Updated version of the fstest article.

website/articles/fstest-mapfs-file-does-not-exist.html

@@ -17,22 +17,54 @@
     <main>
         <article>
             <section id="introduction">
+                <h1>The problem</h1>
                 <p>
-                    In my tests, I defined an <code>fstest.MapFS</code> like this
+                    I rewrote my tests to use an  <code>fstest.MapFS</code>, defined like this
 <code>
-    <pre>
-contentRoot := fstest.MapFS{
-            "/parentDir/index.html": &fstest.MapFile{Data: []byte("content"), Mode: 0o755},
-            "/parentDir/childDir/1-2-3.html": &fstest.MapFile{Data: []byte("other content"`), Mode: 0o755},
-        }
-    </pre>
+<pre>contentRoot := fstest.MapFS{
+    "/parentDir/index.html": &fstest.MapFile{Data: []byte("content"), Mode: 0o755},
+    "/parentDir/childDir/1-2-3.html": &fstest.MapFile{Data: []byte("other content"`), Mode: 0o755},
+}</pre>
 </code>
-                Simple, and yet when I ran a test against it, I received this error:
+
+                    Simple, and yet when I ran a test against it, I received this error:
+                    <code>
+                        <pre>2024/03/27 19:42:59 http: panic serving [::1]:57135: open parentDir: file does not exist</pre>
+                    </code>
+                </p>
+                </section>
+
+                <section id="problem-details">
+                    <h1>The setup</h1>
+                <p>
+                I wrote this comment a few days ago and haven't thought much about it since:
 <code>
-    <pre>
-2024/03/27 19:42:59 http: panic serving [::1]:57135: open parentDir: file does not exist
-    </pre>
+<pre>
+// /index.html becomes index.html
+// /articles/page.html becomes articles/page.html
+// without this the paths aren't found properly inside the fs.
+pagePath = strings.TrimPrefix(pagePath, "/")
+pageContent, err := fs.ReadFile(a.SiteFiles, pagePath)</pre>
+</code>
+
+                    pagePath is retrieved from an http GET, and <i>always</i> starts with a "/"; I don't want to be seeking for an absolute 
+                    path inside the os.DirFS because the file won't be found. 
+                </p>
+
+                <p>
+                    ReadFile dispatches to the a.SiteFiles implementation of the Open function, so the <code>os.DirFS</code> controls lookup of the file. You
+                    can see that <a href="https://cs.opensource.google/go/go/+/refs/tags/go1.22.1:src/io/fs/readfile.go;l=37">in the official source</a>:
+                    <code>
+<pre>func ReadFile(fsys FS, name string) ([]byte, error) {
+if fsys, ok := fsys.(ReadFileFS); ok {
+    return fsys.ReadFile(name)
+}
+
+file, err := fsys.Open(name)</pre>
 </code>
+                </p>
+
+                <p>
 
 </p>
 </section>
@@ -43,31 +75,23 @@ <h1>What's the misunderstanding?</h1>
                     This one took me a couple of hours to finally understand.
                 </p>
                 <p>
-                    An <code>fstest.MapFS</code> is pretty much a hash map. <i>If you don't have a precise match for the key in the map
-                    then you get a <code>file does not exist</code> error.</i> Here's the implementation that shows that:
+                    An <code>fstest.MapFS</code> is a hash map. Its implementation of <code>Open()</code> is very simple:
 <code>
-    <pre>
-        file := fsys[name]
-    </pre>
-- reference in <a href="https://cs.opensource.google/go/go/+/refs/tags/go1.22.1:src/testing/fstest/mapfs.go;l=51">the go source code</a>.
+    <pre>file := fsys[name]</pre>
+    - reference in <a href="https://cs.opensource.google/go/go/+/refs/tags/go1.22.1:src/testing/fstest/mapfs.go;l=51">the go source code</a>.
 </code>
+
+<i>It simply looks up the key in the hashmap, and I was manipulating that key to remove a leading slash.</i>
                 </p>
                 <h1>What's the fix?</h1>
                 <p>
-
-                    In my case, I was performing some path munging in my logic so that I was searching for paths relatively, not absolutely 
-                    using a leading "/", so my search string was "parentDir/index.html". 
-                </p>
-                <p>
-                    Once I understood that <i>the path in the <code>fstest.MapFS</code> does not need to be an absolute path</i>, because <i>it is only
-                    a key in a hash map</i>, I redefined my structure like this and got my tests passing:
+                    Once I understood that <i>the path in the <code>fstest.MapFS</code> is only a map key</i>, I realised it does not need to be an absolute path. 
+                    I redefined it like this and got my tests passing:
 <code>
-    <pre>
-contentRoot := fstest.MapFS{
-            "/parentDir/index.html": &fstest.MapFile{Data: []byte("content"), Mode: 0o755},
-            "/parentDir/childDir/1-2-3.html": &fstest.MapFile{Data: []byte("other content"`), Mode: 0o755},
-        }
-    </pre>
+    <pre>contentRoot := fstest.MapFS{
+"parentDir/index.html": &fstest.MapFile{Data: []byte("content"), Mode: 0o755},
+"parentDir/childDir/1-2-3.html": &fstest.MapFile{Data: []byte("other content"`), Mode: 0o755},
+}</pre>
 </code>
                 </p>
             </section>