Encode virtual paths as /nix/store/virtual000<N>

This makes lib.isStorePath in nixpkgs return true for source trees and
fixes some cases where /__nix_virtual__ showed up in the NixOS manual.

src/libexpr/eval.cc

@@ -500,6 +500,7 @@ EvalState::EvalState(
     , valueAllocCache(std::make_shared<void *>(nullptr))
     , env1AllocCache(std::make_shared<void *>(nullptr))
 #endif
+    , virtualPathMarker(settings.nixStore + "/virtual00000000000000000")
     , baseEnv(allocEnv(128))
     , staticBaseEnv{std::make_shared<StaticEnv>(false, nullptr)}
 {

src/libexpr/eval.hh

@@ -227,14 +227,16 @@ public:
     void registerAccessor(ref<InputAccessor> accessor);
 
     /* Convert a path to a string representation of the format
-       `/__nix_virtual__/<accessor-number>/<path>`. */
+       `/nix/store/virtual000...<accessor-number>/<path>`. */
     std::string encodePath(const SourcePath & path);
 
     /* Decode a path encoded by `encodePath()`. */
     SourcePath decodePath(std::string_view s, PosIdx pos = noPos);
 
+    const std::string virtualPathMarker;
+
     /* Decode all virtual paths in a string, i.e. all
-       /__nix_virtual__/... substrings are replaced by the
+       /nix/store/virtual000... substrings are replaced by the
        corresponding input accessor. */
     std::string decodePaths(std::string_view s);
 

src/libexpr/paths.cc

@@ -14,18 +14,16 @@ void EvalState::registerAccessor(ref<InputAccessor> accessor)
     inputAccessors.emplace(accessor->number, accessor);
 }
 
-static constexpr std::string_view marker = "/__nix_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 /__nix_virtual__/<N>) and we should depreceate it
+       to /nix/store/virtual000...<N>) and we should deprecate 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();
+        : fmt("%s%08x-source%s", virtualPathMarker, path.accessor->number, path.path.absOrEmpty());
 }
 
 SourcePath EvalState::decodePath(std::string_view s, PosIdx pos)
@@ -33,17 +31,17 @@ 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)) {
+    if (hasPrefix(s, virtualPathMarker)) {
         auto fail = [s]() {
             throw Error("cannot decode virtual path '%s'", s);
         };
 
-        s = s.substr(marker.size());
+        s = s.substr(virtualPathMarker.size());
 
         try {
             auto slash = s.find('/');
             if (slash == std::string::npos) fail();
-            size_t number = std::stoi(std::string(s, 0, slash));
+            size_t number = std::stoi(std::string(s, 0, slash), nullptr, 16);
             s = s.substr(slash);
 
             auto accessor = inputAccessors.find(number);
@@ -70,7 +68,7 @@ std::string EvalState::decodePaths(std::string_view s)
     size_t pos = 0;
 
     while (true) {
-        auto m = s.find(marker, pos);
+        auto m = s.find(virtualPathMarker, pos);
         if (m == s.npos) {
             res.append(s.substr(pos));
             return res;

src/libutil/canon-path.hh

@@ -58,6 +58,14 @@ public:
     const std::string & abs() const
     { return path; }
 
+    /* Like abs(), but return an empty string if this path is
+       '/'. Thus the returned string never ends in a slash. */
+    const std::string & absOrEmpty() const
+    {
+        const static std::string epsilon;
+        return isRoot() ? epsilon : path;
+    }
+
     const char * c_str() const
     { return path.c_str(); }