thodg/libgit2

diff --git a/include/git2/errors.h b/include/git2/errors.h
index 973d560..bcf2f80 100644
--- a/include/git2/errors.h
+++ b/include/git2/errors.h
@@ -40,6 +40,7 @@ typedef enum {
 	GIT_EINVALIDSPEC    = -12,	/*< Name/ref spec was not in a valid format */
 	GIT_EMERGECONFLICT  = -13,	/*< Merge conflicts prevented operation */
 	GIT_ELOCKED         = -14,	/*< Lock file prevented operation */
+	GIT_EMODIFIED       = -15,	/*< Reference value does not match expected */
 
 	GIT_PASSTHROUGH     = -30,	/*< Internal only */
 	GIT_ITEROVER        = -31,	/*< Signals end of iteration with iterator */
diff --git a/include/git2/refs.h b/include/git2/refs.h
index f9aaea8..970faf7 100644
--- a/include/git2/refs.h
+++ b/include/git2/refs.h
@@ -68,6 +68,46 @@ GIT_EXTERN(int) git_reference_name_to_id(
 GIT_EXTERN(int) git_reference_dwim(git_reference **out, git_repository *repo, const char *shorthand);
 
 /**
+ * Conditionally create a new symbolic reference.
+ *
+ * A symbolic reference is a reference name that refers to another
+ * reference name.  If the other name moves, the symbolic name will move,
+ * too.  As a simple example, the "HEAD" reference might refer to
+ * "refs/heads/master" while on the "master" branch of a repository.
+ *
+ * The symbolic reference will be created in the repository and written to
+ * the disk.  The generated reference object must be freed by the user.
+ *
+ * Valid reference names must follow one of two patterns:
+ *
+ * 1. Top-level names must contain only capital letters and underscores,
+ *    and must begin and end with a letter. (e.g. "HEAD", "ORIG_HEAD").
+ * 2. Names prefixed with "refs/" can be almost anything.  You must avoid
+ *    the characters '~', '^', ':', '\\', '?', '[', and '*', and the
+ *    sequences ".." and "@{" which have special meaning to revparse.
+ *
+ * This function will return an error if a reference already exists with the
+ * given name unless `force` is true, in which case it will be overwritten.
+ *
+ * The signature and message for the reflog will be ignored if the
+ * reference does not belong in the standard set (HEAD, branches and
+ * remote-tracking branches) and it does not have a reflog.
+ *
+ * It will also return an error if the reference's value at the time
+ * of updating does not match the one passed.
+ *
+ * @param out Pointer to the newly created reference
+ * @param repo Repository where that reference will live
+ * @param name The name of the reference
+ * @param target The target of the reference
+ * @param force Overwrite existing references
+ * @param signature The identity that will used to populate the reflog entry
+ * @param log_message The one line long message to be appended to the reflog
+ * @return 0 on success, GIT_EEXISTS, GIT_EINVALIDSPEC, GIT_EMODIFIED or an error code
+ */
+GIT_EXTERN(int) git_reference_symbolic_create_matching(git_reference **out, git_repository *repo, const char *name, const char *target, int force, const git_signature *signature, const char *log_message, const char *old_value);
+
+/**
  * Create a new symbolic reference.
  *
  * A symbolic reference is a reference name that refers to another
@@ -144,6 +184,50 @@ GIT_EXTERN(int) git_reference_symbolic_create(git_reference **out, git_repositor
 GIT_EXTERN(int) git_reference_create(git_reference **out, git_repository *repo, const char *name, const git_oid *id, int force, const git_signature *signature, const char *log_message);
 
 /**
+ * Conditionally create new direct reference
+ *
+ * A direct reference (also called an object id reference) refers directly
+ * to a specific object id (a.k.a. OID or SHA) in the repository.  The id
+ * permanently refers to the object (although the reference itself can be
+ * moved).  For example, in libgit2 the direct ref "refs/tags/v0.17.0"
+ * refers to OID 5b9fac39d8a76b9139667c26a63e6b3f204b3977.
+ *
+ * The direct reference will be created in the repository and written to
+ * the disk.  The generated reference object must be freed by the user.
+ *
+ * Valid reference names must follow one of two patterns:
+ *
+ * 1. Top-level names must contain only capital letters and underscores,
+ *    and must begin and end with a letter. (e.g. "HEAD", "ORIG_HEAD").
+ * 2. Names prefixed with "refs/" can be almost anything.  You must avoid
+ *    the characters '~', '^', ':', '\\', '?', '[', and '*', and the
+ *    sequences ".." and "@{" which have special meaning to revparse.
+ *
+ * This function will return an error if a reference already exists with the
+ * given name unless `force` is true, in which case it will be overwritten.
+ *
+ * The signature and message for the reflog will be ignored if the
+ * reference does not belong in the standard set (HEAD, branches and
+ * remote-tracking branches) and and it does not have a reflog.
+ *
+ * It will also return an error if the reference's value at the time
+ * of updating does not match the one passed.
+ *
+ * @param out Pointer to the newly created reference
+ * @param repo Repository where that reference will live
+ * @param name The name of the reference
+ * @param id The object id pointed to by the reference.
+ * @param force Overwrite existing references
+ * @param force Overwrite existing references
+ * @param signature The identity that will used to populate the reflog entry
+ * @param log_message The one line long message to be appended to the reflog
+ * @param old_id The old value which the reference should have
+ * @return 0 on success, GIT_EMODIFIED if the value of the reference
+ * has changed, GIT_EEXISTS, GIT_EINVALIDSPEC or an error code
+ */
+GIT_EXTERN(int) git_reference_create_matching(git_reference **out, git_repository *repo, const char *name, const git_oid *id, int force, const git_signature *signature, const char *log_message, const git_oid *old_id);
+
+/**
  * Get the OID pointed to by a direct reference.
  *
  * Only available if the reference is direct (i.e. an object id reference,
@@ -254,22 +338,19 @@ GIT_EXTERN(int) git_reference_symbolic_set_target(
 	const char *log_message);
 
 /**
- * Create a new reference with the same name as the given reference but a
+ * Conditionally create a new reference with the same name as the given reference but a
  * different OID target. The reference must be a direct reference, otherwise
  * this will fail.
  *
  * The new reference will be written to disk, overwriting the given reference.
  *
- * The signature and message for the reflog will be ignored if the
- * reference does not belong in the standard set (HEAD, branches and
- * remote-tracking branches) and and it does not have a reflog.
- *
  * @param out Pointer to the newly created reference
  * @param ref The reference
  * @param id The new target OID for the reference
  * @param signature The identity that will used to populate the reflog entry
  * @param log_message The one line long message to be appended to the reflog
- * @return 0 or an error code
+ * @return 0 on success, GIT_EMODIFIED if the value of the reference
+ * has changed, or an error code
  */
 GIT_EXTERN(int) git_reference_set_target(
 	git_reference **out,
@@ -317,12 +398,26 @@ GIT_EXTERN(int) git_reference_rename(
  * will be immediately removed on disk but the memory will not be freed.
  * Callers must call `git_reference_free`.
  *
+ * This function will return an error if the reference has changed
+ * from the time it was looked up.
+ *
  * @param ref The reference to remove
- * @return 0 or an error code
+ * @return 0, GIT_EMODIFIED or an error code
  */
 GIT_EXTERN(int) git_reference_delete(git_reference *ref);
 
 /**
+ * Delete an existing reference by name
+ *
+ * This method removes the named reference from the repository without
+ * looking at its old value.
+ *
+ * @param ref The reference to remove
+ * @return 0, GIT_EMODIFIED or an error code
+ */
+GIT_EXTERN(int) git_reference_remove(git_repository *repo, const char *name);
+
+/**
  * Fill a list with all the references that can be found in a repository.
  *
  * The string array will be filled with the names of all references; these
diff --git a/include/git2/sys/refdb_backend.h b/include/git2/sys/refdb_backend.h
index 5bbd4ba..aa5ef9e 100644
--- a/include/git2/sys/refdb_backend.h
+++ b/include/git2/sys/refdb_backend.h
@@ -94,7 +94,8 @@ struct git_refdb_backend {
 	 */
 	int (*write)(git_refdb_backend *backend,
 		     const git_reference *ref, int force,
-		     const git_signature *who, const char *message);
+		     const git_signature *who, const char *message,
+		     const git_oid *old, const char *old_target);
 
 	int (*rename)(
 		git_reference **out, git_refdb_backend *backend,
@@ -105,7 +106,7 @@ struct git_refdb_backend {
 	 * Deletes the given reference from the refdb.  A refdb implementation
 	 * must provide this function.
 	 */
-	int (*del)(git_refdb_backend *backend, const char *ref_name);
+	int (*del)(git_refdb_backend *backend, const char *ref_name, const git_oid *old_id, const char *old_target);
 
 	/**
 	 * Suggests that the given refdb compress or optimize its references.
diff --git a/src/refdb.c b/src/refdb.c
index 411423d..984c3c7 100644
--- a/src/refdb.c
+++ b/src/refdb.c
@@ -167,14 +167,14 @@ void git_refdb_iterator_free(git_reference_iterator *iter)
 	iter->free(iter);
 }
 
-int git_refdb_write(git_refdb *db, git_reference *ref, int force, const git_signature *who, const char *message)
+int git_refdb_write(git_refdb *db, git_reference *ref, int force, const git_signature *who, const char *message, const git_oid *old_id, const char *old_target)
 {
 	assert(db && db->backend);
 
 	GIT_REFCOUNT_INC(db);
 	ref->db = db;
 
-	return db->backend->write(db->backend, ref, force, who, message);
+	return db->backend->write(db->backend, ref, force, who, message, old_id, old_target);
 }
 
 int git_refdb_rename(
@@ -201,10 +201,10 @@ int git_refdb_rename(
 	return 0;
 }
 
-int git_refdb_delete(struct git_refdb *db, const char *ref_name)
+int git_refdb_delete(struct git_refdb *db, const char *ref_name, const git_oid *old_id, const char *old_target)
 {
 	assert(db && db->backend);
-	return db->backend->del(db->backend, ref_name);
+	return db->backend->del(db->backend, ref_name, old_id, old_target);
 }
 
 int git_refdb_reflog_read(git_reflog **out, git_refdb *db,  const char *name)
diff --git a/src/refdb.h b/src/refdb.h
index 91eecb7..cbad86f 100644
--- a/src/refdb.h
+++ b/src/refdb.h
@@ -42,8 +42,8 @@ int git_refdb_iterator_next(git_reference **out, git_reference_iterator *iter);
 int git_refdb_iterator_next_name(const char **out, git_reference_iterator *iter);
 void git_refdb_iterator_free(git_reference_iterator *iter);
 
-int git_refdb_write(git_refdb *refdb, git_reference *ref, int force, const git_signature *who, const char *message);
-int git_refdb_delete(git_refdb *refdb, const char *ref_name);
+int git_refdb_write(git_refdb *refdb, git_reference *ref, int force, const git_signature *who, const char *message, const git_oid *old_id, const char *old_target);
+int git_refdb_delete(git_refdb *refdb, const char *ref_name, const git_oid *old_id, const char *old_target);
 
 int git_refdb_reflog_read(git_reflog **out, git_refdb *db,  const char *name);
 int git_refdb_reflog_write(git_reflog *reflog);
diff --git a/src/refdb_fs.c b/src/refdb_fs.c
index 89c77c1..ea758de 100644
--- a/src/refdb_fs.c
+++ b/src/refdb_fs.c
@@ -688,30 +688,32 @@ static int reference_path_available(
 	return 0;
 }
 
-static int loose_lock(git_filebuf *file, refdb_fs_backend *backend, const git_reference *ref)
+static int loose_lock(git_filebuf *file, refdb_fs_backend *backend, const char *name)
 {
+        int error;
 	git_buf ref_path = GIT_BUF_INIT;
 
+	assert(file && backend && name);
+
 	/* Remove a possibly existing empty directory hierarchy
 	 * which name would collide with the reference name
 	 */
-	if (git_futils_rmdir_r(ref->name, backend->path, GIT_RMDIR_SKIP_NONEMPTY) < 0)
+	if (git_futils_rmdir_r(name, backend->path, GIT_RMDIR_SKIP_NONEMPTY) < 0)
 		return -1;
 
-	if (git_buf_joinpath(&ref_path, backend->path, ref->name) < 0)
+	if (git_buf_joinpath(&ref_path, backend->path, name) < 0)
 		return -1;
 
-	if (git_filebuf_open(file, ref_path.ptr, GIT_FILEBUF_FORCE, GIT_REFS_FILE_MODE) < 0) {
-		git_buf_free(&ref_path);
-		return -1;
-	}
+	error = git_filebuf_open(file, ref_path.ptr, GIT_FILEBUF_FORCE, GIT_REFS_FILE_MODE);
 
 	git_buf_free(&ref_path);
-	return 0;
+        return error;
 }
 
 static int loose_commit(git_filebuf *file, const git_reference *ref)
 {
+	assert(file && ref);
+
 	if (ref->type == GIT_REF_OID) {
 		char oid[GIT_OID_HEXSZ + 1];
 		git_oid_nfmt(oid, sizeof(oid), &ref->target.oid);
@@ -928,16 +930,54 @@ static bool should_write_reflog(git_repository *repo, const char *name)
 	return 0;
 }
 
+static int cmp_old_ref(int *cmp, git_refdb_backend *backend, const char *name,
+	const git_oid *old_id, const char *old_target)
+{
+	int error = 0;
+	git_reference *old_ref = NULL;
+
+	*cmp = 0;
+	/* It "matches" if there is no old value to compare against */
+	if (!old_id && !old_target)
+		return 0;
+
+	if ((error = refdb_fs_backend__lookup(&old_ref, backend, name)) < 0)
+		goto out;
+
+	/* If the types don't match, there's no way the values do */
+	if (old_id && old_ref->type != GIT_REF_OID) {
+		*cmp = -1;
+		goto out;
+	}
+	if (old_target && old_ref->type != GIT_REF_SYMBOLIC) {
+		*cmp = 1;
+		goto out;
+	}
+
+	if (old_id && old_ref->type == GIT_REF_OID)
+		*cmp = git_oid_cmp(old_id, &old_ref->target.oid);
+
+	if (old_target && old_ref->type == GIT_REF_SYMBOLIC)
+		*cmp = git__strcmp(old_target, old_ref->target.symbolic);
+
+out:
+	git_reference_free(old_ref);
+
+	return error;
+}
+
 static int refdb_fs_backend__write(
 	git_refdb_backend *_backend,
 	const git_reference *ref,
 	int force,
 	const git_signature *who,
-	const char *message)
+	const char *message,
+	const git_oid *old_id,
+	const char *old_target)
 {
 	refdb_fs_backend *backend = (refdb_fs_backend *)_backend;
 	git_filebuf file = GIT_FILEBUF_INIT;
-	int error;
+	int error = 0, cmp = 0;
 
 	assert(backend);
 
@@ -945,10 +985,19 @@ static int refdb_fs_backend__write(
 	if (error < 0)
 		return error;
 
-	/* We need to perform the reflog append under the ref's lock */
-	if ((error = loose_lock(&file, backend, ref)) < 0)
+	/* We need to perform the reflog append and old value check under the ref's lock */
+	if ((error = loose_lock(&file, backend, ref->name)) < 0)
 		return error;
 
+	if ((error = cmp_old_ref(&cmp, _backend, ref->name, old_id, old_target)) < 0)
+		goto on_error;
+
+	if (cmp) {
+		giterr_set(GITERR_REFERENCE, "old reference value does not match");
+		error = GIT_EMODIFIED;
+		goto on_error;
+	}
+
 	if (should_write_reflog(backend->repo, ref->name) &&
 	    (error = reflog_append(backend, ref, who, message)) < 0) {
 		git_filebuf_cleanup(&file);
@@ -956,20 +1005,40 @@ static int refdb_fs_backend__write(
 	}
 
 	return loose_commit(&file, ref);
+
+on_error:
+        git_filebuf_cleanup(&file);
+        return error;
 }
 
 static int refdb_fs_backend__delete(
 	git_refdb_backend *_backend,
-	const char *ref_name)
+	const char *ref_name,
+	const git_oid *old_id, const char *old_target)
 {
 	refdb_fs_backend *backend = (refdb_fs_backend *)_backend;
 	git_buf loose_path = GIT_BUF_INIT;
-	size_t pack_pos;
-	int error = 0;
+	size_t pack_pos
+;	git_filebuf file = GIT_FILEBUF_INIT;
+	int error = 0, cmp = 0;
 	bool loose_deleted = 0;
 
 	assert(backend && ref_name);
 
+	if ((error = loose_lock(&file, backend, ref_name)) < 0)
+		return error;
+
+	error = cmp_old_ref(&cmp, _backend, ref_name, old_id, old_target);
+	//git_filebuf_cleanup(&file);
+	if (error < 0)
+		goto cleanup;
+
+	if (cmp) {
+		giterr_set(GITERR_REFERENCE, "old reference value does not match");
+		error = GIT_EMODIFIED;
+		goto cleanup;
+	}
+
 	/* If a loose reference exists, remove it from the filesystem */
 	if (git_buf_joinpath(&loose_path, backend->path, ref_name) < 0)
 		return -1;
@@ -982,14 +1051,14 @@ static int refdb_fs_backend__delete(
 	git_buf_free(&loose_path);
 
 	if (error != 0)
-		return error;
+		goto cleanup;
 
-	if (packed_reload(backend) < 0)
-		return -1;
+	if ((error = packed_reload(backend)) < 0)
+		goto cleanup;
 
 	/* If a packed reference exists, remove it from the packfile and repack */
-	if (git_sortedcache_wlock(backend->refcache) < 0)
-		return -1;
+	if ((error = git_sortedcache_wlock(backend->refcache)) < 0)
+		goto cleanup;
 
 	if (!(error = git_sortedcache_lookup_index(
 			&pack_pos, backend->refcache, ref_name)))
@@ -997,10 +1066,17 @@ static int refdb_fs_backend__delete(
 
 	git_sortedcache_wunlock(backend->refcache);
 
-	if (error == GIT_ENOTFOUND)
-		return loose_deleted ? 0 : ref_error_notfound(ref_name);
+	if (error == GIT_ENOTFOUND) {
+		error = loose_deleted ? 0 : ref_error_notfound(ref_name);
+		goto cleanup;
+	}
+
+	error = packed_write(backend);
 
-	return packed_write(backend);
+cleanup:
+	git_filebuf_cleanup(&file);
+
+	return error;
 }
 
 static int refdb_reflog_fs__rename(git_refdb_backend *_backend, const char *old_name, const char *new_name);
@@ -1026,7 +1102,7 @@ static int refdb_fs_backend__rename(
 		(error = refdb_fs_backend__lookup(&old, _backend, old_name)) < 0)
 		return error;
 
-	if ((error = refdb_fs_backend__delete(_backend, old_name)) < 0) {
+	if ((error = refdb_fs_backend__delete(_backend, old_name, NULL, NULL)) < 0) {
 		git_reference_free(old);
 		return error;
 	}
@@ -1037,7 +1113,7 @@ static int refdb_fs_backend__rename(
 		return -1;
 	}
 
-	if ((error = loose_lock(&file, backend, new)) < 0) {
+	if ((error = loose_lock(&file, backend, new->name)) < 0) {
 		git_reference_free(new);
 		return error;
 	}
diff --git a/src/refs.c b/src/refs.c
index ca5f24e..bdf2da3 100644
--- a/src/refs.c
+++ b/src/refs.c
@@ -116,7 +116,26 @@ void git_reference_free(git_reference *reference)
 
 int git_reference_delete(git_reference *ref)
 {
-	return git_refdb_delete(ref->db, ref->name);
+	const git_oid *old_id = NULL;
+	const char *old_target = NULL;
+
+	if (ref->type == GIT_REF_OID)
+		old_id = &ref->target.oid;
+	else
+		old_target = ref->target.symbolic;
+
+	return git_refdb_delete(ref->db, ref->name, old_id, old_target);
+}
+
+int git_reference_remove(git_repository *repo, const char *name)
+{
+	git_refdb *db;
+	int error;
+
+	if ((error = git_repository_refdb__weakptr(&db, repo)) < 0)
+		return error;
+
+	return git_refdb_delete(db, name, NULL, NULL);
 }
 
 int git_reference_lookup(git_reference **ref_out,
@@ -331,7 +350,9 @@ static int reference__create(
 	const char *symbolic,
 	int force,
 	const git_signature *signature,
-	const char *log_message)
+	const char *log_message,
+	const git_oid *old_id,
+	const char *old_target)
 {
 	char normalized[GIT_REFNAME_MAX];
 	git_refdb *refdb;
@@ -380,7 +401,7 @@ static int reference__create(
 
 	GITERR_CHECK_ALLOC(ref);
 
-	if ((error = git_refdb_write(refdb, ref, force, signature, log_message)) < 0) {
+	if ((error = git_refdb_write(refdb, ref, force, signature, log_message, old_id, old_target)) < 0) {
 		git_reference_free(ref);
 		return error;
 	}
@@ -406,19 +427,21 @@ static int log_signature(git_signature **out, git_repository *repo)
 	return 0;
 }
 
-int git_reference_create(
+int git_reference_create_matching(
 	git_reference **ref_out,
 	git_repository *repo,
 	const char *name,
-	const git_oid *oid,
+	const git_oid *id,
 	int force,
 	const git_signature *signature,
-	const char *log_message)
+	const char *log_message,
+	const git_oid *old_id)
+
 {
 	int error;
 	git_signature *who = NULL;
 	
-	assert(oid);
+	assert(id);
 
 	if (!signature) {
 		if ((error = log_signature(&who, repo)) < 0)
@@ -428,21 +451,34 @@ int git_reference_create(
 	}
 
 	error = reference__create(
-		ref_out, repo, name, oid, NULL, force, signature, log_message);
+		ref_out, repo, name, id, NULL, force, signature, log_message, old_id, NULL);
 
 	git_signature_free(who);
 	return error;
 }
 
-int git_reference_symbolic_create(
+int git_reference_create(
 	git_reference **ref_out,
 	git_repository *repo,
 	const char *name,
-	const char *target,
+	const git_oid *id,
 	int force,
 	const git_signature *signature,
 	const char *log_message)
 {
+        return git_reference_create_matching(ref_out, repo, name, id, force, signature, log_message, NULL);
+}
+
+int git_reference_symbolic_create_matching(
+	git_reference **ref_out,
+	git_repository *repo,
+	const char *name,
+	const char *target,
+	int force,
+	const git_signature *signature,
+	const char *log_message,
+	const char *old_target)
+{
 	int error;
 	git_signature *who = NULL;
 
@@ -456,12 +492,24 @@ int git_reference_symbolic_create(
 	}
 
 	error = reference__create(
-		ref_out, repo, name, NULL, target, force, signature, log_message);
+		ref_out, repo, name, NULL, target, force, signature, log_message, NULL, old_target);
 
 	git_signature_free(who);
 	return error;
 }
 
+int git_reference_symbolic_create(
+	git_reference **ref_out,
+	git_repository *repo,
+	const char *name,
+	const char *target,
+	int force,
+	const git_signature *signature,
+	const char *log_message)
+{
+	return git_reference_symbolic_create_matching(ref_out, repo, name, target, force, signature, log_message, NULL);
+}
+
 static int ensure_is_an_updatable_direct_reference(git_reference *ref)
 {
 	if (ref->type == GIT_REF_OID)
@@ -479,14 +527,16 @@ int git_reference_set_target(
 	const char *log_message)
 {
 	int error;
+	git_repository *repo;
 
 	assert(out && ref && id);
 
+	repo = ref->db->repo;
+
 	if ((error = ensure_is_an_updatable_direct_reference(ref)) < 0)
 		return error;
 
-	return git_reference_create(
-		out, ref->db->repo, ref->name, id, 1, signature, log_message);
+	return git_reference_create_matching(out, repo, ref->name, id, 1, signature, log_message, &ref->target.oid);
 }
 
 static int ensure_is_an_updatable_symbolic_reference(git_reference *ref)
@@ -512,8 +562,8 @@ int git_reference_symbolic_set_target(
 	if ((error = ensure_is_an_updatable_symbolic_reference(ref)) < 0)
 		return error;
 
-	return git_reference_symbolic_create(
-		out, ref->db->repo, ref->name, target, 1, signature, log_message);
+	return git_reference_symbolic_create_matching(
+		out, ref->db->repo, ref->name, target, 1, signature, log_message, ref->target.symbolic);
 }
 
 static int reference__rename(git_reference **out, git_reference *ref, const char *new_name, int force,
diff --git a/tests/refs/delete.c b/tests/refs/delete.c
index 5e4afb1..9d1c3fd 100644
--- a/tests/refs/delete.c
+++ b/tests/refs/delete.c
@@ -91,3 +91,17 @@ void test_refs_delete__packed_only(void)
 	git_reference_free(ref);
 	git_refdb_free(refdb);
 }
+
+void test_refs_delete__remove(void)
+{
+	git_reference *ref;
+
+	/* Check that passing no old values lets us delete */
+
+	cl_git_pass(git_reference_lookup(&ref, g_repo, packed_test_head_name));
+	git_reference_free(ref);
+
+	cl_git_pass(git_reference_remove(g_repo, packed_test_head_name));
+
+	cl_git_fail(git_reference_lookup(&ref, g_repo, packed_test_head_name));
+}
diff --git a/tests/refs/races.c b/tests/refs/races.c
new file mode 100644
index 0000000..02d57ef
--- /dev/null
+++ b/tests/refs/races.c
@@ -0,0 +1,152 @@
+#include "clar_libgit2.h"
+
+#include "repository.h"
+#include "git2/reflog.h"
+#include "reflog.h"
+#include "ref_helpers.h"
+
+static const char *commit_id = "099fabac3a9ea935598528c27f866e34089c2eff";
+static const char *refname = "refs/heads/master";
+static const char *other_refname = "refs/heads/foo";
+static const char *other_commit_id = "a65fedf39aefe402d3bb6e24df4d4f5fe4547750";
+
+static git_repository *g_repo;
+
+void test_refs_races__initialize(void)
+{
+   g_repo = cl_git_sandbox_init("testrepo");
+}
+
+void test_refs_races__cleanup(void)
+{
+   cl_git_sandbox_cleanup();
+}
+
+void test_refs_races__create_matching(void)
+{
+	git_reference *ref, *ref2, *ref3;
+	git_oid id, other_id;
+
+	git_oid_fromstr(&id, commit_id);
+	git_oid_fromstr(&other_id, other_commit_id);
+
+	cl_git_fail_with(GIT_EMODIFIED, git_reference_create_matching(&ref, g_repo, refname, &other_id, 1, NULL, NULL, &other_id));
+
+	cl_git_pass(git_reference_lookup(&ref, g_repo, refname));
+	cl_git_pass(git_reference_create_matching(&ref2, g_repo, refname, &other_id, 1, NULL, NULL, &id));
+	cl_git_fail_with(GIT_EMODIFIED, git_reference_set_target(&ref3, ref, &other_id, NULL, NULL));
+
+	git_reference_free(ref);
+	git_reference_free(ref2);
+	git_reference_free(ref3);
+}
+
+void test_refs_races__symbolic_create_matching(void)
+{
+	git_reference *ref, *ref2, *ref3;
+	git_oid id, other_id;
+
+	git_oid_fromstr(&id, commit_id);
+	git_oid_fromstr(&other_id, other_commit_id);
+
+	cl_git_fail_with(GIT_EMODIFIED, git_reference_symbolic_create_matching(&ref, g_repo, "HEAD", other_refname, 1, NULL, NULL, other_refname));
+
+	cl_git_pass(git_reference_lookup(&ref, g_repo, "HEAD"));
+	cl_git_pass(git_reference_symbolic_create_matching(&ref2, g_repo, "HEAD", other_refname, 1, NULL, NULL, refname));
+	cl_git_fail_with(GIT_EMODIFIED, git_reference_symbolic_set_target(&ref3, ref, other_refname, NULL, NULL));
+
+	git_reference_free(ref);
+	git_reference_free(ref2);
+	git_reference_free(ref3);
+}
+
+void test_refs_races__delete(void)
+{
+	git_reference *ref, *ref2;
+	git_oid id, other_id;
+
+	git_oid_fromstr(&id, commit_id);
+	git_oid_fromstr(&other_id, other_commit_id);
+
+	/* We can delete a value that matches */
+	cl_git_pass(git_reference_lookup(&ref, g_repo, refname));
+	cl_git_pass(git_reference_delete(ref));
+	git_reference_free(ref);
+
+	/* We cannot delete a symbolic value that doesn't match */
+	cl_git_pass(git_reference_lookup(&ref, g_repo, "HEAD"));
+	cl_git_pass(git_reference_symbolic_create_matching(&ref2, g_repo, "HEAD", other_refname, 1, NULL, NULL, refname));
+	cl_git_fail_with(GIT_EMODIFIED, git_reference_delete(ref));
+
+	git_reference_free(ref);
+	git_reference_free(ref2);
+
+	cl_git_pass(git_reference_create(&ref, g_repo, refname, &id, 1, NULL, NULL));
+	git_reference_free(ref);
+
+	/* We cannot delete an oid value that doesn't match */
+	cl_git_pass(git_reference_lookup(&ref, g_repo, refname));
+	cl_git_pass(git_reference_create_matching(&ref2, g_repo, refname, &other_id, 1, NULL, NULL, &id));
+	cl_git_fail_with(GIT_EMODIFIED, git_reference_delete(ref));
+
+	git_reference_free(ref);
+	git_reference_free(ref2);
+}
+
+void test_refs_races__switch_oid_to_symbolic(void)
+{
+	git_reference *ref, *ref2, *ref3;
+	git_oid id, other_id;
+
+	git_oid_fromstr(&id, commit_id);
+	git_oid_fromstr(&other_id, other_commit_id);
+
+	/* Removing a direct ref when it's currently symbolic should fail */
+	cl_git_pass(git_reference_lookup(&ref, g_repo, refname));
+	cl_git_pass(git_reference_symbolic_create(&ref2, g_repo, refname, other_refname, 1, NULL, NULL));
+	cl_git_fail_with(GIT_EMODIFIED, git_reference_delete(ref));
+
+	git_reference_free(ref);
+	git_reference_free(ref2);
+
+	cl_git_pass(git_reference_create(&ref, g_repo, refname, &id, 1, NULL, NULL));
+	git_reference_free(ref);
+
+	/* Updating a direct ref when it's currently symbolic should fail */
+	cl_git_pass(git_reference_lookup(&ref, g_repo, refname));
+	cl_git_pass(git_reference_symbolic_create(&ref2, g_repo, refname, other_refname, 1, NULL, NULL));
+	cl_git_fail_with(GIT_EMODIFIED, git_reference_set_target(&ref3, ref, &other_id, NULL, NULL));
+
+	git_reference_free(ref);
+	git_reference_free(ref2);
+	git_reference_free(ref3);
+}
+
+void test_refs_races__switch_symbolic_to_oid(void)
+{
+	git_reference *ref, *ref2, *ref3;
+	git_oid id, other_id;
+
+	git_oid_fromstr(&id, commit_id);
+	git_oid_fromstr(&other_id, other_commit_id);
+
+	/* Removing a symbolic ref when it's currently direct should fail */
+	cl_git_pass(git_reference_lookup(&ref, g_repo, "HEAD"));
+	cl_git_pass(git_reference_create(&ref2, g_repo, "HEAD", &id, 1, NULL, NULL));
+	cl_git_fail_with(GIT_EMODIFIED, git_reference_delete(ref));
+
+	git_reference_free(ref);
+	git_reference_free(ref2);
+
+	cl_git_pass(git_reference_symbolic_create(&ref, g_repo, "HEAD", refname, 1, NULL, NULL));
+	git_reference_free(ref);
+
+	/* Updating a symbolic ref when it's currently direct should fail */
+	cl_git_pass(git_reference_lookup(&ref, g_repo, "HEAD"));
+	cl_git_pass(git_reference_create(&ref2, g_repo, "HEAD", &id, 1, NULL, NULL));
+	cl_git_fail_with(GIT_EMODIFIED, git_reference_symbolic_set_target(&ref3, ref, other_refname, NULL, NULL));
+
+	git_reference_free(ref);
+	git_reference_free(ref2);
+	git_reference_free(ref3);
+}