fix(ext/formdata): support multiple headers in FormData (#20801)

Fixes https://github.com/denoland/deno/issues/20793

cli/tests/unit/body_test.ts

@@ -155,3 +155,35 @@ Deno.test(async function bodyArrayBufferMultipleParts() {
   const body = buildBody(stream);
   assertEquals((await body.arrayBuffer()).byteLength, size);
 });
+
+// https://github.com/denoland/deno/issues/20793
+Deno.test(
+  { permissions: { net: true } },
+  async function bodyMultipartFormDataMultipleHeaders() {
+    const boundary = "----formdata-polyfill-0.970665446687947";
+    const payload = [
+      "------formdata-polyfill-0.970665446687947",
+      'Content-Disposition: form-data; name="x"; filename="blob"',
+      "Content-Length: 1",
+      "Content-Type: application/octet-stream",
+      "last-modified: Wed, 04 Oct 2023 20:28:45 GMT",
+      "",
+      "y",
+      "------formdata-polyfill-0.970665446687947--",
+    ].join("\r\n");
+
+    const body = buildBody(
+      new TextEncoder().encode(payload),
+      new Headers({
+        "Content-Type": `multipart/form-data; boundary=${boundary}`,
+      }),
+    );
+
+    const formData = await body.formData();
+    const file = formData.get("x");
+    assert(file instanceof File);
+    const text = await file.text();
+    assertEquals(text, "y");
+    assertEquals(file.size, 1);
+  },
+);

ext/fetch/21_formdata.js

@@ -448,25 +448,24 @@ class MultipartParser {
       const prevByte = this.body[i - 1];
       const isNewLine = byte === LF && prevByte === CR;
 
-      if (state === 1 || state === 2 || state == 3) {
+      if (state === 1) {
         headerText += StringFromCharCode(byte);
       }
+
       if (state === 0 && isNewLine) {
         state = 1;
-      } else if (state === 1 && isNewLine) {
+      } else if (
+        state === 1
+      ) {
+        if (
+          isNewLine && this.body[i + 1] === CR &&
+          this.body[i + 2] === LF
+        ) {
+          // end of the headers section
           state = 2;
-        const headersDone = this.body[i + 1] === CR &&
+          fileStart = i + 3; // After \r\n
-          this.body[i + 2] === LF;
-
-        if (headersDone) {
-          state = 3;
         }
-      } else if (state === 2 && isNewLine) {
+      } else if (state === 2) {
-        state = 3;
-      } else if (state === 3 && isNewLine) {
-        state = 4;
-        fileStart = i + 1;
-      } else if (state === 4) {
         if (this.boundaryChars[boundaryIndex] !== byte) {
           boundaryIndex = 0;
         } else {
@@ -487,7 +486,7 @@ class MultipartParser {
           const latin1Filename = MapPrototypeGet(disposition, "filename");
           const latin1Name = MapPrototypeGet(disposition, "name");
 
-          state = 5;
+          state = 3;
           // Reset
           boundaryIndex = 0;
           headerText = "";
@@ -510,7 +509,7 @@ class MultipartParser {
             formData.append(name, core.decode(content));
           }
         }
-      } else if (state === 5 && isNewLine) {
+      } else if (state === 3 && isNewLine) {
         state = 1;
       }
     }