Fix @defer with payload containing "---" (#10891)

* Fix `@defer` with payload containing "---"

When using `@defer` if the response payload contains "---" then the
query will fail with an error message such as:

		ApolloError: Unterminated string in JSON at position 15378

* chore(tests): updates HttpLink test to validate that boundaries
are not parsed within response data, only read at the beginning
of each chunk.

* chore: update changeset

---------

Co-authored-by: alessia <alessia@apollographql.com>

.changeset/angry-weeks-marry.md

@@ -0,0 +1,5 @@
+---
+"@apollo/client": patch
+---
+
+Fixes a bug in how multipart responses are read when using `@defer`. When reading a multipart body, `HttpLink` no longer attempts to parse the boundary (e.g. `"---"` or other boundary string) within the response data itself, only when reading the beginning of each mulitpart chunked message.

src/link/http/__tests__/HttpLink.ts

@@ -1360,7 +1360,10 @@ describe('HttpLink', () => {
         'Content-Type: application/json; charset=utf-8',
         'Content-Length: 58',
         '',
-        '{"hasNext":false, "incremental": [{"data":{"name":"stubby"},"path":["stub"],"extensions":{"timestamp":1633038919}}]}',
+        // Intentionally using the boundary value `---` within the “name” to
+        // validate that boundary delimiters are not parsed within the response
+        // data itself, only read at the beginning of each chunk.
+        '{"hasNext":false, "incremental": [{"data":{"name":"stubby---"},"path":["stub"],"extensions":{"timestamp":1633038919}}]}',
         '-----',
       ].join("\r\n");
 
@@ -1418,7 +1421,7 @@ describe('HttpLink', () => {
                 expect(result).toEqual({
                   incremental: [{
                     data: {
-                      name: 'stubby',
+                      name: 'stubby---',
                     },
                     extensions: {
                       timestamp: 1633038919,
@@ -1544,7 +1547,7 @@ describe('HttpLink', () => {
                 expect(result).toEqual({
                   incremental: [{
                     data: {
-                      name: 'stubby',
+                      name: 'stubby---',
                     },
                     extensions: {
                       timestamp: 1633038919,

src/link/http/parseAndCheckHttpResponse.ts

@@ -39,40 +39,43 @@ export async function readMultipartBody<
         .trim()
     : "-";
 
-  let boundary = `--${boundaryVal}`;
+  const boundary = `\r\n--${boundaryVal}`;
   let buffer = "";
   const iterator = responseIterator(response);
   let running = true;
 
   while (running) {
     const { value, done } = await iterator.next();
     const chunk = typeof value === "string" ? value : decoder.decode(value);
+    const searchFrom = buffer.length - boundary.length + 1;
     running = !done;
     buffer += chunk;
-    let bi = buffer.indexOf(boundary);
+    let bi = buffer.indexOf(boundary, searchFrom);
 
     while (bi > -1) {
       let message: string;
       [message, buffer] = [
         buffer.slice(0, bi),
         buffer.slice(bi + boundary.length),
       ];
-      if (message.trim()) {
-        const i = message.indexOf("\r\n\r\n");
-        const headers = parseHeaders(message.slice(0, i));
-        const contentType = headers["content-type"];
-        if (
-          contentType &&
-          contentType.toLowerCase().indexOf("application/json") === -1
-        ) {
-          throw new Error(
-            "Unsupported patch content type: application/json is required."
-          );
-        }
-        const body = message.slice(i);
+      const i = message.indexOf("\r\n\r\n");
+      const headers = parseHeaders(message.slice(0, i));
+      const contentType = headers["content-type"];
+      if (
+        contentType &&
+        contentType.toLowerCase().indexOf("application/json") === -1
+      ) {
+        throw new Error(
+          "Unsupported patch content type: application/json is required."
+        );
+      }
+      // nb: Technically you'd want to slice off the beginning "\r\n" but since
+      // this is going to be `JSON.parse`d there is no need.
+      const body = message.slice(i);
 
+      if (body) {
         try {
-          const result = parseJsonBody<T>(response, body.replace("\r\n", ""));
+          const result = parseJsonBody<T>(response, body);
           if (
             Object.keys(result).length > 1 ||
             "data" in result ||