diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 87b1e73a5..5b69a2174 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -17,6 +17,7 @@
 #include "nix/expr/print.hh"
 #include "nix/fetchers/filtering-source-accessor.hh"
 #include "nix/util/memory-source-accessor.hh"
+#include "nix/util/mounted-source-accessor.hh"
 #include "nix/expr/gc-small-vector.hh"
 #include "nix/util/url.hh"
 #include "nix/fetchers/fetch-to-store.hh"
diff --git a/src/libfetchers/git.cc b/src/libfetchers/git.cc
index f750d907d..f6f5c30ee 100644
--- a/src/libfetchers/git.cc
+++ b/src/libfetchers/git.cc
@@ -15,6 +15,7 @@
 #include "nix/fetchers/fetch-settings.hh"
 #include "nix/util/json-utils.hh"
 #include "nix/util/archive.hh"
+#include "nix/util/mounted-source-accessor.hh"
 
 #include <regex>
 #include <string.h>
diff --git a/src/libutil/include/nix/util/meson.build b/src/libutil/include/nix/util/meson.build
index 07a4f1d11..dcfaa8e3f 100644
--- a/src/libutil/include/nix/util/meson.build
+++ b/src/libutil/include/nix/util/meson.build
@@ -47,6 +47,7 @@ headers = files(
   'logging.hh',
   'lru-cache.hh',
   'memory-source-accessor.hh',
+  'mounted-source-accessor.hh',
   'muxable-pipe.hh',
   'os-string.hh',
   'pool.hh',
diff --git a/src/libutil/include/nix/util/mounted-source-accessor.hh b/src/libutil/include/nix/util/mounted-source-accessor.hh
new file mode 100644
index 000000000..518ae4f09
--- /dev/null
+++ b/src/libutil/include/nix/util/mounted-source-accessor.hh
@@ -0,0 +1,20 @@
+#pragma once
+
+#include "source-accessor.hh"
+
+namespace nix {
+
+struct MountedSourceAccessor : SourceAccessor
+{
+    virtual void mount(CanonPath mountPoint, ref<SourceAccessor> accessor) = 0;
+
+    /**
+     * Return the accessor mounted on `mountPoint`, or `nullptr` if
+     * there is no such mount point.
+     */
+    virtual std::shared_ptr<SourceAccessor> getMount(CanonPath mountPoint) = 0;
+};
+
+ref<MountedSourceAccessor> makeMountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> mounts);
+
+} // namespace nix
diff --git a/src/libutil/include/nix/util/source-accessor.hh b/src/libutil/include/nix/util/source-accessor.hh
index aa937da48..7419ef392 100644
--- a/src/libutil/include/nix/util/source-accessor.hh
+++ b/src/libutil/include/nix/util/source-accessor.hh
@@ -214,8 +214,6 @@ ref<SourceAccessor> getFSSourceAccessor();
  */
 ref<SourceAccessor> makeFSSourceAccessor(std::filesystem::path root);
 
-ref<SourceAccessor> makeMountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> mounts);
-
 /**
  * Construct an accessor that presents a "union" view of a vector of
  * underlying accessors. Earlier accessors take precedence over later.
diff --git a/src/libutil/mounted-source-accessor.cc b/src/libutil/mounted-source-accessor.cc
index 4c32147f9..5c0ecc1ff 100644
--- a/src/libutil/mounted-source-accessor.cc
+++ b/src/libutil/mounted-source-accessor.cc
@@ -1,18 +1,22 @@
-#include "nix/util/source-accessor.hh"
+#include "nix/util/mounted-source-accessor.hh"
+
+#include <boost/unordered/concurrent_flat_map.hpp>
 
 namespace nix {
 
-struct MountedSourceAccessor : SourceAccessor
+struct MountedSourceAccessorImpl : MountedSourceAccessor
 {
-    std::map<CanonPath, ref<SourceAccessor>> mounts;
+    boost::concurrent_flat_map<CanonPath, ref<SourceAccessor>> mounts;
 
-    MountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> _mounts)
-        : mounts(std::move(_mounts))
+    MountedSourceAccessorImpl(std::map<CanonPath, ref<SourceAccessor>> _mounts)
     {
         displayPrefix.clear();
 
         // Currently we require a root filesystem. This could be relaxed.
-        assert(mounts.contains(CanonPath::root));
+        assert(_mounts.contains(CanonPath::root));
+
+        for (auto & [path, accessor] : _mounts)
+            mount(path, accessor);
 
         // FIXME: return dummy parent directories automatically?
     }
@@ -52,10 +56,9 @@ struct MountedSourceAccessor : SourceAccessor
         // Find the nearest parent of `path` that is a mount point.
         std::vector<std::string> subpath;
         while (true) {
-            auto i = mounts.find(path);
-            if (i != mounts.end()) {
+            if (auto mount = getMount(path)) {
                 std::reverse(subpath.begin(), subpath.end());
-                return {i->second, CanonPath(subpath)};
+                return {ref(mount), CanonPath(subpath)};
             }
 
             assert(!path.isRoot());
@@ -69,11 +72,24 @@ struct MountedSourceAccessor : SourceAccessor
         auto [accessor, subpath] = resolve(path);
         return accessor->getPhysicalPath(subpath);
     }
+
+    void mount(CanonPath mountPoint, ref<SourceAccessor> accessor) override
+    {
+        mounts.emplace(std::move(mountPoint), std::move(accessor));
+    }
+
+    std::shared_ptr<SourceAccessor> getMount(CanonPath mountPoint) override
+    {
+        if (auto res = getConcurrent(mounts, mountPoint))
+            return *res;
+        else
+            return nullptr;
+    }
 };
 
-ref<SourceAccessor> makeMountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> mounts)
+ref<MountedSourceAccessor> makeMountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> mounts)
 {
-    return make_ref<MountedSourceAccessor>(std::move(mounts));
+    return make_ref<MountedSourceAccessorImpl>(std::move(mounts));
 }
 
 } // namespace nix
diff --git a/src/nix/env.cc b/src/nix/env.cc
index c8fb5bee0..0a211399a 100644
--- a/src/nix/env.cc
+++ b/src/nix/env.cc
@@ -7,6 +7,7 @@
 #include "nix/util/strings.hh"
 #include "nix/util/executable-path.hh"
 #include "nix/util/environment-variables.hh"
+#include "nix/util/mounted-source-accessor.hh"
 
 using namespace nix;