[RFC WIP PATCH 1/3] tag: factor out tag reading from write_tag_body()

[Lucas Oshiro]

Improve code readability by moving tag reading to a new function called
get_tag_body, which will be used in the implementation of the git tag
--amend functionality.

Add warning in write_tag_body() in case the tag body is not successfully
recovered.

Co-authored-by: Bárbara Fernandes <barbara....@gmail.com>
Signed-off-by: Bárbara Fernandes <barbara....@gmail.com>
Signed-off-by: Lucas Oshiro <lucasseikiosh...@gmail.com>
Helped-by: Matheus Tavares <matheus.bernard...@usp.br>
---
 builtin/tag.c | 42 ++++++++++++++++++++++++++++++++++--------
 1 file changed, 34 insertions(+), 8 deletions(-)

diff --git a/builtin/tag.c b/builtin/tag.c
index e0a4c25382..e1e3549af9 100644
--- a/builtin/tag.c
+++ b/builtin/tag.c
@@ -170,26 +170,52 @@ static int git_tag_config(const char *var, const char 
*value, void *cb)
        return git_color_default_config(var, value, cb);
 }
 
-static void write_tag_body(int fd, const struct object_id *oid)
+/* 
+ * Returns the tag body of the given oid or NULL, in case of error. If size is
+ * not NULL it is assigned the body size in bytes (excluding the '\0').
+ */
+static char *get_tag_body(const struct object_id *oid, size_t *size) 
 {
-       unsigned long size;
+       unsigned long buf_size;
        enum object_type type;
-       char *buf, *sp;
+       char *buf, *sp, *tag_body;
+       size_t tag_body_size, signature_offset;
 
-       buf = read_object_file(oid, &type, &size);
+       buf = read_object_file(oid, &type, &buf_size);
        if (!buf)
-               return;
+               return NULL;
        /* skip header */
        sp = strstr(buf, "\n\n");
 
-       if (!sp || !size || type != OBJ_TAG) {
+       if (!sp || !buf_size || type != OBJ_TAG) {
                free(buf);
-               return;
+               return NULL;
        }
        sp += 2; /* skip the 2 LFs */
-       write_or_die(fd, sp, parse_signature(sp, buf + size - sp));
+       signature_offset = parse_signature(sp, buf + buf_size - sp);
+       sp[signature_offset] = '\0';
 
+       /* detach sp from buf */
+       tag_body_size = strlen(sp) + 1;
+       tag_body = xmalloc(tag_body_size);
+       xsnprintf(tag_body, tag_body_size, "%s", sp);
        free(buf);
+       if (size)
+               *size = tag_body_size - 1; /* exclude '\0' */
+       return tag_body;
+}
+
+static void write_tag_body(int fd, const struct object_id *oid)
+{
+       size_t size;
+       const char *tag_body = get_tag_body(oid, &size);
+
+       if (!tag_body) {
+               warning("failed to get tag body for %s", oid->hash);
+               return;
+       }
+       printf("tag_body: <%s>\n", tag_body);
+       write_or_die(fd, tag_body, size);
 }
 
 static int build_tag_object(struct strbuf *buf, int sign, struct object_id 
*result)
-- 
2.23.0