[PATCH v2] merge-ort: fix calling merge_finalize() with no intermediate merge

From: Elijah Newren <newren@gmail.com>

If some code sets up the data structures for a merge, but then never actually performs one before calling merge_finalize(), then merge_finalize() wouldn't notice that result->priv was NULL and return early, resulting in following that NULL pointer and getting a segfault. There is currently no code in the git codebase that does this, but this issue was found during testing of some proposed patches that had the following structure:

    struct merge_options merge_opt;
    struct merge_result result;

    init_merge_options(&merge_opt, the_repository);
    memset(&result, 0, sizeof(result));

    <do N merges, for some value of N>

    merge_finalize(&merge_opt, &result);

where some flags could cause the code to have N=0, i.e. doing no merges. Add a check for result->priv being NULL and return early to avoid a segfault in these kinds of cases.

While at it, ensure the FREE_AND_NULL() in the function does something useful with the nulling aspect, namely sets result->priv to NULL rather than a mere temporary.

Reported-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
---
    merge-ort: fix calling merge_finalize() with no intermediate merge
    
    Changes since v1:
    
     * Moved code into an if-block instead of returning early, as suggested
       by Stolee.
    
    See
    https://lore.kernel.org/git/CABPp-BHCdjOutYqdMO1NbYKNA0BgkXRgwUEKK=MX0kXM-5G_DQ@mail.gmail.com/

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1518%2Fnewren%2Ffix-merge-finalize-with-no-merge-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1518/newren/fix-merge-finalize-with-no-merge-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1518

Range-diff vs v1:

 1:  e636027f60c ! 1:  2cb1c63f05b merge-ort: fix calling merge_finalize() with no intermediate merge
     @@ merge-ort.c: void merge_switch_to_result(struct merge_options *opt,
       
      -	clear_or_reinit_internal_opts(opti, 0);
      -	FREE_AND_NULL(opti);
     -+	if (!result->priv)
     -+		return;
     -+	clear_or_reinit_internal_opts(result->priv, 0);
     -+	FREE_AND_NULL(result->priv);
     ++	if (result->priv) {
     ++		clear_or_reinit_internal_opts(result->priv, 0);
     ++		FREE_AND_NULL(result->priv);
     ++	}
       }
       
       /*** Function Grouping: helper functions for merge_incore_*() ***/

 merge-ort.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)