git fetcher: Add exportIgnore parameter

Enabled for fetchGit, which historically had this behavior,
among other behaviors we do not want in fetchGit.

fetchTree disables this parameter by default. It can choose the
simpler behavior, as it is still experimental.

I am not confident that the filtering implementation is future
proof. It should reuse a source filtering wrapper, which I believe
Eelco has already written, but not merged yet.

src/libfetchers/git-utils.cc

@@ -7,6 +7,7 @@
 
 #include <boost/core/span.hpp>
 
+#include <git2/attr.h>
 #include <git2/blob.h>
 #include <git2/commit.h>
 #include <git2/config.h>
@@ -21,6 +22,7 @@
 #include <git2/submodule.h>
 #include <git2/tree.h>
 
+#include <iostream>
 #include <unordered_set>
 #include <queue>
 #include <regex>
@@ -307,7 +309,7 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
         return std::nullopt;
     }
 
-    std::vector<std::tuple<Submodule, Hash>> getSubmodules(const Hash & rev) override;
+    std::vector<std::tuple<Submodule, Hash>> getSubmodules(const Hash & rev, bool exportIgnore) override;
 
     std::string resolveSubmoduleUrl(
         const std::string & url,
@@ -340,7 +342,7 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
         return true;
     }
 
-    ref<InputAccessor> getAccessor(const Hash & rev) override;
+    ref<InputAccessor> getAccessor(const Hash & rev, bool exportIgnore) override;
 
     static int sidebandProgressCallback(const char * str, int len, void * payload)
     {
@@ -460,10 +462,12 @@ struct GitInputAccessor : InputAccessor
 {
     ref<GitRepoImpl> repo;
     Tree root;
+    bool exportIgnore;
 
-    GitInputAccessor(ref<GitRepoImpl> repo_, const Hash & rev)
+    GitInputAccessor(ref<GitRepoImpl> repo_, const Hash & rev, bool exportIgnore)
         : repo(repo_)
         , root(peelObject<Tree>(*repo, lookupObject(*repo, hashToOID(rev)).get(), GIT_OBJECT_TREE))
+        , exportIgnore(exportIgnore)
     {
     }
 
@@ -492,7 +496,7 @@ struct GitInputAccessor : InputAccessor
             return Stat { .type = tDirectory };
 
         auto entry = lookup(path);
-        if (!entry)
+        if (!entry || isExportIgnored(path))
             return std::nullopt;
 
         auto mode = git_tree_entry_filemode(entry);
@@ -527,6 +531,12 @@ struct GitInputAccessor : InputAccessor
 
                 for (size_t n = 0; n < count; ++n) {
                     auto entry = git_tree_entry_byindex(tree.get(), n);
+                    if (exportIgnore) {
+                        if (isExportIgnored(path + git_tree_entry_name(entry))) {
+                            continue;
+                        }
+                    }
+
                     // FIXME: add to cache
                     res.emplace(std::string(git_tree_entry_name(entry)), DirEntry{});
                 }
@@ -556,6 +566,33 @@ struct GitInputAccessor : InputAccessor
 
     std::unordered_map<CanonPath, TreeEntry> lookupCache;
 
+    bool isExportIgnored(const CanonPath & path) {
+        if (!exportIgnore)
+            return false;
+
+        const char *exportIgnoreEntry = nullptr;
+
+        // GIT_ATTR_CHECK_INDEX_ONLY:
+        // > It will use index only for creating archives or for a bare repo
+        // > (if an index has been specified for the bare repo).
+        // -- https://github.com/libgit2/libgit2/blob/HEAD/include/git2/attr.h#L113C62-L115C48
+        if (git_attr_get(&exportIgnoreEntry,
+                *repo,
+                GIT_ATTR_CHECK_INDEX_ONLY,
+                std::string(path.rel()).c_str(),
+                "export-ignore")) {
+            if (git_error_last()->klass == GIT_ENOTFOUND)
+                return false;
+            else
+                throw Error("looking up '%s': %s", showPath(path), git_error_last()->message);
+        }
+        else {
+            // Official git will silently reject export-ignore lines that have
+            // values. We do the same.
+            return GIT_ATTR_IS_TRUE(exportIgnoreEntry);
+        }
+    }
+
     /* Recursively look up 'path' relative to the root. */
     git_tree_entry * lookup(const CanonPath & path)
     {
@@ -569,6 +606,10 @@ struct GitInputAccessor : InputAccessor
                     throw Error("looking up '%s': %s", showPath(path), git_error_last()->message);
             }
 
+            if (entry && isExportIgnored(path)) {
+                entry.reset();
+            }
+
             i = lookupCache.emplace(path, std::move(entry)).first;
         }
 
@@ -644,17 +685,17 @@ struct GitInputAccessor : InputAccessor
     }
 };
 
-ref<InputAccessor> GitRepoImpl::getAccessor(const Hash & rev)
+ref<InputAccessor> GitRepoImpl::getAccessor(const Hash & rev, bool exportIgnore)
 {
-    return make_ref<GitInputAccessor>(ref<GitRepoImpl>(shared_from_this()), rev);
+    return make_ref<GitInputAccessor>(ref<GitRepoImpl>(shared_from_this()), rev, exportIgnore);
 }
 
-std::vector<std::tuple<GitRepoImpl::Submodule, Hash>> GitRepoImpl::getSubmodules(const Hash & rev)
+std::vector<std::tuple<GitRepoImpl::Submodule, Hash>> GitRepoImpl::getSubmodules(const Hash & rev, bool exportIgnore)
 {
     /* Read the .gitmodules files from this revision. */
     CanonPath modulesFile(".gitmodules");
 
-    auto accessor = getAccessor(rev);
+    auto accessor = getAccessor(rev, exportIgnore);
     if (!accessor->pathExists(modulesFile)) return {};
 
     /* Parse it and get the revision of each submodule. */