Don't crash if flush fails during unwinding

include/fmt/core.h

@@ -176,6 +176,13 @@ FMT_END_NAMESPACE_STD
 #    define FMT_EXCEPTIONS 1
 #  endif
 #endif
+#if FMT_EXCEPTIONS
+#  define FMT_TRY try
+#  define FMT_CATCH(x) catch (x)
+#else
+#  define FMT_TRY if (true)
+#  define FMT_CATCH(x) if (false)
+#endif
 
 // Disable [[noreturn]] on MSVC/NVCC because of bogus unreachable code warnings.
 #if FMT_EXCEPTIONS && FMT_HAS_CPP_ATTRIBUTE(noreturn) && !FMT_MSC_VERSION && \
@@ -965,7 +972,12 @@ class iterator_buffer : public Traits, public buffer<T> {
       : Traits(other),
         buffer<T>(grow, data_, 0, buffer_size),
         out_(other.out_) {}
-  ~iterator_buffer() { flush(); }
+  ~iterator_buffer() {
+    FMT_TRY { flush(); }
+    FMT_CATCH(...) {
+      // Don't crash if flush fails during unwinding.
+    }
+  }
 
   auto out() -> OutputIt {
     flush();

include/fmt/format.h

@@ -137,14 +137,6 @@ FMT_END_NAMESPACE
 #  endif
 #endif
 
-#if FMT_EXCEPTIONS
-#  define FMT_TRY try
-#  define FMT_CATCH(x) catch (x)
-#else
-#  define FMT_TRY if (true)
-#  define FMT_CATCH(x) if (false)
-#endif
-
 #ifndef FMT_MAYBE_UNUSED
 #  if FMT_HAS_CPP17_ATTRIBUTE(maybe_unused)
 #    define FMT_MAYBE_UNUSED [[maybe_unused]]

test/core-test.cc

@@ -851,3 +851,26 @@ FMT_END_NAMESPACE
 TEST(core_test, trappy_conversion) {
   EXPECT_EQ(fmt::format("{}", its_a_trap()), "42");
 }
+
+TEST(core_test, throw_in_buffer_dtor) {
+  enum { buffer_size = 256 };
+
+  struct throwing_iterator {
+    int& count;
+
+    auto operator=(char) -> throwing_iterator& {
+      if (++count > buffer_size) throw std::exception();
+      return *this;
+    }
+    auto operator*() -> throwing_iterator& { return *this; }
+    auto operator++() -> throwing_iterator& { return *this; }
+    auto operator++(int) -> throwing_iterator { return *this; }
+  };
+
+  try {
+    int count = 0;
+    fmt::format_to(throwing_iterator{count}, fmt::runtime("{:{}}{"), "",
+                   buffer_size + 1);
+  } catch (const std::exception&) {
+  }
+}