libgit2 does not return a valid blob - c

Question

I'm trying to get a blob of a repository with libgit2:
#include <git2.h>
#include <stdio.h>
int main() {
git_libgit2_init();
git_repository *repo = NULL;
int error = git_repository_open(&repo, "/home/martin/Dokumente/TestRepository");
if (error < 0) {
const git_error *e = git_error_last();
printf("Error %d/%d: %s\n", error, e->klass, e->message);
exit(error);
}
git_diff *diff = NULL;
git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
opts.flags |= GIT_DIFF_IGNORE_WHITESPACE;
opts.flags |= GIT_DIFF_INCLUDE_UNTRACKED;
error = git_diff_index_to_workdir(&diff, repo, NULL, &opts);
if (error < 0) {
const git_error *e = git_error_last();
printf("Error %d/%d: %s\n", error, e->klass, e->message);
exit(error);
}
git_patch* patch = nullptr;
git_patch_from_diff(&patch, diff, 0);
bool oldFile = false;
const git_diff_delta *dd = git_patch_get_delta(patch);
const git_oid &id = (!oldFile) ? dd->new_file.id : dd->old_file.id;
git_object *obj = nullptr;
git_object_lookup(&obj, repo, &id, GIT_OBJECT_ANY);
git_blob* blob = reinterpret_cast<git_blob *>(obj);
const char* pointer = (const char*)git_blob_rawcontent(blob);
// cleanup
git_object_free(obj);
git_patch_free(patch);
git_diff_free(diff);
git_repository_free(repo);
return 0;
}
The repository
create a new repository
commit a file like:
1
2
3
4
remove the 4 again, but do not commit
let the program run
Expected:
The program runs fine.
Observed:
obj is still a nullptr after executing
git_object_lookup()
When setting the variable oldFile to true, the program runs fine and the pointer "pointer" contains the raw blob.
Does anybody know why I don't get a valid object from git_object_lookup() back?

Answer 1

The problem is that you're trying to get an object of id dd->new_file.id. This file is in the working directory, as it hasn't been added or committed yet. This means it isn't in the repository yet. When you run git_object_lookup(), it can't find the object as it hasn't been added to the tree. The OID doesn't correspond to any match so it returns null.
If you want to get the current working directory data, you must first create the object in the tree using git_blob_create_from_workdir, and then when trying to access it, it would be found. So your new code might look like:
bool oldFile = false;
const git_diff_delta *dd = git_patch_get_delta(patch);
git_oid id;
if (!oldFile) {
error = git_blob_create_from_workdir(&id, repo, dd->new_file.path);
if (error < 0) {
const git_error *e = git_error_last();
printf("Error %d/%d: %s\n", error, e->klass, e->message);
exit(error);
}
} else {
id = dd->old_file.id;
}
git_object *obj = nullptr;
git_object_lookup(&obj, repo, &id, GIT_OBJECT_ANY);
git_blob* blob = reinterpret_cast<git_blob *>(obj);
const char* pointer = (const char*)git_blob_rawcontent(blob);

Answer 2

When you diff between the index and the workdir, the new side of the delta represents the file in the working directory. Its id is the hash of the file on disk. Unless you explicitly insert that blob into the repository's object store by some other means, there's no reason for it to be there yet.