[release/6.x] Handle both inclusive-end and exclusive-end content-range headers, via the content-length header (#7646)

Author: eddyashton

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Fixed
 
+- Snapshot fetching requests now handle either inclusive-end or exclusive-end `content-range` headers, for compatibility with 7.x nodes.
 - Primaries now indicate all of their chunk-ending transactions in the transaction header, for better interop with 7.x nodes. Upgrades must create a snapshot from the network after it has upgraded to this version, and provide that to joining 7.x nodes.
 
 ## [6.0.20]

src/snapshots/fetch.h

@@ -115,6 +115,65 @@ namespace snapshots
       }
     }
 
+    {
+      // Use content-length to determine whether the sender used an exclusive or
+      // inclusive range end
+      auto length_it = headers.find(ccf::http::headers::CONTENT_LENGTH);
+      if (length_it == headers.end())
+      {
+        throw std::runtime_error(
+          "Response is missing expected content-length header");
+      }
+
+      size_t content_length = 0;
+
+      {
+        const auto& length_s = length_it->second;
+
+        const auto [p, ec] = std::from_chars(
+          length_s.data(), length_s.data() + length_s.size(), content_length);
+
+        if (ec != std::errc())
+        {
+          throw std::runtime_error(fmt::format(
+            "Could not parse length from content-length header: {}",
+            length_it->second));
+        }
+      }
+
+      const auto range_length =
+        parsed_values.range_end - parsed_values.range_start;
+      if (range_length == content_length)
+      {
+        LOG_DEBUG_FMT(
+          "Server sent an exclusive-end content-range header. "
+          "content-length={}, content-range={}. This is expected for 6.x to "
+          "6.x nodes",
+          length_it->second,
+          it->second);
+      }
+      else if (range_length + 1 == content_length)
+      {
+        LOG_INFO_FMT(
+          "Server sent an inclusive-end content-range header. "
+          "content-length={}, content-range={}. Adjusting this to local "
+          "exclusive-end representation. This should be a temporary "
+          "mismatch, between 6.x and 7.x nodes in a mixed network",
+          length_it->second,
+          it->second);
+        parsed_values.range_end += 1;
+      }
+      else
+      {
+        throw std::runtime_error(fmt::format(
+          "content-range ({}, {} bytes) and content-length ({}) headers do not "
+          "agree",
+          it->second,
+          range_length,
+          length_it->second));
+      }
+    }
+
     return parsed_values;
   }