Better error reporting when talking to HTTP/1.1 servers

[jroper]

If the gRPC client connects to an HTTP/1.1 server, either because we're using HTTP/2 over plain text (h2c), or because the server doesn't support ALPN and the gRPC client still doesn't fail in that case, it would be nice if the client could give a nicer error message than error reading server preface: http2: frame too large.

Detecting that the server is an HTTP/1.1 server is actually trivial, the HTTP/2 preface looks like a valid HTTP request with an unknown method, so HTTP/1.1 servers will usually respond to HTTP/2 requests with:

HTTP/1.1 405 Method Not Allowed

An HTTP/2 client expects the first three bytes sent in the stream, ie HTT, to be the length of the HTTP/2 settings frame. So, it will read HTT as the number 0x485454, which is around 4.7mb, and fail with the above mentioned error. What can be done is special case the value of 0x485454, and generate a different error message, eg error reading server preface: http2: frame too large - possible HTTP/1.1 server.

[Aditya-Sood]

hi @zasweq, if @jroper is not planning to raise a PR can I pick this up?

[jroper]

I have no plans at this stage to raise a PR.

[prakrit55]

Hey @Aditya-Sood, I want to get started with it. Would you lend me one ?😅😃

[Aditya-Sood]

@prakrit55 I've picked this up, will let you know if I need assistance

[jroper]

If anyone is looking to contribute, the other bug report that I linked to I think is far more important than this issue to fix, and the fix is somewhat simple:

#434

[easwars]

We've made progress on #434 and will be enforcing ALPN by default in the near future.