Skip attempts to access /proc/self/fd on FreeBSD

[mjguzik]

Summary

In contrast to Linux it does not provide entries which can be readlinked -- these are just regular files, not giving the expected outcome. That's on top of procfs not being mounted by default to begin with.

This is probably the case on other BSDs as well, so I expect there will be more ifdefs added down the road.

[mjguzik]

• change multiline comment to '//' instead of /* */

[mjguzik]

Side note that hasProcSelfFD should get dropped -- the code can go straight for readlink and fallback if that fails. The access() call performed by hasProcSelfFD adds nothing really. Maybe I'll submit a patch later. Note that even in this case the readlink would have to be made conditional on FreeBSD.

llvm/lib/Support/Unix/Path.inc

 
 ErrorOr<basic_file_status> directory_entry::status() const {
   file_status s;
   if (auto EC = fs::status(Path, s, FollowSymlinks))
     return EC;
   return s;
 }
 
-#if !defined(F_GETPATH)
+//
+// FreeBSD optionally provides /proc/self/fd, but it is incompatible with
+// Linux. The thing to use is realpath.
+//
+#if !defined(__FreeBSD__)
+#define TRY_PROC_SELF_FD
+#endif
+
+#if !defined(F_GETPATH) && defined(TRY_PROC_SELF_FD)
 static bool hasProcSelfFD() {
   // If we have a /proc filesystem mounted, we can quickly establish the
   // real name of the file with readlink
   static const bool Result = (::access("/proc/self/fd", R_OK) == 0);
   return Result;
 }
 #endif
 

 #if defined(F_GETPATH)
   // When F_GETPATH is availble, it is the quickest way to get
   // the real path name.
   char Buffer[PATH_MAX];
   if (::fcntl(ResultFD, F_GETPATH, Buffer) != -1)
     RealPath->append(Buffer, Buffer + strlen(Buffer));
 #else
   char Buffer[PATH_MAX];
+#if defined(TRY_PROC_SELF_FD)
   if (hasProcSelfFD()) {
     char ProcPath[64];
     snprintf(ProcPath, sizeof(ProcPath), "/proc/self/fd/%d", ResultFD);
     ssize_t CharCount = ::readlink(ProcPath, Buffer, sizeof(Buffer));
     if (CharCount > 0)
       RealPath->append(Buffer, Buffer + CharCount);
   } else {
+#endif
     SmallString<128> Storage;
     StringRef P = Name.toNullTerminatedStringRef(Storage);
 
     // Use ::realpath to get the real path name
     if (::realpath(P.begin(), Buffer) != nullptr)
       RealPath->append(Buffer, Buffer + strlen(Buffer));
+#if defined(TRY_PROC_SELF_FD)
   }
 #endif
+#endif
   return std::error_code();
 }
 
 Expected<file_t> openNativeFileForRead(const Twine &Name, OpenFlags Flags,
                                        SmallVectorImpl<char> *RealPath) {
   file_t ResultFD;
   std::error_code EC = openFileForRead(Name, ResultFD, Flags, RealPath);
   if (EC)