Reuse curl handle to attempt to reuse sessions (#7321)

Co-authored-by: Amaury Chamayou <amchamay@microsoft.com>

Author: cjen1-msft

src/http/curl.h

@@ -63,6 +63,20 @@ namespace ccf::curl
       }
     }
 
+    // No implicit copying: unique ownership of the CURL handle
+    UniqueCURL(const UniqueCURL&) = delete;
+    UniqueCURL& operator=(const UniqueCURL&) = delete;
+
+    // Move semantics
+    UniqueCURL(UniqueCURL&& other) noexcept : p(std::move(other.p)) {}
+    UniqueCURL& operator=(UniqueCURL&& other) noexcept
+    {
+      p = std::move(other.p);
+      return *this;
+    }
+
+    ~UniqueCURL() = default;
+
     operator CURL*() const
     {
       return p.get();
@@ -486,6 +500,11 @@ namespace ccf::curl
       return curl_handle;
     }
 
+    [[nodiscard]] UniqueCURL& get_easy_handle_ptr()
+    {
+      return curl_handle;
+    }
+
     [[nodiscard]] RESTVerb get_method() const
     {
       return method;

src/snapshots/fetch.h

@@ -49,8 +49,8 @@ namespace snapshots
       // Make initial request, which returns a redirect response to specific
       // snapshot
       std::string snapshot_url;
+      ccf::curl::UniqueCURL curl_easy;
       {
-        ccf::curl::UniqueCURL curl_easy;
         curl_easy.set_opt(CURLOPT_CAINFO, path_to_peer_cert.c_str());
 
         auto initial_url = fmt::format(
@@ -106,12 +106,12 @@ namespace snapshots
 
         snapshot_url =
           fmt::format("https://{}{}", peer_address, location_it->second);
+        curl_easy = std::move(request.get_easy_handle_ptr());
       }
 
       // Make follow-up request to redirected URL, to fetch total content size
       size_t content_size = 0;
       {
-        ccf::curl::UniqueCURL curl_easy;
         curl_easy.set_opt(CURLOPT_CAINFO, path_to_peer_cert.c_str());
 
         ccf::curl::UniqueSlist headers;
@@ -174,6 +174,7 @@ namespace snapshots
             snapshot_size_request.get_url(),
             ec));
         }
+        curl_easy = std::move(snapshot_size_request.get_easy_handle_ptr());
       }
 
       // Fetch 4MB chunks at a time
@@ -192,7 +193,6 @@ namespace snapshots
 
         while (true)
         {
-          ccf::curl::UniqueCURL curl_easy;
           curl_easy.set_opt(CURLOPT_CAINFO, path_to_peer_cert.c_str());
 
           ccf::curl::UniqueSlist headers;
@@ -236,6 +236,7 @@ namespace snapshots
 
           snapshot_response =
             std::move(snapshot_range_request.get_response_ptr());
+          curl_easy = std::move(snapshot_range_request.get_easy_handle_ptr());
 
           if (range_end == content_size)
           {