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https://issues.apache.org/jira/browse/HTTPCLIENT-1992?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16861615#comment-16861615
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Serkan Turgut commented on HTTPCLIENT-1992:
-------------------------------------------
Thank you [~olegk], I submitted the PR.
[https://github.com/apache/httpcomponents-client/pull/153]
> Impossible to access trailer-headers available in chunked transfer-encoding
> ---------------------------------------------------------------------------
>
> Key: HTTPCLIENT-1992
> URL: https://issues.apache.org/jira/browse/HTTPCLIENT-1992
> Project: HttpComponents HttpClient
> Issue Type: Bug
> Components: HttpClient (classic)
> Affects Versions: 5.0 Beta4, 5.0 Beta5
> Reporter: Serkan Turgut
> Priority: Major
> Time Spent: 10m
> Remaining Estimate: 0h
>
> Message trailers in chunked messaging are being
> [parsed|http://hc.apache.org/httpcomponents-core-5.0.x/httpcore5/xref/org/apache/hc/core5/http/impl/io/ChunkedInputStream.html#L284]
> and [kept in
> ChunkedInputStream|http://hc.apache.org/httpcomponents-core-5.0.x/httpcore5/xref/org/apache/hc/core5/http/impl/io/ChunkedInputStream.html#L88].
> The parsed trailer headers are exposed with
> [getFooters()|http://hc.apache.org/httpcomponents-core-5.0.x/httpcore5/xref/org/apache/hc/core5/http/impl/io/ChunkedInputStream.html#L321]
> method. However, there is no caller of this method in the entire call stack
> during a chunked messaging (both in version 4 and 5).
> HttpClient 5 brings the support for message trailers by creating an API in
> HttpEntity interface named as
> [getTrailers().|https://hc.apache.org/httpcomponents-core-5.0.x/httpcore5/apidocs/org/apache/hc/core5/http/HttpEntity.html#getTrailers(]).
> Even though there are multiple implementations of these HttpEntity, none of
> them reach to ChunkedInputStream.getFooters() to get the trailers and expose
> them to caller of HttpEntity.getTrailers().
> * AbstractHttpEntity
> *
> ** Following implementations return hardcoded 'null' for getTrailers()
> because the default implementation in AbstractHttpEntity returns null
> ** BasicHttpEntity
> ** BufferedHttpEntity
> ** ByteArrayEntity
> ** ByteBufferEntity
> ** EntityTemplate
> ** FileEntity
> ** InputStreamEntity
> ** SerializableEntity
> ** StringEntity
> ** ResponseEntityProxy
> ** HttpEntityWithTrailers
> *** Only returns the trailer list given its constructor instead of getting
> them from ChunkedInputStream.getFooters()
> ** HttpEntityWrapper
> *** Call wrappterEntity.getTrailers(), if the underlying entity is one of
> them in this list, this class also returns null
>
> Currently the call chain is following (omitted irrelevant calls);
> #1 MainClientExec.execute() -> creates ResponseEntityProxy regardless the
> response is chunked or not.
> #2 ResponseEntityProxy.enhance() -> wraps the underlying stream with
> EofSensorInputStream which hides ChunkedInputStream.getFooters() method from
> outside. Since EofSensorInputStream is unaware of chunked messaging and
> trailers, there is no way to reach ChuckedInputStream.getFooters() from
> outside after this point.
> #3 EofSensorInputStream -> This is to watch the stream and free it if it's
> EOF.
> #4 ChunkedInputStream.read()
> #5 ChunkedInputStream.nextChunk()
> #6 ChunkedInputStream.parseTrailerHeaders() -> Parses the trailer headers
> and puts into ChunkedInputSteam.footers field, exposed by getFooters() method
> but no one is calling this method.
> Code snip to observe the behavior;
> {code:java}
> CloseableHttpClient httpClient =
> HttpClients.custom().addResponseInterceptorLast(new HttpResponseInterceptor()
> {
> @Override
> public void process(HttpResponse response, EntityDetails entity,
> HttpContext context) throws HttpException, IOException {
> if (response instanceof CloseableHttpResponse) {
> CloseableHttpResponse closeableHttpResponse =
> (CloseableHttpResponse) response;
> HttpEntity httpEntity = closeableHttpResponse.getEntity();
> Supplier<List<? extends Header>> trailers =
> httpEntity.getTrailers();
> // --> 'trailers' is always NULL
> }
> }
> }).build();
> // Make Http requests to any server which can responds in 'Transfer-Encoding:
> chunked' and send trailers, observe the trailer supplier above is always null.
> {code}
> *Proposed solution (the patch is also ready, I will submit a pull request if
> agree on the problem and proposal)*
> There needs to be a way to propagate what has been parsed in
> ChunkedInputStream.parseTrailerHeaders() by calling
> ChunkedInputStream.getFooters() and pass those headers to callers of
> HttpEntity.getTrailers(). To overcome this issue, the proposal is the
> following;
> * Implement a version of ResponseEntityProxy named as
> ResponseChunkedEntityProxy for chunked messaging so it recognizes the
> underlying stream is ChunkedInputStream and It can return a Supplier which
> delegates the call to ChunkedInputStream.getFooters() when
> ResponseChunkedEntityProxy.getTrailers() called and return the supplier for
> trailers to be used by users of HttpClient at the end of the communication.
> * Change
> [MainClientExec.execute|https://hc.apache.org/httpcomponents-client-5.0.x/httpclient5/xref/org/apache/hc/client5/http/impl/classic/MainClientExec.html#L138]
> in a way to use ResponseChunkedEntityProxy when the response header
> 'Transfer-Encoding: chunked' presents, so the HttpEntity.getTrailers() API
> can return trailers parsed by ChunkedInputStream.
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