Experimentally return treeHash on git and github fetchers

src/libfetchers/git-utils.cc

@@ -57,7 +57,7 @@ bool operator == (const git_oid & oid1, const git_oid & oid2)
 
 namespace nix {
 
-struct GitInputAccessor;
+struct GitInputAccessorImpl;
 
 // Some wrapper types that ensure that the git_*_free functions get called.
 template<auto del>
@@ -334,9 +334,11 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
     }
 
     /**
-     * A 'GitInputAccessor' with no regard for export-ignore or any other transformations.
+     * A 'GitInputAccessorImpl' with no regard for export-ignore or any other transformations.
      */
-    ref<GitInputAccessor> getRawAccessor(const Hash & rev);
+    ref<GitInputAccessorImpl> getRawAccessor(const Hash & rev);
+
+    ref<GitInputAccessor> getPlainAccessor(const Hash & rev) override;
 
     ref<InputAccessor> getAccessor(const Hash & rev, bool exportIgnore) override;
 
@@ -477,17 +479,24 @@ ref<GitRepo> GitRepo::openRepo(const std::filesystem::path & path, bool create,
 /**
  * Raw git tree input accessor.
  */
-struct GitInputAccessor : InputAccessor
+struct GitInputAccessorImpl : GitInputAccessor
 {
     ref<GitRepoImpl> repo;
     Tree root;
 
-    GitInputAccessor(ref<GitRepoImpl> repo_, const Hash & rev)
+    GitInputAccessorImpl(ref<GitRepoImpl> repo_, const Hash & rev)
         : repo(repo_)
         , root(peelObject<Tree>(*repo, lookupObject(*repo, hashToOID(rev)).get(), GIT_OBJECT_TREE))
     {
     }
 
+    Hash getTreeHash() override
+    {
+        auto * oid = git_tree_id(root.get());
+        assert(oid);
+        return toHash(*oid);
+    }
+
     std::string readBlob(const CanonPath & path, bool symlink)
     {
         auto blob = getBlob(path, symlink);
@@ -922,17 +931,22 @@ struct GitFileSystemObjectSinkImpl : GitFileSystemObjectSink
     }
 };
 
-ref<GitInputAccessor> GitRepoImpl::getRawAccessor(const Hash & rev)
+ref<GitInputAccessorImpl> GitRepoImpl::getRawAccessor(const Hash & rev)
 {
     auto self = ref<GitRepoImpl>(shared_from_this());
-    return make_ref<GitInputAccessor>(self, rev);
+    return make_ref<GitInputAccessorImpl>(self, rev);
+}
+
+ref<GitInputAccessor> GitRepoImpl::getPlainAccessor(const Hash & rev)
+{
+    return getRawAccessor(rev);
 }
 
 ref<InputAccessor> GitRepoImpl::getAccessor(const Hash & rev, bool exportIgnore)
 {
-    auto self = ref<GitRepoImpl>(shared_from_this());
-    ref<GitInputAccessor> rawGitAccessor = getRawAccessor(rev);
+    ref<GitInputAccessorImpl> rawGitAccessor = getRawAccessor(rev);
     if (exportIgnore) {
+        auto self = ref<GitRepoImpl>(shared_from_this());
         return make_ref<GitExportIgnoreInputAccessor>(self, rawGitAccessor, rev);
     }
     else {

src/libfetchers/git-utils.hh

@@ -16,6 +16,16 @@ struct GitFileSystemObjectSink : FileSystemObjectSink
     virtual Hash sync() = 0;
 };
 
+/**
+ * Git Input Accessor
+ *
+ * Created from `GitRepo`. Support some additional operations.
+ */
+struct GitInputAccessor : InputAccessor
+{
+    virtual Hash getTreeHash() = 0;
+};
+
 struct GitRepo
 {
     virtual ~GitRepo()
@@ -75,6 +85,8 @@ struct GitRepo
 
     virtual bool hasObject(const Hash & oid) = 0;
 
+    virtual ref<GitInputAccessor> getPlainAccessor(const Hash & rev) = 0;
+
     virtual ref<InputAccessor> getAccessor(const Hash & rev, bool exportIgnore) = 0;
 
     virtual ref<InputAccessor> getAccessor(const WorkdirInfo & wd, bool exportIgnore, MakeNotAllowedError makeNotAllowedError) = 0;

src/libfetchers/git.cc

@@ -206,6 +206,7 @@ struct GitInputScheme : InputScheme
             "shallow",
             "submodules",
             "exportIgnore",
+            "treeHash",
             "lastModified",
             "revCount",
             "narHash",
@@ -601,6 +602,8 @@ struct GitInputScheme : InputScheme
 
         auto rev = *input.getRev();
 
+        auto gotTreeHash = repo->getPlainAccessor(rev)->getTreeHash();
+
         Attrs infoAttrs({
             {"rev", rev.gitRev()},
             {"lastModified", getLastModified(repoInfo, repoDir, rev)},
@@ -646,6 +649,11 @@ struct GitInputScheme : InputScheme
                 mounts.insert_or_assign(CanonPath::root, accessor);
                 accessor = makeMountedInputAccessor(std::move(mounts));
             }
+        } else {
+            /* If we don't have submodules and aren't doing export
+               ignore, then the tree hash is useful info to provide. */
+            if (experimentalFeatureSettings.isEnabled(Xp::GitHashing) && !exportIgnore)
+                input.attrs.insert_or_assign("treeHash", gotTreeHash.gitRev());
         }
 
         assert(!origRev || origRev == rev);

src/libfetchers/github.cc

@@ -276,9 +276,8 @@ struct GitArchiveInputScheme : InputScheme
     {
         auto [input, tarballInfo] = downloadArchive(store, _input);
 
-        #if 0
-        input.attrs.insert_or_assign("treeHash", tarballInfo.treeHash.gitRev());
-        #endif
+        if (experimentalFeatureSettings.isEnabled(Xp::GitHashing))
+            input.attrs.insert_or_assign("treeHash", tarballInfo.treeHash.gitRev());
         input.attrs.insert_or_assign("lastModified", uint64_t(tarballInfo.lastModified));
 
         auto accessor = getTarballCache()->getAccessor(tarballInfo.treeHash, false);