logging: Protect ShrinkDebugFile by m_cs

We should not be logging while shrinking the debug file, so make sure
that's true by using our mutex.

src/logging.cpp

@@ -514,6 +514,8 @@ void BCLog::Logger::LogPrint_(util::log::Entry entry)
 
 void BCLog::Logger::ShrinkDebugFile()
 {
+    STDLOCK(m_cs);
+
     // Amount of debug.log to save at end when shrinking (must fit in memory)
     constexpr size_t RECENT_DEBUG_HISTORY_SIZE = 10 * 1000000;
 
@@ -535,7 +537,14 @@ void BCLog::Logger::ShrinkDebugFile()
         // Restart the file with some of the end
         std::vector<char> vch(RECENT_DEBUG_HISTORY_SIZE, 0);
         if (fseek(file, -((long)vch.size()), SEEK_END)) {
-            LogWarning("Failed to shrink debug log file: fseek(...) failed");
+            // LogWarning, except with m_cs held
+            LogPrint_({
+                .category = BCLog::ALL,
+                .level = Level::Warning,
+                .should_ratelimit = true,
+                .source_loc = SourceLocation{__func__},
+                .message = "Failed to shrink debug log file: fseek(...) failed",
+            });
             fclose(file);
             return;
         }

src/logging.h

@@ -227,7 +227,7 @@ namespace BCLog {
          */
         void DisableLogging() EXCLUSIVE_LOCKS_REQUIRED(!m_cs);
 
-        void ShrinkDebugFile();
+        void ShrinkDebugFile() EXCLUSIVE_LOCKS_REQUIRED(!m_cs);
 
         std::unordered_map<LogFlags, Level> CategoryLevels() const EXCLUSIVE_LOCKS_REQUIRED(!m_cs)
         {