net/http: sniffing for non-zero GET request body is broken

[neild]

When making a request which usually lacks a response body (GET, HEAD, DELETE, etc.), if the Request.Body is non-nil, the content length is not set, and Transfer-Encoding: chunked is not set, we attempt to read one byte from the body before sending the request. If the read succeeds within 200ms and indicates a zero-length body, we set Request.Body = nil.

If the read takes more than 200ms, we set Transfer-Encoding: chunked and proceed.

See: https://go.googlesource.com/go/+/refs/heads/master/src/net/http/transfer.go#191

The latter path (when the probe does not complete within 200ms) is broken when the body is not zero-length.

In this path, we replace Request.Body with io.MultiReader(finishAsyncByteRead{t}, t.Body). finishAsyncByteRead.Read does not return io.EOF after reading the initial byte, so the io.MultiReader retries the read, which never completes.

https://go.googlesource.com/go/+/refs/heads/master/src/net/http/transfer.go#1066

The impact is that sending a request can hang when all of the following apply:

• The request method is one of GET, HEAD, DELETE, OPTIONS, PROPFIND, or SEARCH.
• The Request.Body is non-nil.
• The content length is not set, or is set to -1.
• Transfer-Encoding: chunked is not set.
• The request body does not respond to a read within 200ms.

Thanks to @EdSchouten for identifying this problem in CL 328711.

[gopherbot]

Change https://golang.org/cl/340256 mentions this issue: net/http: fix hang in probing for a zero-length request body

[born2ngopi]

is the request like this @neild ?

curl localhost:9999 --request GET --data '{"name":"abcd"}' --header "Content-Length: -1"

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