname.endsWith("/")) {
+              res.writeHead(302, {Location: pathname + "/" + url.search});
+              res.end();
+              return;
+            }
             pathname = join(pathname, "index");
             path = join(path, "index");
           }

test/input/build/multi/subsection/index.md

@@ -1,3 +1,5 @@
 # Sub-Section
 
-This is a sub-section of the multi-section page.
+This is a sub-section of the multi-section page.
+
+We can link to [this sub-section](./) and to [the parent section](../).

test/output/build/multi/subsection/index.html

@@ -33,6 +33,7 @@
 <main id="observablehq-main" class="observablehq">
 <h1 id="sub-section" tabindex="-1"><a class="observablehq-header-anchor" href="#sub-section">Sub-Section</a></h1>
 <p>This is a sub-section of the multi-section page.</p>
+<p>We can link to <a href="./">this sub-section</a> and to <a href="../">the parent section</a>.</p>
 </main>
 <footer id="observablehq-footer">
 <nav><a rel="prev" href="../"><span>Home</span></a></nav>