[hurd] Make getMainExecutable get the real binary path

[sthibaul]

Summary

On GNU/Hurd, llvm-config is returning bogus value, such as:

$ llvm-config-6.0 --includedir
/usr/include

while it should be:
$ llvm-config-6.0 --includedir
/usr/lib/llvm-6.0/include

This is because getMainExecutable does not get the actual installation path. On GNU/Hurd, /proc/self/exe is indeed a symlink to the path that was used to start the program, and not the eventual binary file. Llvm's getMainExecutable thus needs to run realpath over it to get the actual place where llvm was installed (/usr/lib/llvm-6.0/bin/llvm-config), and not /usr/bin/llvm-config-6.0. This will not change the result on Linux, where /proc/self/exe already points to the eventual file.

[sthibaul]

Note: I added the realpath call for all OSes hitting this part of the code, in case the behavior would be the same on some other OSes. We could instead just call realpath(exe_path, NULL); only in the __GNU__ case (i.e. GNU/Hurd) instead if it is preferred.

[rnk]

lgtm

I think doing this everywhere is fine.

lib/Support/Unix/Path.inc

188

Can you add a comment explaining that Hurd needs this realpath call as you have in the commit message? Otherwise some well-meaning person will try to simplify the code and break Hurd.

[sthibaul]

(I can not commit this myself)

lib/Support/Unix/Path.inc

   if (sys::fs::exists(aPath)) {
       // /proc is not always mounted under Linux (chroot for example).
       ssize_t len = readlink(aPath.str().c_str(), exe_path, sizeof(exe_path));
-      if (len >= 0)
-          return std::string(exe_path, len);
+      if (len >= 0) {
+          if (len >= sizeof(exe_path))
+              len = sizeof(exe_path)-1;
+          exe_path[len] = '\0';
+
+          // At least on GNU/Hurd, /proc/self/exe is a symlink to the path that
+          // was used to start the program, and not the eventual binary file.
+          // We thus needs to run realpath over it to get the actual place
+          // where llvm was installed.
+#if _POSIX_VERSION >= 200112 || defined(__GLIBC__)
+          char *real_path = realpath(exe_path, NULL);
+          std::string ret = std::string(real_path);
+          free(real_path);
+          return ret;
+#else
+          char real_path[MAXPATHLEN];
+          realpath(exe_path, real_path);
+          return std::string(real_path);
+#endif
+      }
   } else {
       // Fall back to the classical detection.
       if (getprogpath(exe_path, argv0))