[PATCH] builtin/commit.c: prevent bad commits

[Dan Aloni <alonid@gmail.com>]

These commits which have hashes starting with the hex string 'bad',
always give me the chills. Why should a perfectly good commit be
jinxed?

Statistically, one of 4096 commits may be 'bad'. This change adds a
'--prevent-bad' switch to the commit command in order to prevent such
commit hashes from being generated. Internally, the commit is retried
with a slight commit meta-data modification - a newline is added to the
end of the commit message. The meta-data change results in a different
hash, that if we are lucky enough (4095/4096 chance) may not be 'bad'.

Note that this change does not affect actual software quality maintained
using Git. Thus, it is recommended keep testing all generated versions
regardless of commit hash jinxes.

Signed-off-by: Dan Aloni <alonid@gmail.com>
---
 builtin/commit.c | 34 +++++++++++++++++++++++++++++-----
 1 file changed, 29 insertions(+), 5 deletions(-)

diff --git a/builtin/commit.c b/builtin/commit.c
index 37fcb55ab0a0..afaa7cefaedf 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -89,6 +89,7 @@ static int edit_flag = -1; /* unspecified */
 static int quiet, verbose, no_verify, allow_empty, dry_run, renew_authorship;
 static int config_commit_verbose = -1; /* unspecified */
 static int no_post_rewrite, allow_empty_message;
+static int prevent_bad;
 static char *untracked_files_arg, *force_date, *ignore_submodule_arg, *ignored_arg;
 static char *sign_commit;
 
@@ -1449,6 +1450,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		OPT_BOOL('z', "null", &s.null_termination,
 			 N_("terminate entries with NUL")),
 		OPT_BOOL(0, "amend", &amend, N_("amend previous commit")),
+		OPT_BOOL(0, "prevent-bad", &prevent_bad, N_("prevent a bad commit")),
 		OPT_BOOL(0, "no-post-rewrite", &no_post_rewrite, N_("bypass post-rewrite hook")),
 		{ OPTION_STRING, 'u', "untracked-files", &untracked_files_arg, N_("mode"), N_("show untracked files, optional modes: all, normal, no. (Default: all)"), PARSE_OPT_OPTARG, NULL, (intptr_t)"all" },
 		/* end commit contents options */
@@ -1583,12 +1585,34 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		append_merge_tag_headers(parents, &tail);
 	}
 
-	if (commit_tree_extended(sb.buf, sb.len, &active_cache_tree->oid,
-				 parents, &oid, author_ident.buf, sign_commit,
-				 extra)) {
-		rollback_index_files();
-		die(_("failed to write commit object"));
+	for (;;) {
+		char *oid_hex;
+		struct commit_list *copy_parents;
+
+		copy_parents = copy_commit_list(parents);
+
+		if (commit_tree_extended(sb.buf, sb.len, &active_cache_tree->oid,
+					 parents, &oid, author_ident.buf, sign_commit,
+					 extra)) {
+			rollback_index_files();
+			die(_("failed to write commit object"));
+		}
+
+		oid_hex = oid_to_hex(&oid);
+		if (prevent_bad &&
+		    oid_hex[0] == 'b' &&
+		    oid_hex[1] == 'a' &&
+		    oid_hex[2] == 'd' )
+		{
+			parents = copy_parents;
+			strbuf_add(&sb, "\n", 1);
+			continue;
+		}
+
+		free_commit_list(copy_parents);
+		break;
 	}
+
 	strbuf_release(&author_ident);
 	free_commit_extra_headers(extra);
 
-- 
2.14.3