GET form submissions to the current path

Closes #107

The [HTML specification][] outlines the algorithm for handling form
submissions, and describes the steps involved in handling a `method=get`
request with `application/x-www-form-urlencoded` encoding.

However, it doesn't explicitly mention how to handle a `<form
method="get">` submission when the `action` attribute is omitted and the
current URL includes a query parameter that would be included within the
`<form>` element fields' values.

Determining the test cases to reproduce browser default behavior
involved was some trial and error troubleshooting in real browsers with
default (JavaScript-less) `<form>` submission handling.

The algorithm appears to be as follows:

* If there are no query parameters, append all field values as query
   parameters

* If there are existing query parameters whose keys exist as `<form>`
  fields, override them based on the form field's value

  * If there are existing query parameters whose keys exist as
    _multiple_ `<form>` fields, override the first occurrence, and then
    append subsequent values

[HTML specification]: https://html.spec.whatwg.org/#form-submission-algorithm

src/http/fetch_request.ts

@@ -137,8 +137,17 @@ export class FetchRequest {
 }
 
 function mergeFormDataEntries(url: URL, entries: [string, FormDataEntryValue][]): URL {
+  const currentSearchParams = new URLSearchParams(url.search)
+
   for (const [ name, value ] of entries) {
-    url.searchParams.append(name, value.toString())
+    if (value instanceof File) continue
+
+    if (currentSearchParams.has(name)) {
+      currentSearchParams.delete(name)
+      url.searchParams.set(name, value)
+    } else {
+      url.searchParams.append(name, value)
+    }
   }
 
   return url

src/tests/fixtures/form.html

@@ -43,6 +43,22 @@
         <input type="hidden" name="greeting" value="Hello from a form">
         <input type="submit">
       </form>
+      <form action="/src/tests/fixtures/form.html?query=1" method="get" class="conflicting-values">
+        <input type="hidden" name="query" value="2">
+        <input type="submit">
+      </form>
+    </div>
+    <hr>
+    <div id="no-action">
+      <form class="single">
+        <input type="hidden" name="query" value="1">
+        <input type="submit">
+      </form>
+      <form class="multiple">
+        <input type="hidden" name="query" value="1">
+        <input type="hidden" name="query" value="2">
+        <input type="submit">
+      </form>
     </div>
     <hr>
     <div id="reject">

src/tests/functional/form_submission_tests.ts

@@ -15,7 +15,7 @@ export class FormSubmissionTests extends TurboDriveTestCase {
     this.assert.ok(await this.formSubmitted)
     this.assert.equal(await this.pathname, "/src/tests/fixtures/form.html")
     this.assert.equal(await this.visitAction, "advance")
-    this.assert.equal((await this.searchParams).get("greeting"), "Hello from a redirect")
+    this.assert.equal(await this.getSearchParam("greeting"), "Hello from a redirect")
   }
 
   async "test standard GET form submission"() {
@@ -25,7 +25,16 @@ export class FormSubmissionTests extends TurboDriveTestCase {
     this.assert.notOk(await this.formSubmitted)
     this.assert.equal(await this.pathname, "/src/tests/fixtures/one.html")
     this.assert.equal(await this.visitAction, "advance")
-    this.assert.equal((await this.searchParams).get("greeting"), "Hello from a form")
+    this.assert.equal(await this.getSearchParam("greeting"), "Hello from a form")
+  }
+
+  async "test standard GET form submission appending keys"() {
+    await this.goToLocation("/src/tests/fixtures/form.html?query=1")
+    await this.clickSelector("#standard form.conflicting-values input[type=submit]")
+    await this.nextBody
+
+    this.assert.equal(await this.pathname, "/src/tests/fixtures/form.html")
+    this.assert.equal(await await this.getSearchParam("query"), "2")
   }
 
   async "test standard form submission with empty created response"() {
@@ -52,26 +61,62 @@ export class FormSubmissionTests extends TurboDriveTestCase {
     await this.clickSelector("#standard form[method=post][enctype] input[type=submit]")
     await this.nextBeat
 
-    const enctype = (await this.searchParams).get("enctype")
+    const enctype = await this.getSearchParam("enctype")
     this.assert.ok(enctype?.startsWith("multipart/form-data"), "submits a multipart/form-data request")
   }
 
   async "test standard GET form submission ignores enctype"() {
     await this.clickSelector("#standard form[method=get][enctype] input[type=submit]")
     await this.nextBeat
 
-    const enctype = (await this.searchParams).get("enctype")
+    const enctype = await this.getSearchParam("enctype")
     this.assert.notOk(enctype, "GET form submissions ignore enctype")
   }
 
   async "test standard POST form submission without an enctype"() {
     await this.clickSelector("#standard form[method=post].no-enctype input[type=submit]")
     await this.nextBeat
 
-    const enctype = (await this.searchParams).get("enctype")
+    const enctype = await this.getSearchParam("enctype")
     this.assert.ok(enctype?.startsWith("application/x-www-form-urlencoded"), "submits a application/x-www-form-urlencoded request")
   }
 
+  async "test no-action form submission with single parameter"() {
+    await this.clickSelector("#no-action form.single input[type=submit]")
+    await this.nextBody
+
+    this.assert.equal(await this.pathname, "/src/tests/fixtures/form.html")
+    this.assert.equal(await await this.getSearchParam("query"), "1")
+
+    await this.clickSelector("#no-action form.single input[type=submit]")
+    await this.nextBody
+
+    this.assert.equal(await this.pathname, "/src/tests/fixtures/form.html")
+    this.assert.equal(await await this.getSearchParam("query"), "1")
+
+    await this.goToLocation("/src/tests/fixtures/form.html?query=2")
+    await this.clickSelector("#no-action form.single input[type=submit]")
+    await this.nextBody
+
+    this.assert.equal(await this.pathname, "/src/tests/fixtures/form.html")
+    this.assert.equal(await await this.getSearchParam("query"), "1")
+  }
+
+  async "test no-action form submission with multiple parameters"() {
+    await this.goToLocation("/src/tests/fixtures/form.html?query=2")
+    await this.clickSelector("#no-action form.multiple input[type=submit]")
+    await this.nextBody
+
+    this.assert.equal(await this.pathname, "/src/tests/fixtures/form.html")
+    this.assert.deepEqual(await this.getAllSearchParams("query"), [ "1", "2" ])
+
+    await this.clickSelector("#no-action form.multiple input[type=submit]")
+    await this.nextBody
+
+    this.assert.equal(await this.pathname, "/src/tests/fixtures/form.html")
+    this.assert.deepEqual(await this.getAllSearchParams("query"), [ "1", "2" ])
+  }
+
   async "test invalid form submission with unprocessable entity status"() {
     await this.clickSelector("#reject form.unprocessable_entity input[type=submit]")
     await this.nextBody
@@ -103,7 +148,7 @@ export class FormSubmissionTests extends TurboDriveTestCase {
     await this.clickSelector("#submitter form[method=post]:not([enctype]) input[formenctype]")
     await this.nextBeat
 
-    const enctype = (await this.searchParams).get("enctype")
+    const enctype = await this.getSearchParam("enctype")
     this.assert.ok(enctype?.startsWith("multipart/form-data"), "submits a multipart/form-data request")
   }
 

src/tests/helpers/functional_test_case.ts

@@ -94,6 +94,14 @@ export class FunctionalTestCase extends InternTestCase {
     return this.evaluate(() => location.search).then(search => new URLSearchParams(search))
   }
 
+  async getSearchParam(key: string): Promise<string> {
+    return (await this.searchParams).get(key) || ""
+  }
+
+  async getAllSearchParams(key: string): Promise<string[]> {
+    return (await this.searchParams).getAll(key) || []
+  }
+
   get hash(): Promise<string> {
     return this.evaluate(() => location.hash)
   }