Add realpath variants of the commands that return paths.

Closes #1670.

src/globals.cc

@@ -3,6 +3,7 @@
 #include "globals.h"
 
 #include <cstdlib>
+#include <stdlib.h>
 #include <torrent/exceptions.h>
 
 rpc::ip_table_list ip_tables;
@@ -28,3 +29,32 @@ expand_path(const std::string& path) {
 
   return path;
 }
+
+// Resolves a path to a canonical one with no symlinks or relative components,
+// so it can safely be handed to an external script. Returns an empty string if
+// the path does not name an existing file or directory.
+std::string
+resolve_path(const std::string& path) {
+  if (path.empty())
+    return std::string();
+
+  char* resolved = ::realpath(expand_path(path).c_str(), nullptr);
+
+  if (resolved == nullptr)
+    return std::string();
+
+  std::string result(resolved);
+  std::free(resolved);
+
+  return result;
+}
+
+std::string
+resolve_path_or_throw(const std::string& path) {
+  auto result = resolve_path(path);
+
+  if (result.empty())
+    throw torrent::input_error("Could not resolve path: '" + path + "'.");
+
+  return result;
+}