Add static webpack etag support

Webpack (which is what vue uses to compress its HTML) is capable of
generating hashes of files when it produces the dist files[1].

This gets generated in the form of
<filename>.<hash>.<extension>

This commit attempts to detect these patterns, and enable etag caching
to speed up webui load times.  It detects these patterns, grabs the hash
for the file, and returns it in the Etag header[2].

The behavior is implemented such that:
If the file has an etag, the etag header is returned.
If the request has an If-None-Match header, and that header matches,
only 304 is returned.

Tested:
Tests were run on qemu S7106 bmcweb with default error logging level,
and HTTP/2 enabled, along with svg optimization patches.

Run scripts/generate_auth_certificate.py to set up TLS certificates.
(valid TLS certs are required for HTTP caching to work properly in some
browsers).
Load the webui.  Note that DOM load takes 1.10 seconds, Load takes 1.10
seconds, and all requests return 200 OK.
Refresh the GUI.
Note that most resources now return 304, and DOM time is reduced to
279 milliseconds and load is reduced to 280 milliseconds.  DOM load
(which is what the BMC has control over) is decreased by a factor of
3-4X.
Setting chrome to "Fast 5g" throttling in the network tab shows a more
pronounced difference, 1.28S load time vs 3.96S.

BMC also shows 477KB transferred on the wire, versus 2.3KB
transferred on the wire.  This has the potential to significantly
reduce the load on the BMC when the webui refreshes.

[1] https://webpack.js.org/guides/caching/
[2] https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag

Change-Id: I68aa7ef75533506d98e8fce10bb04a494dc49669
Signed-off-by: Ed Tanous <ed@tanous.net>

2 files changed, 23 insertions, 11 deletions

diff --git a/http/http2_connection.hpp b/http/http2_connection.hpp
index 4b2d186f07..2c60c1268e 100644
--- a/http/http2_connection.hpp
+++ b/http/http2_connection.hpp
@@ -167,17 +167,18 @@ class HTTP2Connection :
             close();
             return -1;
         }
-        Response& thisRes = it->second.res;
-        thisRes = std::move(completedRes);
-        crow::Request& thisReq = it->second.req;
-        std::vector<nghttp2_nv> hdr;
+        Http2StreamData& stream = it->second;
+        Response& res = stream.res;
+        res = std::move(completedRes);
+        crow::Request& thisReq = stream.req;
 
-        completeResponseFields(thisReq, thisRes);
-        thisRes.addHeader(boost::beast::http::field::date, getCachedDateStr());
-        thisRes.preparePayload();
+        completeResponseFields(thisReq, res);
+        res.addHeader(boost::beast::http::field::date, getCachedDateStr());
+        res.preparePayload();
 
-        boost::beast::http::fields& fields = thisRes.fields();
-        std::string code = std::to_string(thisRes.resultInt());
+        boost::beast::http::fields& fields = res.fields();
+        std::string code = std::to_string(res.resultInt());
+        std::vector<nghttp2_nv> hdr;
         hdr.emplace_back(
             headerFromStringViews(":status", code, NGHTTP2_NV_FLAG_NONE));
         for (const boost::beast::http::fields::value_type& header : fields)
@@ -185,8 +186,6 @@ class HTTP2Connection :
             hdr.emplace_back(headerFromStringViews(
                 header.name_string(), header.value(), NGHTTP2_NV_FLAG_NONE));
         }
-        Http2StreamData& stream = it->second;
-        crow::Response& res = stream.res;
         http::response<bmcweb::HttpBody>& fbody = res.response;
         stream.writer.emplace(fbody.base(), fbody.body());
 
@@ -279,6 +278,13 @@ class HTTP2Connection :
         else
 #endif // BMCWEB_INSECURE_DISABLE_AUTHX
         {
+            std::string_view expected = thisReq.getHeaderValue(
+                boost::beast::http::field::if_none_match);
+            BMCWEB_LOG_DEBUG("Setting expected hash {}", expected);
+            if (!expected.empty())
+            {
+                asyncResp->res.setExpectedHash(expected);
+            }
             handler->handle(thisReq, asyncResp);
         }
         return 0;
diff --git a/http/http_response.hpp b/http/http_response.hpp
index c93d60d685..18266ec93d 100644
--- a/http/http_response.hpp
+++ b/http/http_response.hpp
@@ -80,6 +80,7 @@ struct Response
         }
         response = std::move(r.response);
         jsonValue = std::move(r.jsonValue);
+        expectedHash = std::move(r.expectedHash);
 
         // Only need to move completion handler if not already completed
         // Note, there are cases where we might move out of a Response object
@@ -144,6 +145,11 @@ struct Response
         return fields()[key];
     }
 
+    std::string_view getHeaderValue(boost::beast::http::field key) const
+    {
+        return fields()[key];
+    }
+
     void keepAlive(bool k)
     {
         response.keep_alive(k);