Fix accessing 'toString path'

src/libexpr/paths.cc

@@ -14,4 +14,53 @@ void EvalState::registerAccessor(ref<InputAccessor> accessor)
     inputAccessors.emplace(accessor->number, accessor);
 }
 
+static constexpr std::string_view marker = "/__virtual__/";
+
+std::string EvalState::encodePath(const SourcePath & path)
+{
+    /* For backward compatibility, return paths in the root FS
+       normally. Encoding any other path is not very reproducible (due
+       to /__virtual__/<N>) and we should depreceate it eventually. So
+       print a warning about use of an encoded path in
+       decodePath(). */
+    return path.accessor == rootFS
+        ? path.path.abs()
+        : std::string(marker) + std::to_string(path.accessor->number) + path.path.abs();
+}
+
+SourcePath EvalState::decodePath(std::string_view s, PosIdx pos)
+{
+    if (!hasPrefix(s, "/"))
+        throwEvalError(pos, "string '%1%' doesn't represent an absolute path", s);
+
+    if (hasPrefix(s, marker)) {
+        auto fail = [s]() {
+            throw Error("cannot decode virtual path '%s'", s);
+        };
+
+        s = s.substr(marker.size());
+
+        try {
+            auto slash = s.find('/');
+            if (slash == std::string::npos) fail();
+            size_t number = std::stoi(std::string(s, 0, slash));
+            s = s.substr(slash);
+
+            auto accessor = inputAccessors.find(number);
+            if (accessor == inputAccessors.end()) fail();
+
+            SourcePath path {accessor->second, CanonPath(s)};
+
+            static bool warned = false;
+            warnOnce(warned, fmt("applying 'toString' to path '%s' and then accessing it is deprecated, at %s", path, positions[pos]));
+
+            return path;
+        } catch (std::invalid_argument & e) {
+            fail();
+            abort();
+        }
+    } else
+        return {rootFS, CanonPath(s)};
+}
+
 }