Handle framing automatically when downgrading proxied H2 traffic to H1

If you send an HTTP/2 request that likely has a body (we can't tell
initially) with no framing headers, we enable chunked encoding to ensure
any received body can be handled as expected. This might be subtly
breaking in some cases but should be semantically identical to the
existing behaviour for all valid requests.

Author: pimterry

src/rules/requests/request-handlers.ts

@@ -738,7 +738,7 @@ export class PassThroughHandler extends PassThroughHandlerDefinition {
                 MODIFIABLE_PSEUDOHEADERS.includes(key.toLowerCase() as any)
             );
         } else if (isH2Downstream && !shouldTryH2Upstream) {
-            rawHeaders = h2HeadersToH1(rawHeaders);
+            rawHeaders = h2HeadersToH1(rawHeaders, method);
         }
 
         let serverReq: http.ClientRequest;

src/util/header-utils.ts

@@ -218,7 +218,7 @@ export function updateRawHeaders(
 
 // See https://httptoolkit.com/blog/translating-http-2-into-http-1/ for details on the
 // transformations required between H2 & H1 when proxying.
-export function h2HeadersToH1(h2Headers: RawHeaders): RawHeaders {
+export function h2HeadersToH1(h2Headers: RawHeaders, method: string): RawHeaders {
     let h1Headers = h2Headers.filter(([key]) => key[0] !== ':');
 
     if (!findRawHeader(h1Headers, 'host') && findRawHeader(h2Headers, ':authority')) {
@@ -233,6 +233,18 @@ export function h2HeadersToH1(h2Headers: RawHeaders): RawHeaders {
         h1Headers.push(['Cookie', cookieHeaders.join('; ')]);
     }
 
+    // We don't know if the request has a body yet - but just in case, we ensure it could:
+    if (
+        // If the request is a method that probably has a body
+        method !== 'GET' &&
+        method !== 'HEAD' &&
+        !( // And you haven't set any kind of framing headers:
+            findRawHeader(h1Headers, 'content-length') ||
+            findRawHeader(h1Headers, 'transfer-encoding')?.includes('chunked'))
+    ) { // Add transfer-encoding chunked, which should support all possible cases:
+        h1Headers.push(['Transfer-Encoding', 'chunked']);
+    }
+
     return h1Headers;
 }