str: introduce `git_str` for internal, `git_buf` is externalethomson/gitstr

libgit2 has two distinct requirements that were previously solved by
`git_buf`.  We require:

1. A general purpose string class that provides a number of utility APIs
   for manipulating data (eg, concatenating, truncating, etc).
2. A structure that we can use to return strings to callers that they
   can take ownership of.

By using a single class (`git_buf`) for both of these purposes, we have
confused the API to the point that refactorings are difficult and
reasoning about correctness is also difficult.

Move the utility class `git_buf` to be called `git_str`: this represents
its general purpose, as an internal string buffer class.  The name also
is an homage to Junio Hamano ("gitstr").

The public API remains `git_buf`, and has a much smaller footprint.  It
is generally only used as an "out" param with strict requirements that
follow the documentation.  (Exceptions exist for some legacy APIs to
avoid breaking callers unnecessarily.)

Utility functions exist to convert a user-specified `git_buf` to a
`git_str` so that we can call internal functions, then converting it
back again.

1 files changed, 50 insertions, 0 deletions

diff --git a/src/buf.h b/src/buf.h
new file mode 100644
index 000000000..4bc7f2709
--- /dev/null
+++ b/src/buf.h
@@ -0,0 +1,50 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_buf_h__
+#define INCLUDE_buf_h__
+
+#include "git2/buffer.h"
+#include "common.h"
+
+/*
+ * Adapts a private API that takes a `git_str` into a public API that
+ * takes a `git_buf`.
+ */
+
+#define GIT_BUF_WRAP_PRIVATE(buf, fn, ...) \
+  { \
+	git_str str = GIT_STR_INIT; \
+	int error; \
+	if ((error = git_buf_tostr(&str, buf)) == 0 && \
+	    (error = fn(&str, __VA_ARGS__)) == 0) \
+		error = git_buf_fromstr(buf, &str); \
+	git_str_dispose(&str); \
+	return error; \
+}
+
+/**
+ * "Sanitizes" a buffer from user input.  This simply ensures that the
+ * `git_buf` has nice defaults if the user didn't set the members to
+ * anything, so that if we return early we don't leave it populated
+ * with nonsense.
+ */
+extern int git_buf_sanitize(git_buf *from_user);
+
+/**
+ * Populate a `git_str` from a `git_buf` for passing to libgit2 internal
+ * functions.  Sanitizes the given `git_buf` before proceeding.  The
+ * `git_buf` will no longer point to this memory.
+ */
+extern int git_buf_tostr(git_str *out, git_buf *buf);
+
+/**
+ * Populate a `git_buf` from a `git_str` for returning to a user.
+ * The `git_str` will no longer point to this memory.
+ */
+extern int git_buf_fromstr(git_buf *out, git_str *str);
+
+#endif