Require shallow clones to be requested explicitly

If you do a fetchTree on a Git repository, whether the result contains
a revCount attribute should not depend on whether that repository
happens to be a shallow clone or not. That would complicate caching a
lot and would be semantically messy. So applying fetchTree/fetchGit to
a shallow repository is now an error unless you pass the attribute
'shallow = true'. If 'shallow = true', we don't return revCount, even
if the repository is not actually shallow.

Note that Nix itself is not doing shallow clones at the moment. But it
could do so as an optimisation if the user specifies 'shallow = true'.

Issue #2988.

1 files changed, 13 insertions, 3 deletions

diff --git a/src/libstore/fetchers/git.cc b/src/libstore/fetchers/git.cc
index 179a5fb83..c5d4f019c 100644
--- a/src/libstore/fetchers/git.cc
+++ b/src/libstore/fetchers/git.cc
@@ -83,6 +83,7 @@ struct GitInput : Input
     ParsedURL url;
     std::optional<std::string> ref;
     std::optional<Hash> rev;
+    bool shallow = false;
 
     GitInput(const ParsedURL & url) : url(url)
     { }
@@ -114,6 +115,7 @@ struct GitInput : Input
         if (url2.scheme != "git") url2.scheme = "git+" + url2.scheme;
         if (rev) url2.query.insert_or_assign("rev", rev->gitRev());
         if (ref) url2.query.insert_or_assign("ref", *ref);
+        if (shallow) url2.query.insert_or_assign("shallow", "1");
         return url2.to_string();
     }
 
@@ -125,6 +127,8 @@ struct GitInput : Input
             attrs.emplace("ref", *ref);
         if (rev)
             attrs.emplace("rev", rev->gitRev());
+        if (shallow)
+            attrs.emplace("shallow", true);
         return attrs;
     }
 
@@ -361,9 +365,12 @@ struct GitInput : Input
 
         bool isShallow = chomp(runProgram("git", true, { "-C", repoDir, "rev-parse", "--is-shallow-repository" })) == "true";
 
+        if (isShallow && !shallow)
+            throw Error("'%s' is a shallow Git repository, but a non-shallow repository is needed", actualUrl);
+
         if (auto tree = lookupGitInfo(store, name, *input->rev)) {
             assert(*input->rev == tree->first);
-            if (isShallow) tree->second.info.revCount.reset();
+            if (shallow) tree->second.info.revCount.reset();
             return {std::move(tree->second), input};
         }
 
@@ -393,7 +400,7 @@ struct GitInput : Input
             .storePath = std::move(storePath),
             .info = TreeInfo {
                 .revCount =
-                    !isShallow
+                    !shallow
                     ? std::optional(std::stoull(runProgram("git", true, { "-C", repoDir, "rev-list", "--count", input->rev->gitRev() })))
                     : std::nullopt,
                 .lastModified = lastModified
@@ -440,7 +447,7 @@ struct GitInputScheme : InputScheme
         if (maybeGetStrAttr(attrs, "type") != "git") return {};
 
         for (auto & [name, value] : attrs)
-            if (name != "type" && name != "url" && name != "ref" && name != "rev")
+            if (name != "type" && name != "url" && name != "ref" && name != "rev" && name != "shallow")
                 throw Error("unsupported Git input attribute '%s'", name);
 
         auto input = std::make_unique<GitInput>(parseURL(getStrAttr(attrs, "url")));
@@ -451,6 +458,9 @@ struct GitInputScheme : InputScheme
         }
         if (auto rev = maybeGetStrAttr(attrs, "rev"))
             input->rev = Hash(*rev, htSHA1);
+
+        input->shallow = maybeGetBoolAttr(attrs, "shallow").value_or(false);
+
         return input;
     }
 };