Remove fetchTree 'shallow' hack

builtins.fetchTree was setting `shallow = true` when fetching from git.
That's bad because it makes it behave inconsistently from non-fetchTree
fetches, e.g. when updating an input.

Instead, the Git fetcher now will do a shallow fetch automatically if
`revCount` is already set (e.g. when fetching a lock).

Fixes NixOS#14588.

Author: edolstra

src/libexpr/primops/fetchTree.cc

@@ -150,11 +150,6 @@ static void fetchTree(
             attrs.emplace("exportIgnore", Explicit<bool>{true});
         }
 
-        // fetchTree should fetch git repos with shallow = true by default
-        if (type == "git" && !params.isFetchGit && !attrs.contains("shallow")) {
-            attrs.emplace("shallow", Explicit<bool>{true});
-        }
-
         if (!params.allowNameArgument)
             if (auto nameIter = attrs.find("name"); nameIter != attrs.end())
                 state.error<EvalError>("argument 'name' isn’t supported in call to '%s'", fetcher)

src/libfetchers/git.cc

@@ -627,6 +627,16 @@ struct GitInputScheme : InputScheme
         }
     }
 
+    /**
+     * Decide whether we can do a shallow clone, which is faster. This is possible if the user explicitly specified
+     * `shallow = true`, or if we already have a `revCount`.
+     */
+    bool canDoShallow(const Input & input) const
+    {
+        bool shallow = getShallowAttr(input);
+        return shallow || input.getRevCount().has_value();
+    }
+
     std::pair<ref<SourceAccessor>, Input>
     getAccessorFromCommit(const Settings & settings, ref<Store> store, RepoInfo & repoInfo, Input && input) const
     {
@@ -635,7 +645,7 @@ struct GitInputScheme : InputScheme
         auto origRev = input.getRev();
 
         auto originalRef = input.getRef();
-        bool shallow = getShallowAttr(input);
+        bool shallow = canDoShallow(input);
         auto ref = originalRef ? *originalRef : getDefaultRef(repoInfo, shallow);
         input.attrs.insert_or_assign("ref", ref);
 
@@ -680,7 +690,6 @@ struct GitInputScheme : InputScheme
             }
 
             if (doFetch) {
-                bool shallow = getShallowAttr(input);
                 try {
                     auto fetchRef = getAllRefsAttr(input)             ? "refs/*:refs/*"
                                     : input.getRev()                  ? input.getRev()->gitRev()
@@ -727,13 +736,6 @@ struct GitInputScheme : InputScheme
 
         auto repo = GitRepo::openRepo(repoDir);
 
-        auto isShallow = repo->isShallow();
-
-        if (isShallow && !getShallowAttr(input))
-            throw Error(
-                "'%s' is a shallow Git repository, but shallow repositories are only allowed when `shallow = true;` is specified",
-                repoInfo.locationToArg());
-
         // FIXME: check whether rev is an ancestor of ref?
 
         auto rev = *input.getRev();
@@ -744,10 +746,16 @@ struct GitInputScheme : InputScheme
         if (!input.attrs.contains("lastModified"))
             input.attrs.insert_or_assign("lastModified", getLastModified(settings, repoInfo, repoDir, rev));
 
-        if (!getShallowAttr(input)) {
-            /* Like lastModified, skip revCount if supplied by the caller. */
-            if (!input.attrs.contains("revCount"))
-                input.attrs.insert_or_assign("revCount", getRevCount(settings, repoInfo, repoDir, rev));
+        /* Like lastModified, skip revCount if supplied by the caller. */
+        if (!shallow && !input.attrs.contains("revCount")) {
+            auto isShallow = repo->isShallow();
+
+            if (isShallow && !shallow)
+                throw Error(
+                    "'%s' is a shallow Git repository, but shallow repositories are only allowed when `shallow = true;` is specified",
+                    repoInfo.locationToArg());
+
+            input.attrs.insert_or_assign("revCount", getRevCount(settings, repoInfo, repoDir, rev));
         }
 
         printTalkative("using revision %s of repo '%s'", rev.gitRev(), repoInfo.locationToArg());