[PATCH v2] for-each-repo: do nothing on empty config

From: Derrick Stolee <dstolee@microsoft.com>

'git for-each-repo --config=X' should return success without calling any subcommands when the config key 'X' has no value. The current implementation instead segfaults.

A user could run into this issue if they used 'git maintenance start' to initialize their cron schedule using 'git for-each-repo --config=maintenance.repo ...' but then using 'git maintenance unregister' to remove the config option. (Note: 'git maintenance stop' would remove the config _and_ remove the cron schedule.)

Add a simple test to ensure this works.

Reported-by: Andreas Bühmann <dev@uuml.de>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
    for-each-repo: do nothing on empty config
    
    Thanks, Andreas, for drawing my attention to this bug.
    
    [1] https://github.com/gitgitgadget/git/issues/833

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-834%2Fderrickstolee%2Ffor-each-repo-empty-config-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-834/derrickstolee/for-each-repo-empty-config-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/834

Range-diff vs v1:

 1:  36dccbb8c20 ! 1:  a1f1300bacb for-each-repo: do nothing on empty config
     @@ Commit message
          Add a simple test to ensure this works.
      
          Reported-by: Andreas Bühmann <dev@uuml.de>
     +    Helped-by: Eric Sunshine <sunshine@sunshineco.com>
          Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
      
       ## builtin/for-each-repo.c ##
     @@ builtin/for-each-repo.c: int cmd_for_each_repo(int argc, const char **argv, cons
       	values = repo_config_get_value_multi(the_repository,
       					     config_key);
       
     ++	/*
     ++	 * Do nothing on an empty list, which is equivalent to the case
     ++	 * where the config variable does not exist at all.
     ++	 */
      +	if (!values)
     -+		return result;
     ++		return 0;
      +
       	for (i = 0; !result && i < values->nr; i++)
       		result = run_command_on_repo(values->items[i].string, &args);
     @@ t/t0068-for-each-repo.sh: test_expect_success 'run based on configured value' '
       '
       
      +test_expect_success 'do nothing on empty config' '
     -+	git for-each-repo --config=bogus.config -- these args would fail
     ++	# will fail if any subcommand is run
     ++	git for-each-repo --config=bogus.config -- false
      +'
      +
       test_done

 builtin/for-each-repo.c  | 7 +++++++
 t/t0068-for-each-repo.sh | 5 +++++
 2 files changed, 12 insertions(+)