From 4a01081e83b1092a426c2ded19b02f56db46effd Mon Sep 17 00:00:00 2001
From: Veloman Yunkan <veloman.yunkan@gmail.com>
Date: Fri, 19 Nov 2021 14:58:00 +0400
Subject: [PATCH] Thread-safe Book::Illustration::getData()

---
 include/book.h |  2 ++
 src/book.cpp   | 11 +++++++----
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/include/book.h b/include/book.h
index 5d049ad6c..cdcbf51fe 100644
--- a/include/book.h
+++ b/include/book.h
@@ -23,6 +23,7 @@
 #include <string>
 #include <vector>
 #include <memory>
+#include <mutex>
 
 namespace pugi {
 class xml_node;
@@ -57,6 +58,7 @@ class Book
 
    private:
     mutable std::string data;
+    mutable std::mutex mutex;
   };
 
   typedef std::vector<std::shared_ptr<const Illustration>> Illustrations;
diff --git a/src/book.cpp b/src/book.cpp
index bf7c3e18f..63ebbec19 100644
--- a/src/book.cpp
+++ b/src/book.cpp
@@ -227,10 +227,13 @@ const Book::Illustration& Book::getDefaultIllustration() const
 const std::string& Book::Illustration::getData() const
 {
   if (data.empty() && !url.empty()) {
-    try {
-      data = download(url);
-    } catch(...) {
-      std::cerr << "Cannot download favicon from " << url;
+    const std::lock_guard<std::mutex> l(mutex);
+    if ( data.empty() ) {
+      try {
+        data = download(url);
+      } catch(...) {
+        std::cerr << "Cannot download favicon from " << url;
+      }
     }
   }
   return data;