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.

6 files changed, 53 insertions, 9 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 852e8aa11..6fcb2917c 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -158,7 +158,7 @@ string showType(ValueType type)
 {
     switch (type) {
         case tInt: return "an integer";
-        case tBool: return "a boolean";
+        case tBool: return "a Boolean";
         case tString: return "a string";
         case tPath: return "a path";
         case tNull: return "null";
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index a9becddd9..2b3f1b76d 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -58,8 +58,10 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
             state.forceValue(*attr.value);
             if (attr.value->type == tString)
                 attrs.emplace(attr.name, attr.value->string.s);
+            else if (attr.value->type == tBool)
+                attrs.emplace(attr.name, attr.value->boolean);
             else
-                throw TypeError("fetchTree argument '%s' is %s while a string is expected",
+                throw TypeError("fetchTree argument '%s' is %s while a string or Boolean is expected",
                     attr.name, showType(*attr.value));
         }
 
diff --git a/src/libstore/fetchers/attrs.cc b/src/libstore/fetchers/attrs.cc
index 83b6d9164..40c02de42 100644
--- a/src/libstore/fetchers/attrs.cc
+++ b/src/libstore/fetchers/attrs.cc
@@ -14,6 +14,8 @@ Attrs jsonToAttrs(const nlohmann::json & json)
             attrs.emplace(i.key(), i.value().get<int64_t>());
         else if (i.value().is_string())
             attrs.emplace(i.key(), i.value().get<std::string>());
+        else if (i.value().is_boolean())
+            attrs.emplace(i.key(), i.value().get<bool>());
         else
             throw Error("unsupported input attribute type in lock file");
     }
@@ -29,6 +31,8 @@ nlohmann::json attrsToJson(const Attrs & attrs)
             json[attr.first] = *v;
         } else if (auto v = std::get_if<std::string>(&attr.second)) {
             json[attr.first] = *v;
+        } else if (auto v = std::get_if<Explicit<bool>>(&attr.second)) {
+            json[attr.first] = v->t;
         } else abort();
     }
     return json;
@@ -40,7 +44,7 @@ std::optional<std::string> maybeGetStrAttr(const Attrs & attrs, const std::strin
     if (i == attrs.end()) return {};
     if (auto v = std::get_if<std::string>(&i->second))
         return *v;
-    throw Error("input attribute '%s' is not a string", name);
+    throw Error("input attribute '%s' is not a string %s", name, attrsToJson(attrs).dump());
 }
 
 std::string getStrAttr(const Attrs & attrs, const std::string & name)
@@ -57,7 +61,7 @@ std::optional<int64_t> maybeGetIntAttr(const Attrs & attrs, const std::string &
     if (i == attrs.end()) return {};
     if (auto v = std::get_if<int64_t>(&i->second))
         return *v;
-    throw Error("input attribute '%s' is not a string", name);
+    throw Error("input attribute '%s' is not an integer", name);
 }
 
 int64_t getIntAttr(const Attrs & attrs, const std::string & name)
@@ -68,4 +72,21 @@ int64_t getIntAttr(const Attrs & attrs, const std::string & name)
     return *s;
 }
 
+std::optional<bool> maybeGetBoolAttr(const Attrs & attrs, const std::string & name)
+{
+    auto i = attrs.find(name);
+    if (i == attrs.end()) return {};
+    if (auto v = std::get_if<int64_t>(&i->second))
+        return *v;
+    throw Error("input attribute '%s' is not a Boolean", name);
+}
+
+bool getBoolAttr(const Attrs & attrs, const std::string & name)
+{
+    auto s = maybeGetBoolAttr(attrs, name);
+    if (!s)
+        throw Error("input attribute '%s' is missing", name);
+    return *s;
+}
+
 }
diff --git a/src/libstore/fetchers/attrs.hh b/src/libstore/fetchers/attrs.hh
index b7f98d71b..2c9e772d2 100644
--- a/src/libstore/fetchers/attrs.hh
+++ b/src/libstore/fetchers/attrs.hh
@@ -8,7 +8,14 @@
 
 namespace nix::fetchers {
 
-typedef std::variant<std::string, int64_t> Attr;
+/* Wrap bools to prevent string literals (i.e. 'char *') from being
+   cast to a bool in Attr. */
+template<typename T>
+struct Explicit {
+    T t;
+};
+
+typedef std::variant<std::string, int64_t, Explicit<bool>> Attr;
 typedef std::map<std::string, Attr> Attrs;
 
 Attrs jsonToAttrs(const nlohmann::json & json);
@@ -23,4 +30,8 @@ std::optional<int64_t> maybeGetIntAttr(const Attrs & attrs, const std::string &
 
 int64_t getIntAttr(const Attrs & attrs, const std::string & name);
 
+std::optional<bool> maybeGetBoolAttr(const Attrs & attrs, const std::string & name);
+
+bool getBoolAttr(const Attrs & attrs, const std::string & name);
+
 }
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;
     }
 };
diff --git a/src/libstore/fetchers/mercurial.cc b/src/libstore/fetchers/mercurial.cc
index a9c86f1b4..3a767ef17 100644
--- a/src/libstore/fetchers/mercurial.cc
+++ b/src/libstore/fetchers/mercurial.cc
@@ -260,7 +260,7 @@ struct MercurialInput : Input
 
         Attrs infoAttrs({
             {"rev", input->rev->gitRev()},
-            {"revCount", revCount},
+            {"revCount", (int64_t) revCount},
         });
 
         if (!this->rev)