Introduce FSInputAccessor and use it

Backported from the lazy-trees branch. Note that this doesn't yet use
the access control features of FSInputAccessor.

src/libfetchers/fs-input-accessor.cc

@@ -0,0 +1,141 @@
+#include "fs-input-accessor.hh"
+#include "store-api.hh"
+
+namespace nix {
+
+struct FSInputAccessorImpl : FSInputAccessor
+{
+    CanonPath root;
+    std::optional<std::set<CanonPath>> allowedPaths;
+    MakeNotAllowedError makeNotAllowedError;
+
+    FSInputAccessorImpl(
+        const CanonPath & root,
+        std::optional<std::set<CanonPath>> && allowedPaths,
+        MakeNotAllowedError && makeNotAllowedError)
+        : root(root)
+        , allowedPaths(std::move(allowedPaths))
+        , makeNotAllowedError(std::move(makeNotAllowedError))
+    {
+    }
+
+    std::string readFile(const CanonPath & path) override
+    {
+        auto absPath = makeAbsPath(path);
+        checkAllowed(absPath);
+        return nix::readFile(absPath.abs());
+    }
+
+    bool pathExists(const CanonPath & path) override
+    {
+        auto absPath = makeAbsPath(path);
+        return isAllowed(absPath) && nix::pathExists(absPath.abs());
+    }
+
+    Stat lstat(const CanonPath & path) override
+    {
+        auto absPath = makeAbsPath(path);
+        checkAllowed(absPath);
+        auto st = nix::lstat(absPath.abs());
+        return Stat {
+            .type =
+                S_ISREG(st.st_mode) ? tRegular :
+                S_ISDIR(st.st_mode) ? tDirectory :
+                S_ISLNK(st.st_mode) ? tSymlink :
+                tMisc,
+            .isExecutable = S_ISREG(st.st_mode) && st.st_mode & S_IXUSR
+        };
+    }
+
+    DirEntries readDirectory(const CanonPath & path) override
+    {
+        auto absPath = makeAbsPath(path);
+        checkAllowed(absPath);
+        DirEntries res;
+        for (auto & entry : nix::readDirectory(absPath.abs())) {
+            std::optional<Type> type;
+            switch (entry.type) {
+            case DT_REG: type = Type::tRegular; break;
+            case DT_LNK: type = Type::tSymlink; break;
+            case DT_DIR: type = Type::tDirectory; break;
+            }
+            if (isAllowed(absPath + entry.name))
+                res.emplace(entry.name, type);
+        }
+        return res;
+    }
+
+    std::string readLink(const CanonPath & path) override
+    {
+        auto absPath = makeAbsPath(path);
+        checkAllowed(absPath);
+        return nix::readLink(absPath.abs());
+    }
+
+    CanonPath makeAbsPath(const CanonPath & path)
+    {
+        return root + path;
+    }
+
+    void checkAllowed(const CanonPath & absPath) override
+    {
+        if (!isAllowed(absPath))
+            throw makeNotAllowedError
+                ? makeNotAllowedError(absPath)
+                : RestrictedPathError("access to path '%s' is forbidden", absPath);
+    }
+
+    bool isAllowed(const CanonPath & absPath)
+    {
+        if (!absPath.isWithin(root))
+            return false;
+
+        if (allowedPaths) {
+            auto p = absPath.removePrefix(root);
+            if (!p.isAllowed(*allowedPaths))
+                return false;
+        }
+
+        return true;
+    }
+
+    void allowPath(CanonPath path) override
+    {
+        if (allowedPaths)
+            allowedPaths->insert(std::move(path));
+    }
+
+    bool hasAccessControl() override
+    {
+        return (bool) allowedPaths;
+    }
+
+    std::optional<CanonPath> getPhysicalPath(const CanonPath & path) override
+    {
+        return makeAbsPath(path);
+    }
+};
+
+ref<FSInputAccessor> makeFSInputAccessor(
+    const CanonPath & root,
+    std::optional<std::set<CanonPath>> && allowedPaths,
+    MakeNotAllowedError && makeNotAllowedError)
+{
+    return make_ref<FSInputAccessorImpl>(root, std::move(allowedPaths), std::move(makeNotAllowedError));
+}
+
+ref<FSInputAccessor> makeStorePathAccessor(
+    ref<Store> store,
+    const StorePath & storePath,
+    MakeNotAllowedError && makeNotAllowedError)
+{
+    return makeFSInputAccessor(CanonPath(store->toRealPath(storePath)), {}, std::move(makeNotAllowedError));
+}
+
+SourcePath getUnfilteredRootPath(CanonPath path)
+{
+    static auto rootFS = makeFSInputAccessor(CanonPath::root);
+    return {rootFS, path};
+}
+
+}