[hurd] Make getMainExecutable get the real binary path

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.

Patch by Samuel Thibault!

While making changes here, I reformatted this block a bit to reduce
indentation and match 2 space indent style.

Differential Revision: https://reviews.llvm.org/D53557

llvm-svn: 345104

llvm/lib/Support/Unix/Path.inc

@@ -179,14 +179,34 @@ std::string getMainExecutable(const char *argv0, void *MainAddr) {
   char exe_path[MAXPATHLEN];
   StringRef aPath("/proc/self/exe");
   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);
+    // /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 "";
+
+    // Null terminate the string for realpath. readlink never null
+    // terminates its output.
+    len = std::min(len, long(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))
-        return exe_path;
+    // Fall back to the classical detection.
+    if (getprogpath(exe_path, argv0))
+      return exe_path;
   }
 #elif defined(HAVE_DLFCN_H) && defined(HAVE_DLADDR)
   // Use dladdr to get executable path if available.