Add a UnionSourceAccessor

src/libutil/meson.build

@@ -167,6 +167,7 @@ sources = files(
   'tarfile.cc',
   'terminal.cc',
   'thread-pool.cc',
+  'union-source-accessor.cc',
   'unix-domain-socket.cc',
   'url.cc',
   'users.cc',

src/libutil/source-accessor.hh

@@ -216,4 +216,10 @@ 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.
+ */
+ref<SourceAccessor> makeUnionSourceAccessor(std::vector<ref<SourceAccessor>> && accessors);
+
 }

src/libutil/union-source-accessor.cc

@@ -0,0 +1,82 @@
+#include "source-accessor.hh"
+
+namespace nix {
+
+struct UnionSourceAccessor : SourceAccessor
+{
+    std::vector<ref<SourceAccessor>> accessors;
+
+    UnionSourceAccessor(std::vector<ref<SourceAccessor>> _accessors)
+        : accessors(std::move(_accessors))
+    {
+        displayPrefix.clear();
+    }
+
+    std::string readFile(const CanonPath & path) override
+    {
+        for (auto & accessor : accessors) {
+            auto st = accessor->maybeLstat(path);
+            if (st && st->type == Type::tRegular)
+                return accessor->readFile(path);
+        }
+        throw FileNotFound("path '%s' does not exist", showPath(path));
+    }
+
+    std::optional<Stat> maybeLstat(const CanonPath & path) override
+    {
+        for (auto & accessor : accessors) {
+            auto st = accessor->maybeLstat(path);
+            if (st)
+                return st;
+        }
+        return std::nullopt;
+    }
+
+    DirEntries readDirectory(const CanonPath & path) override
+    {
+        DirEntries result;
+        for (auto & accessor : accessors) {
+            auto st = accessor->maybeLstat(path);
+            if (!st || st->type != Type::tDirectory)
+                continue;
+            for (auto & entry : accessor->readDirectory(path))
+                // Don't override entries from previous accessors.
+                result.insert(entry);
+        }
+        return result;
+    }
+
+    std::string readLink(const CanonPath & path) override
+    {
+        for (auto & accessor : accessors) {
+            auto st = accessor->maybeLstat(path);
+            if (st && st->type == Type::tSymlink)
+                return accessor->readLink(path);
+        }
+        throw FileNotFound("path '%s' does not exist", showPath(path));
+    }
+
+    std::string showPath(const CanonPath & path) override
+    {
+        for (auto & accessor : accessors)
+            return accessor->showPath(path);
+        return SourceAccessor::showPath(path);
+    }
+
+    std::optional<std::filesystem::path> getPhysicalPath(const CanonPath & path) override
+    {
+        for (auto & accessor : accessors) {
+            auto p = accessor->getPhysicalPath(path);
+            if (p)
+                return p;
+        }
+        return std::nullopt;
+    }
+};
+
+ref<SourceAccessor> makeUnionSourceAccessor(std::vector<ref<SourceAccessor>> && accessors)
+{
+    return make_ref<UnionSourceAccessor>(std::move(accessors));
+}
+
+}