The "Accept-Encoding" header is added twice when the " compression" option is enabled

[pastrehe]

feign-bom.version 11.9.1
spring-boot 2.7.2

When feign.compression.response.enabled is set to true in configuration, feign adds "Accept-Encoding" header(s) when sending request. Currently, two header fields are added (Accept-Encoding: gzip and Accept-Encoding: deflate).
According to rfc7230 section 3.2.2
A sender MUST NOT generate multiple header fields with the same field
name in a message unless either the entire field value for that
header field is defined as a comma-separated list [i.e., #(values)]
or the header field is a well-known exception
Expected behaivour is to have one header field with the name "Accept-Encoding" and value "gzip, deflate".
ATM, having 2 "Accept-Encoding" headers causing remote side to "flatten" them and pick only one value randomly ("gzip" or "deflate")

[vitalijr2]

As far as I undertand you use httpclient. Could you confirm, please?

I had made a test project with Spring 2.7.2 and Spring Cloud's OpenFeing and met the same issue:

DEBUG i.g.r.t.Monobank - [Monobank#getRates] ---> GET https://api.monobank.ua/bank/currency HTTP/1.1
DEBUG i.g.r.t.Monobank - [Monobank#getRates] Accept-Encoding: gzip
DEBUG i.g.r.t.Monobank - [Monobank#getRates] Accept-Encoding: deflate
DEBUG i.g.r.t.Monobank - [Monobank#getRates] Content-Type: application/json
DEBUG i.g.r.t.Monobank - [Monobank#getRates] ---> END HTTP (0-byte body)

I suppose that it is Apache HTTP Client's issue and does not relates with Spring or Feing.

Update:

I have made another simple feign application with httpclient:

• if there are any annotated headers the client doesn't send "Accept-Encoding"
• if I add them manually like that @Headers({"Accept: application/json", "Accept-Encoding: gzip", "Accept-Encoding: deflate"}) it sends the correct request:

[DEBUG] feign.Logger - [Monobank#getRates] ---> GET https://api.monobank.ua/bank/currency HTTP/1.1
[DEBUG] feign.Logger - [Monobank#getRates] Accept: application/json
[DEBUG] feign.Logger - [Monobank#getRates] Accept-Encoding: gzip, deflate
[DEBUG] feign.Logger - [Monobank#getRates] ---> END HTTP (0-byte body)

[vitalijr2]

This loop adds another header for each its value

feign/core/src/main/java/feign/Request.java

Line 247 in 307a322

builder.append(field).append(": ").append(value).append('\n');

[muzinian]

Hi,this fix may cause add Content-Length twice if Content-Encoding not be set.

feign/core/src/main/java/feign/Client.java

Lines 184 to 196 in 7336c3e

	if (field.equals(CONTENT_LENGTH)) {
	if (!gzipEncodedRequest && !deflateEncodedRequest) {
	contentLength = Integer.valueOf(value);
	connection.addRequestProperty(field, value);
	}
	}
	// Avoid add "Accept-encoding" twice or more when "compression" option is enabled
	if (field.equals(ACCEPT_ENCODING)) {
	connection.addRequestProperty(field, String.join(", ", request.headers().get(field)));
	break;
	} else {
	connection.addRequestProperty(field, value);
	}

[izdt]

Hi,this fix may cause add Content-Length twice if Content-Encoding not be set.

feign/core/src/main/java/feign/Client.java

Lines 184 to 196 in 7336c3e

	if (field.equals(CONTENT_LENGTH)) {
	if (!gzipEncodedRequest && !deflateEncodedRequest) {
	contentLength = Integer.valueOf(value);
	connection.addRequestProperty(field, value);
	}
	}
	// Avoid add "Accept-encoding" twice or more when "compression" option is enabled
	if (field.equals(ACCEPT_ENCODING)) {
	connection.addRequestProperty(field, String.join(", ", request.headers().get(field)));
	break;
	} else {
	connection.addRequestProperty(field, value);
	}

Actually "Content-Length" will not be added because this key is in the restrictedHeaderSet of HttpURLConnection, @see sun.net.www.protocol.http.HttpURLConnection.isRestrictedHeader.
But "else" should be added to the code.

[muzinian]

Hi,this fix may cause add Content-Length twice if Content-Encoding not be set.

feign/core/src/main/java/feign/Client.java

Lines 184 to 196 in 7336c3e

	if (field.equals(CONTENT_LENGTH)) {
	if (!gzipEncodedRequest && !deflateEncodedRequest) {
	contentLength = Integer.valueOf(value);
	connection.addRequestProperty(field, value);
	}
	}
	// Avoid add "Accept-encoding" twice or more when "compression" option is enabled
	if (field.equals(ACCEPT_ENCODING)) {
	connection.addRequestProperty(field, String.join(", ", request.headers().get(field)));
	break;
	} else {
	connection.addRequestProperty(field, value);
	}

Actually "Content-Length" will not be added because this key is in the restrictedHeaderSet of HttpURLConnection, @see sun.net.www.protocol.http.HttpURLConnection.isRestrictedHeader. But "else" should be added to the code.

In our case, We may set System.setProperty("sun.net.http.allowRestrictedHeaders", "true") after RestTemplate be constructed.So sun.net.www.protocol.http.HttpURLConnection.isRestrictedHeaderwill always return false.And yes,an else can fix this.