enable tcp keep-alive on curl handler (#28588)

Author: wfurt

src/Common/src/Interop/Unix/System.Net.Http.Native/Interop.Easy.cs

@@ -111,6 +111,7 @@ internal enum CURLoption
             CURLOPT_PROXYTYPE = CurlOptionLongBase + 101,
             CURLOPT_HTTPAUTH = CurlOptionLongBase + 107,
             CURLOPT_TCP_NODELAY = CurlOptionLongBase + 121,
+            CURLOPT_TCP_KEEPALIVE = CurlOptionLongBase + 213,
             CURLOPT_CONNECTTIMEOUT_MS = CurlOptionLongBase + 156,
             CURLOPT_ADDRESS_SCOPE = CurlOptionLongBase + 171,
             CURLOPT_PROTOCOLS = CurlOptionLongBase + 181,

src/Native/Unix/System.Net.Http.Native/pal_easy.cpp

@@ -26,6 +26,7 @@ static_assert(PAL_CURLOPT_NOSIGNAL == CURLOPT_NOSIGNAL, "");
 static_assert(PAL_CURLOPT_PROXYTYPE == CURLOPT_PROXYTYPE, "");
 static_assert(PAL_CURLOPT_HTTPAUTH == CURLOPT_HTTPAUTH, "");
 static_assert(PAL_CURLOPT_TCP_NODELAY == CURLOPT_TCP_NODELAY, "");
+static_assert(PAL_CURLOPT_TCP_KEEPALIVE == CURLOPT_TCP_KEEPALIVE, "");
 static_assert(PAL_CURLOPT_CONNECTTIMEOUT_MS == CURLOPT_CONNECTTIMEOUT_MS, "");
 static_assert(PAL_CURLOPT_ADDRESS_SCOPE == CURLOPT_ADDRESS_SCOPE, "");
 static_assert(PAL_CURLOPT_PROTOCOLS == CURLOPT_PROTOCOLS, "");

src/Native/Unix/System.Net.Http.Native/pal_easy.h

@@ -35,6 +35,7 @@ enum PAL_CURLoption : int32_t
     PAL_CURLOPT_PROXYTYPE = CurlOptionLongBase + 101,
     PAL_CURLOPT_HTTPAUTH = CurlOptionLongBase + 107,
     PAL_CURLOPT_TCP_NODELAY = CurlOptionLongBase + 121,
+    PAL_CURLOPT_TCP_KEEPALIVE = CurlOptionLongBase + 213,
     PAL_CURLOPT_CONNECTTIMEOUT_MS = CurlOptionLongBase + 156,
     PAL_CURLOPT_ADDRESS_SCOPE = CurlOptionLongBase + 171,
     PAL_CURLOPT_PROTOCOLS = CurlOptionLongBase + 181,

src/System.Net.Http/src/System/Net/Http/CurlHandler/CurlHandler.EasyRequest.cs

@@ -300,6 +300,8 @@ private void SetNetworkingOptions()
                 // and when enabled has a measurably negative impact on latency in key scenarios
                 // (e.g. POST'ing small-ish data).
                 SetCurlOption(CURLoption.CURLOPT_TCP_NODELAY, 1L);
+                // Enable TCP keep-alive.
+                SetCurlOption(CURLoption.CURLOPT_TCP_KEEPALIVE, 1L);
             }
 
             private void SetMultithreading()