Merge #21250: build: make HAVE_O_CLOEXEC available outside LevelDB (bugfix)

9bac71350d build: make HAVE_O_CLOEXEC available outside LevelDB (bugfix) (Sebastian Falbesoner)
584fd91d2d init: only use pipe2 if availabile, check in configure (Sebastian Falbesoner)

Pull request description:

  The result of the O_CLOEXEC availability check is currently only set in the Makefile and passed to LevelDB (see `LEVELDB_CPPFLAGS_INT` in `src/Makefile.leveldb.include`), but not defined to be used in our codebase. This means that code within the preprocessor conditional `#if HAVE_O_CLOEXEC` was actually never compiled. On the master branch this is currently used for pipe creation in `src/shutdown.cpp`, PR #21007 moves this part to a new module (I found the issue while testing that PR).

  The fix is similar to the one in #19803, which solved the same problem for HAVE_FDATASYNC.

  In the course of working on the PR it turned out that pipe2 is not available an all platforms, hence a configure check and a corresponding define HAVE_PIPE2 is introduced and used.

  The PR can be tested by anyone with a system that has pipe2 and O_CLOEXEC available by putting gibberish into the HAVE_O_CLOEXEC block: on master, everything should compile fine, on PR, the compiler should abort with an error. At least that's my naive way of testing preprocessor logic, happy to hear more sophisticated ways :-)

ACKs for top commit:
  laanwj:
    Code review ACK 9bac71350d

Tree-SHA512: aec89faf6ba52b6f014c610ebef7b725d9e967207d58b42a4a71afc9f1268fcb673ecc85b33a2a3debba8105a304dd7edaba4208c5373fcef2ab83e48a170051

configure.ac

@@ -925,6 +925,8 @@ AC_CHECK_DECLS([strnlen])
 dnl Check for daemon(3), unrelated to --with-daemon (although used by it)
 AC_CHECK_DECLS([daemon])
 
+AC_CHECK_DECLS([pipe2])
+
 AC_CHECK_DECLS([le16toh, le32toh, le64toh, htole16, htole32, htole64, be16toh, be32toh, be64toh, htobe16, htobe32, htobe64],,,
 		[#if HAVE_ENDIAN_H
                  #include <endian.h>
@@ -1149,6 +1151,7 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <fcntl.h>]],
  [ AC_MSG_RESULT(yes); HAVE_O_CLOEXEC=1 ],
  [ AC_MSG_RESULT(no); HAVE_O_CLOEXEC=0 ]
 )
+AC_DEFINE_UNQUOTED([HAVE_O_CLOEXEC], [$HAVE_O_CLOEXEC], [Define to 1 if O_CLOEXEC flag is available.])
 
 dnl crc32c platform checks
 AC_MSG_CHECKING(for __builtin_prefetch)

src/shutdown.cpp

@@ -32,7 +32,7 @@ static int g_shutdown_pipe[2] = {-1, -1};
 bool InitShutdownState()
 {
 #ifndef WIN32
-#if HAVE_O_CLOEXEC
+#if HAVE_O_CLOEXEC && HAVE_DECL_PIPE2
     // If we can, make sure that the file descriptors are closed on exec()
     // to prevent interference.
     if (pipe2(g_shutdown_pipe, O_CLOEXEC) != 0) {