Support empty Git repositories / workdirs

Fixes #10039.

src/libfetchers/git-utils.cc

@@ -2,6 +2,7 @@
 #include "fs-input-accessor.hh"
 #include "input-accessor.hh"
 #include "filtering-input-accessor.hh"
+#include "memory-input-accessor.hh"
 #include "cache.hh"
 #include "finally.hh"
 #include "processes.hh"
@@ -942,17 +943,21 @@ ref<InputAccessor> GitRepoImpl::getAccessor(const Hash & rev, bool exportIgnore)
 ref<InputAccessor> GitRepoImpl::getAccessor(const WorkdirInfo & wd, bool exportIgnore, MakeNotAllowedError makeNotAllowedError)
 {
     auto self = ref<GitRepoImpl>(shared_from_this());
+    /* In case of an empty workdir, return an empty in-memory tree. We
+       cannot use AllowListInputAccessor because it would return an
+       error for the root (and we can't add the root to the allow-list
+       since that would allow access to all its children). */
     ref<InputAccessor> fileAccessor =
-        AllowListInputAccessor::create(
-                makeFSInputAccessor(path),
-                std::set<CanonPath> { wd.files },
-                std::move(makeNotAllowedError));
-    if (exportIgnore) {
+        wd.files.empty()
+        ? makeEmptyInputAccessor()
+        : AllowListInputAccessor::create(
+            makeFSInputAccessor(path),
+            std::set<CanonPath> { wd.files },
+            std::move(makeNotAllowedError)).cast<InputAccessor>();
+    if (exportIgnore)
         return make_ref<GitExportIgnoreInputAccessor>(self, fileAccessor, std::nullopt);
-    }
-    else {
+    else
         return fileAccessor;
-    }
 }
 
 ref<GitFileSystemObjectSink> GitRepoImpl::getFileSystemObjectSink()

src/libfetchers/git.cc

@@ -158,6 +158,8 @@ std::vector<PublicKey> getPublicKeys(const Attrs & attrs)
 
 }  // end namespace
 
+static const Hash nullRev{HashAlgorithm::SHA1};
+
 struct GitInputScheme : InputScheme
 {
     std::optional<Input> inputFromURL(const ParsedURL & url, bool requireTree) const override
@@ -708,10 +710,12 @@ struct GitInputScheme : InputScheme
             if (auto ref = repo->getWorkdirRef())
                 input.attrs.insert_or_assign("ref", *ref);
 
-            auto rev = repoInfo.workdirInfo.headRev.value();
+            /* Return a rev of 000... if there are no commits yet. */
+            auto rev = repoInfo.workdirInfo.headRev.value_or(nullRev);
 
             input.attrs.insert_or_assign("rev", rev.gitRev());
-            input.attrs.insert_or_assign("revCount", getRevCount(repoInfo, repoInfo.url, rev));
+            input.attrs.insert_or_assign("revCount",
+                rev == nullRev ? 0 : getRevCount(repoInfo, repoInfo.url, rev));
 
             verifyCommit(input, repo);
         } else {

src/libfetchers/memory-input-accessor.cc

@@ -20,4 +20,10 @@ ref<MemoryInputAccessor> makeMemoryInputAccessor()
     return make_ref<MemoryInputAccessorImpl>();
 }
 
+ref<InputAccessor> makeEmptyInputAccessor()
+{
+    static auto empty = makeMemoryInputAccessor().cast<InputAccessor>();
+    return empty;
+}
+
 }

src/libfetchers/memory-input-accessor.hh

@@ -13,4 +13,6 @@ struct MemoryInputAccessor : InputAccessor
 
 ref<MemoryInputAccessor> makeMemoryInputAccessor();
 
+ref<InputAccessor> makeEmptyInputAccessor();
+
 }