Work around libevent bug 306 in evbuffer_add_file.

See <https://github.com/libevent/libevent/issues/306> for details.

src/http-test.cc

@@ -76,12 +76,25 @@ TEST(EvBufferTest, AddFileTest) {
   // Ensure adding the whole file succeeds.
   EvBuffer buf1;
   ASSERT_TRUE(buf1.AddFile(in_fd, 0, 3, &error_message)) << error_message;
+  in_fd = -1;
   EXPECT_EQ(3u, evbuffer_get_length(buf1.get()));
 
   // Ensure adding an empty region succeeds.
   EvBuffer buf2;
   ASSERT_TRUE(buf2.AddFile(in_fd, 0, 0, &error_message)) << error_message;
   EXPECT_EQ(0u, evbuffer_get_length(buf2.get()));
+
+  // Ensure adding part of a file after another string succeeds.
+  in_fd = open(foo_filename.c_str(), O_RDONLY);
+  EvBuffer buf3;
+  buf3.Add("1234");
+  ASSERT_TRUE(buf3.AddFile(in_fd, 1, 2, &error_message)) << error_message;
+  auto size3 = evbuffer_get_length(buf3.get());
+  EXPECT_EQ(6u, size3);
+  std::string buf3_contents = std::string(
+      reinterpret_cast<const char *>(evbuffer_pullup(buf3.get(), size3)),
+      size3);
+  EXPECT_EQ("1234oo", buf3_contents);
 }
 
 class FileSlicesTest : public testing::Test {

src/http.cc

@@ -176,6 +176,23 @@ bool EvBuffer::AddFile(int fd, ev_off_t offset, ev_off_t length,
     return true;
   }
 
+  if (evbuffer_get_length(buf_) > 0) {
+    // Work around https://github.com/libevent/libevent/issues/306 by using a
+    // fresh buffer for evbuffer_add_file.
+    EvBuffer fresh_buffer;
+    if (!fresh_buffer.AddFile(fd, offset, length, error_message)) {
+      return false;
+    }
+
+    // Crash if evbuffer_add_buffer fails, because the ownership of |fd| has
+    // already been transferred, and it's too confusing to support some
+    // failures in which the caller still owns |fd| and some in which it does
+    // not.
+    CHECK_EQ(0, evbuffer_add_buffer(buf_, fresh_buffer.buf_))
+        << "evbuffer_add_buffer failed: " << strerror(errno);
+    return true;
+  }
+
   if (evbuffer_add_file(buf_, fd, offset, length) != 0) {
     int err = errno;
     *error_message = StrCat("evbuffer_add_file failed with offset ", offset,