[PATCH 11/12] builtin/rebase: fix options.strategy memory lifecycle

[andrzej@ahunt.org]

From: Andrzej Hunt <ajrhunt@google.com>

This change:
- xstrdup()'s all string being used for replace_opts.strategy, to
  guarantee that replace_opts owns these strings. This is needed because
  sequencer_remove_state() will free replace_opts.strategy, and it's
  usually called as part of the usage of replace_opts.
- Removes xstrdup()'s being used to populate options.strategy in
  cmd_rebase(), which avoids leaking options.strategy, even in the
  case where strategy is never moved/copied into replace_opts.

These changes are needed because:
- We would always create a new string for options.strategy if we either
  get a strategy via options (OPT_STRING(...strategy...), or via
  GIT_TEST_MERGE_ALGORITHM.
- But only sometimes is this string copied into replace_opts - in which
  case it did get free()'d in sequencer_remove_state().
- The rest of the time, the newly allocated string would remain unused,
  causing a leak. But we can't just add a free because that can result
  in a double-free in those cases where replace_opts was populated.

An alternative approach would be to set options.strategy to NULL when
moving the pointer to replace_opts.strategy, combined with always
free()'ing options.strategy, but that seems like a more
complicated and wasteful approach.

This was first seen when running t0021 with LSAN, but t2012 helped catch
the fact that we can't just free(options.strategy) at the end of
cmd_rebase (as that can cause a double-free). LSAN output from t0021:

LSAN output from t0021:

Direct leak of 4 byte(s) in 1 object(s) allocated from:
    #0 0x486804 in strdup ../projects/compiler-rt/lib/asan/asan_interceptors.cpp:452:3
    #1 0xa71eb8 in xstrdup wrapper.c:29:14
    #2 0x61b1cc in cmd_rebase builtin/rebase.c:1779:22
    #3 0x4ce83e in run_builtin git.c:475:11
    #4 0x4ccafe in handle_builtin git.c:729:3
    #5 0x4cb01c in run_argv git.c:818:4
    #6 0x4cb01c in cmd_main git.c:949:19
    #7 0x6b3fad in main common-main.c:52:11
    #8 0x7f267b512349 in __libc_start_main (/lib64/libc.so.6+0x24349)

SUMMARY: AddressSanitizer: 4 byte(s) leaked in 1 allocation(s).

Signed-off-by: Andrzej Hunt <andrzej@ahunt.org>
---
 builtin/rebase.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 12f093121d..9d81db0f3a 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -139,7 +139,7 @@ static struct replay_opts get_replay_opts(const struct rebase_options *opts)
 	replay.ignore_date = opts->ignore_date;
 	replay.gpg_sign = xstrdup_or_null(opts->gpg_sign_opt);
 	if (opts->strategy)
-		replay.strategy = opts->strategy;
+		replay.strategy = xstrdup_or_null(opts->strategy);
 	else if (!replay.strategy && replay.default_strategy) {
 		replay.strategy = replay.default_strategy;
 		replay.default_strategy = NULL;
@@ -1723,7 +1723,6 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	}
 
 	if (options.strategy) {
-		options.strategy = xstrdup(options.strategy);
 		switch (options.type) {
 		case REBASE_APPLY:
 			die(_("--strategy requires --merge or --interactive"));
@@ -1776,7 +1775,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	if (options.type == REBASE_MERGE &&
 	    !options.strategy &&
 	    getenv("GIT_TEST_MERGE_ALGORITHM"))
-		options.strategy = xstrdup(getenv("GIT_TEST_MERGE_ALGORITHM"));
+		options.strategy = getenv("GIT_TEST_MERGE_ALGORITHM");
 
 	switch (options.type) {
 	case REBASE_MERGE:
-- 
2.26.2