net: make CaptureMessage() mockable

Rename `CaptureMessage()` to `CaptureMessageToFile()` and introduce a
`std::function` variable called `CaptureMessage` whose value can be
changed by unit tests, should they need to inspect message contents.

src/net.cpp

@@ -3085,7 +3085,10 @@ uint64_t CConnman::CalculateKeyedNetGroup(const CAddress& ad) const
     return GetDeterministicRandomizer(RANDOMIZER_ID_NETGROUP).Write(vchNetGroup.data(), vchNetGroup.size()).Finalize();
 }
 
-void CaptureMessage(const CAddress& addr, const std::string& msg_type, const Span<const unsigned char>& data, bool is_incoming)
+void CaptureMessageToFile(const CAddress& addr,
+                          const std::string& msg_type,
+                          const Span<const unsigned char>& data,
+                          bool is_incoming)
 {
     // Note: This function captures the message at the time of processing,
     // not at socket receive/send time.
@@ -3112,3 +3115,9 @@ void CaptureMessage(const CAddress& addr, const std::string& msg_type, const Spa
     ser_writedata32(f, size);
     f.write(AsBytes(data));
 }
+
+std::function<void(const CAddress& addr,
+                   const std::string& msg_type,
+                   const Span<const unsigned char>& data,
+                   bool is_incoming)>
+    CaptureMessage = CaptureMessageToFile;

src/net.h

@@ -31,6 +31,7 @@
 #include <condition_variable>
 #include <cstdint>
 #include <deque>
+#include <functional>
 #include <map>
 #include <memory>
 #include <optional>
@@ -1272,7 +1273,17 @@ private:
 };
 
 /** Dump binary message to file, with timestamp */
-void CaptureMessage(const CAddress& addr, const std::string& msg_type, const Span<const unsigned char>& data, bool is_incoming);
+void CaptureMessageToFile(const CAddress& addr,
+                          const std::string& msg_type,
+                          const Span<const unsigned char>& data,
+                          bool is_incoming);
+
+/** Defaults to `CaptureMessageToFile()`, but can be overridden by unit tests. */
+extern std::function<void(const CAddress& addr,
+                          const std::string& msg_type,
+                          const Span<const unsigned char>& data,
+                          bool is_incoming)>
+    CaptureMessage;
 
 struct NodeEvictionCandidate
 {

test/functional/p2p_message_capture.py

@@ -20,7 +20,7 @@ LENGTH_SIZE = 4
 MSGTYPE_SIZE = 12
 
 def mini_parser(dat_file):
-    """Parse a data file created by CaptureMessage.
+    """Parse a data file created by CaptureMessageToFile.
 
     From the data file we'll only check the structure.