libutil: implement a realPath() utility

Just a wrapper around POSIX realpath().

Change-Id: I2593770285dbae573eace490efce5b272b00b001

src/libutil/file-system.cc

@@ -1,4 +1,5 @@
 #include <sys/time.h>
+#include <cstdlib>
 #include <filesystem>
 #include <atomic>
 
@@ -106,6 +107,24 @@ Path canonPath(PathView path, bool resolveSymlinks)
     return s.empty() ? "/" : std::move(s);
 }
 
+Path realPath(Path const & path)
+{
+    // With nullptr, realpath() malloc's and returns a new c-string.
+    char * resolved = realpath(path.c_str(), nullptr);
+    int saved = errno;
+    if (resolved == nullptr) {
+        throw SysError(saved, "cannot get realpath for '%s'", path);
+    }
+
+    Finally const _free([&] { free(resolved); });
+
+    // There's not really a from_raw_parts() for std::string.
+    // The copy is not a big deal.
+    Path ret(resolved);
+
+    return ret;
+}
+
 void chmodPath(const Path & path, mode_t mode)
 {
     if (chmod(path.c_str(), mode) == -1)

src/libutil/file-system.hh

@@ -46,6 +46,22 @@ Path absPath(Path path,
  */
 Path canonPath(PathView path, bool resolveSymlinks = false);
 
+/**
+ * Resolves a file path to a fully absolute path with no symbolic links.
+ *
+ * @param path The path to resolve. If it is relative, it will be resolved relative
+ * to the process's current directory.
+ *
+ * @note This is not a pure function; it performs this resolution by querying
+ * the filesystem.
+ *
+ * @note @ref path sadly must be (a reference to) an owned string, as std::string_view
+ * are not valid C strings...
+ *
+ * @return The fully resolved path.
+ */
+Path realPath(Path const & path);
+
 /**
  * Change the permissions of a path
  * Not called `chmod` as it shadows and could be confused with