[Support] Use O_CLOEXEC only when declared

[labath]

Summary

Use the O_CLOEXEC flag only when it is available. Some old systems (e.g.
SLES10) do not support this flag. POSIX explicitly guarantees that this
flag can be checked for using #if, so there is no need for a CMake
check.

[labath]

In the original version my patch (D28854) I had fallback code which sets the FD_CLOEXEC flag using fcntl(2). What do you think about using that? It's a bit racy, but it's better than nothing.

I intend to start using these function from lldb, so if anyone will use lldb on the old systems, he'll get descriptor leakage there.

[mgorny]

Yeah, I think that'll work for me.

[labath]

In D28894#650269, @mgorny wrote:

Yeah, I think that'll work for me.

Now I am not sure whether that meant "yeah, that'll work and i'll do it", or "yeah, go ahead and you do it" :P, but based on lack of activity, it seems to be latter.

I'll go ahead and add those then. I'd very much appreciate if you could test it for me though.

[labath]

Add FD_CLOEXEC fallback.

[mgorny]

Haha, I thought you meant you'll do it since you had the code already :-)

[mgorny]

Builds fine for me.

[labath]

@rafael: I am going to assume you are fine with this as well. Let me know if you have concerns.

lib/Support/Unix/Path.inc

static bool hasProcSelfFD() {
   return Result;
 }
 #endif
 
 std::error_code openFileForRead(const Twine &Name, int &ResultFD,
                                 SmallVectorImpl<char> *RealPath) {
   SmallString<128> Storage;
   StringRef P = Name.toNullTerminatedStringRef(Storage);
-  while ((ResultFD = open(P.begin(), O_RDONLY | O_CLOEXEC)) < 0) {
+  int OpenFlags = O_RDONLY;
+#ifdef O_CLOEXEC
+  OpenFlags |= O_CLOEXEC;
+#endif
+  while ((ResultFD = open(P.begin(), OpenFlags)) < 0) {
     if (errno != EINTR)
       return std::error_code(errno, std::generic_category());
   }
+#ifndef O_CLOEXEC
+  int r = fcntl(ResultFD, F_SETFD, FD_CLOEXEC);
+  (void)r;
+  assert(r == 0 && "fcntl(F_SETFD, FD_CLOEXEC) failed");
+#endif
   // Attempt to get the real name of the file, if the user asked
   if(!RealPath)
     return std::error_code();
   RealPath->clear();
 #if defined(F_GETPATH)
   // When F_GETPATH is availble, it is the quickest way to get
   // the real path name.
   char Buffer[MAXPATHLEN];

 }
 
 std::error_code openFileForWrite(const Twine &Name, int &ResultFD,
                             sys::fs::OpenFlags Flags, unsigned Mode) {
   // Verify that we don't have both "append" and "excl".
   assert((!(Flags & sys::fs::F_Excl) || !(Flags & sys::fs::F_Append)) &&
          "Cannot specify both 'excl' and 'append' file creation flags!");
 
-  int OpenFlags = O_CREAT | O_CLOEXEC;
+  int OpenFlags = O_CREAT;
+
+#ifdef O_CLOEXEC
+  OpenFlags |= O_CLOEXEC;
+#endif
 
   if (Flags & F_RW)
     OpenFlags |= O_RDWR;
   else
     OpenFlags |= O_WRONLY;
 
   if (Flags & F_Append)
     OpenFlags |= O_APPEND;
   else
     OpenFlags |= O_TRUNC;
 
   if (Flags & F_Excl)
     OpenFlags |= O_EXCL;
 
   SmallString<128> Storage;
   StringRef P = Name.toNullTerminatedStringRef(Storage);
   while ((ResultFD = open(P.begin(), OpenFlags, Mode)) < 0) {
     if (errno != EINTR)
       return std::error_code(errno, std::generic_category());
   }
+#ifndef O_CLOEXEC
+  int r = fcntl(ResultFD, F_SETFD, FD_CLOEXEC);
+  (void)r;
+  assert(r == 0 && "fcntl(F_SETFD, FD_CLOEXEC) failed");
+#endif
   return std::error_code();
 }
 
 std::error_code getPathFromOpenFD(int FD, SmallVectorImpl<char> &ResultPath) {
   if (FD < 0)
     return make_error_code(errc::bad_file_descriptor);
 
 #if defined(F_GETPATH)