ODB writing fails gracefully when unsupported

If no ODB backends support writing, we should fail gracefully.

src/odb.c

@@ -862,7 +862,7 @@ int git_odb_open_wstream(
 {
 	size_t i, writes = 0;
 	int error = GIT_ERROR;
-	git_hash_ctx *ctx;
+	git_hash_ctx *ctx = NULL;
 
 	assert(stream && db);
 
@@ -883,22 +883,28 @@ int git_odb_open_wstream(
 		}
 	}
 
+	if (error < 0) {
 		if (error == GIT_PASSTHROUGH)
 			error = 0;
-	if (error < 0 && !writes)
+		else if (!writes)
 			error = git_odb__error_unsupported_in_backend("write object");
 
+		goto done;
+	}
+
 	ctx = git__malloc(sizeof(git_hash_ctx));
 	GITERR_CHECK_ALLOC(ctx);
 
+	if ((error = git_hash_ctx_init(ctx)) < 0)
+		goto done;
 
-	git_hash_ctx_init(ctx);
 	hash_header(ctx, size, type);
 	(*stream)->hash_ctx = ctx;
 
 	(*stream)->declared_size = size;
 	(*stream)->received_bytes = 0;
 
+done:
 	return error;
 }
 

tests/odb/backend/nobackend.c

+#include "clar_libgit2.h"
+#include "repository.h"
+#include "git2/sys/repository.h"
+
+static git_repository *_repo;
+
+void test_odb_backend_nobackend__initialize(void)
+{
+	git_config *config;
+	git_odb *odb;
+	git_refdb *refdb;
+
+	cl_git_pass(git_repository_new(&_repo));
+	cl_git_pass(git_config_new(&config));
+	cl_git_pass(git_odb_new(&odb));
+	cl_git_pass(git_refdb_new(&refdb, _repo));
+
+	git_repository_set_config(_repo, config);
+	git_repository_set_odb(_repo, odb);
+	git_repository_set_refdb(_repo, refdb);
+}
+
+void test_odb_backend_nobackend__cleanup(void)
+{
+	git_repository_free(_repo);
+}
+
+void test_odb_backend_nobackend__write_fails_gracefully(void)
+{
+	git_oid id;
+	git_odb *odb;
+	const git_error *err;
+
+	git_repository_odb(&odb, _repo);
+	cl_git_fail(git_odb_write(&id, odb, "Hello world!\n", 13, GIT_OBJ_BLOB));
+
+	err = giterr_last();
+	cl_assert_equal_s(err->message, "Cannot write object - unsupported in the loaded odb backends");
+
+	git_odb_free(odb);
+}