Only use sd_journal_* if we really are outputing to journald

We check that the device and inode numbers are actually the same as the
JOURNAL_STREAM value, instead of just checking that the value exists.

This fixes the logger unit tests

src/logger/logger.cpp

@@ -1,6 +1,10 @@
 #include <logger/logger.hpp>
 #include <config/config.hpp>
 
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
 Logger::Logger(const int log_level):
   log_level(log_level),
   stream(std::cout.rdbuf()),
@@ -8,7 +12,22 @@ Logger::Logger(const int log_level):
   null_stream{&null_buffer}
 {
 #ifdef SYSTEMD_FOUND
-  if (::getenv("JOURNAL_STREAM") != nullptr && this->use_stdout())
+  if (!this->use_stdout())
+    return;
+
+  // See https://www.freedesktop.org/software/systemd/man/systemd.exec.html#%24JOURNAL_STREAM
+  const char* journal_stream = ::getenv("JOURNAL_STREAM");
+  if (journal_stream == nullptr)
+    return;
+
+  struct stat s{};
+  const int res = ::fstat(STDOUT_FILENO, &s);
+  if (res == -1)
+    return;
+
+  const auto stdout_stream = std::to_string(s.st_dev) + ":" + std::to_string(s.st_ino);
+
+  if (stdout_stream == journal_stream)
     this->use_systemd = true;
 #endif
 }

src/logger/logger.hpp

@@ -9,6 +9,7 @@
  */
 
 #include <memory>
+#include <string>
 #include <iostream>
 #include <fstream>
 #include <sstream>