Answer missing files with 404 instead of the index page

app/src/main/java/com/runcode/app/runtime/StaticWebEngine.kt

@@ -232,9 +232,11 @@ class StaticWebEngine(
 
     /**
      * Maps a request path onto a file inside [sourceDir], falling back to the entrypoint so
-     * single-page apps keep working. Returns null when the path escapes the source directory.
+     * single-page apps keep working. Returns null when the path escapes the source directory,
+     * and a file that does not exist when the request names a missing file, which the caller
+     * answers with 404.
      */
-    private fun resolveTarget(sourceDir: File, rawPath: String, entryPoint: String): File? {
+    internal fun resolveTarget(sourceDir: File, rawPath: String, entryPoint: String): File? {
         val decoded = try {
             URLDecoder.decode(rawPath, "UTF-8")
         } catch (_: Exception) {
@@ -246,10 +248,23 @@ class StaticWebEngine(
         if (!isInside(candidate, sourceDir)) return null
         if (candidate.isFile) return candidate
 
+        // A single-page app routes /dashboard through index.html, so an unknown route falls
+        // back to the entrypoint. A request that names a file does not: answering /app.css
+        // with the HTML page, and a 200 at that, broke pages quietly and hid typos in asset
+        // paths. Missing files get a 404 instead.
+        if (namesAFile(requested)) return candidate
+
         val fallback = File(sourceDir, entryPoint).canonicalFile
         return if (isInside(fallback, sourceDir)) fallback else null
     }
 
+    /** True when the last path segment carries an extension, e.g. `app.css` but not `dashboard`. */
+    private fun namesAFile(path: String): Boolean {
+        val name = path.trimEnd('/').substringAfterLast('/')
+        val dot = name.lastIndexOf('.')
+        return dot > 0 && dot < name.length - 1
+    }
+
     private fun isInside(candidate: File, root: File): Boolean {
         return candidate.path == root.path || candidate.path.startsWith(root.path + File.separator)
     }

app/src/test/java/com/runcode/app/runtime/StaticWebRoutingTest.kt

@@ -0,0 +1,65 @@
+package com.runcode.app.runtime
+
+import android.app.Application
+import com.runcode.app.network.PortManager
+import com.runcode.app.system.ProcessMonitor
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNull
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+import java.io.File
+import java.nio.file.Files
+
+@RunWith(RobolectricTestRunner::class)
+@Config(sdk = [28], application = Application::class)
+class StaticWebRoutingTest {
+
+    private lateinit var engine: StaticWebEngine
+    private lateinit var sourceDir: File
+
+    @Before fun setup() {
+        engine = StaticWebEngine(PortManager(), ProcessMonitor())
+        sourceDir = Files.createTempDirectory("static-web").toFile()
+        File(sourceDir, "index.html").writeText("<html></html>")
+        File(sourceDir, "style.css").writeText("body{}")
+        File(sourceDir, "assets").mkdirs()
+        File(sourceDir, "assets/app.js").writeText("// app")
+    }
+
+    private fun resolve(path: String) = engine.resolveTarget(sourceDir, path, "index.html")
+
+    @Test fun `root serves the entrypoint`() {
+        assertEquals(File(sourceDir, "index.html").canonicalFile, resolve(""))
+    }
+
+    @Test fun `existing files are served`() {
+        assertEquals(File(sourceDir, "style.css").canonicalFile, resolve("style.css"))
+        assertEquals(File(sourceDir, "assets/app.js").canonicalFile, resolve("assets/app.js"))
+    }
+
+    @Test fun `a route with no extension falls back to the entrypoint`() {
+        assertEquals(File(sourceDir, "index.html").canonicalFile, resolve("dashboard"))
+        assertEquals(File(sourceDir, "index.html").canonicalFile, resolve("users/42/"))
+    }
+
+    @Test fun `a missing file is not answered with the entrypoint`() {
+        // It used to return index.html with a 200, so a typo in an asset path broke the page
+        // quietly. The caller turns a file that does not exist into a 404.
+        val missing = resolve("missing.css")
+        assertEquals(File(sourceDir, "missing.css").canonicalFile, missing)
+        assertFalse(missing!!.isFile)
+
+        val missingNested = resolve("assets/nope.js")
+        assertEquals(File(sourceDir, "assets/nope.js").canonicalFile, missingNested)
+        assertFalse(missingNested!!.isFile)
+    }
+
+    @Test fun `paths that escape the source directory are refused`() {
+        assertNull(resolve("../secrets.txt"))
+        assertNull(resolve("assets/../../secrets.txt"))
+    }
+}