patchread-cache: report lock error when refreshing index

> In the repo_refresh_and_write_index of read-cache.c, we return -1 to
> indicate that writing the index to disk failed.
> However, callers do not use this information. Commands such as stash print
>   "could not write index"
> and then exit, which does not help to discover the exact problem.

> Signed-off-by: Han Young <hanyang.tony@bytedance.com>
> ---
>  read-cache.c     |  2 +-
>  t/t3903-stash.sh | 15 +++------------
>  2 files changed, 4 insertions(+), 13 deletions(-)
> 
> diff --git a/read-cache.c b/read-cache.c
> index c0bb760ad..50e842bfa 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -1456,7 +1456,7 @@ int repo_refresh_and_write_index(struct repository *repo,
>  	struct lock_file lock_file = LOCK_INIT;
>  	int fd, ret = 0;
>  
> -	fd = repo_hold_locked_index(repo, &lock_file, 0);
> +	fd = repo_hold_locked_index(repo, &lock_file, gentle ? 0 : LOCK_REPORT_ON_ERROR);

>  	if (!gentle && fd < 0)
>  		return -1;
>  	if (refresh_index(repo->index, refresh_flags, pathspec, seen, header_msg))
> diff --git a/t/t3903-stash.sh b/t/t3903-stash.sh
> index 35b85c790..39098ade4 100755
> --- a/t/t3903-stash.sh
> +++ b/t/t3903-stash.sh
> @@ -1571,11 +1571,8 @@ test_expect_success 'stash create reports a locked index' '
>  		echo change >A.file &&
>  		touch .git/index.lock &&
>  
> -		cat >expect <<-EOF &&
> -		error: could not write index
> -		EOF
>  		test_must_fail git stash create 2>err &&
> -		test_cmp expect err
> +		test_grep "error: Unable to create '.*index.lock'" err

>  	)
>  '
>  
> @@ -1588,11 +1585,8 @@ test_expect_success 'stash push reports a locked index' '
>  		echo change >A.file &&
>  		touch .git/index.lock &&
>  
> -		cat >expect <<-EOF &&
> -		error: could not write index
> -		EOF
>  		test_must_fail git stash push 2>err &&
> -		test_cmp expect err
> +		test_grep "error: Unable to create '.*index.lock'" err

>  	)
>  '
>  
> @@ -1606,11 +1600,8 @@ test_expect_success 'stash apply reports a locked index' '
>  		git stash push &&
>  		touch .git/index.lock &&
>  
> -		cat >expect <<-EOF &&
> -		error: could not write index
> -		EOF
>  		test_must_fail git stash apply 2>err &&
> -		test_cmp expect err
> +		test_grep "error: Unable to create '.*index.lock'" err

>>  		test_must_fail git stash create 2>err &&
>> -		test_cmp expect err
>> +		test_grep "error: Unable to create '.*index.lock'" err
>
> The test now checks for the explicit lock error message. The check for
> the "error: could not write index" message is also removed even though
> it should still be present in the output. Should we also continue to
> grep for that message too?

> In the repo_refresh_and_write_index of read-cache.c, we return -1 to
> indicate that writing the index to disk failed.
> However, callers do not use this information. Commands such as stash print
>   "could not write index"
> and then exit, which does not help to discover the exact problem.
>
> We can let repo_hold_locked_index print the error message if the locking
> failed.
>
> Signed-off-by: Han Young <hanyang.tony@bytedance.com>
> ---
> Changes since v1:
> also check the "could not write index" error output
>
>  read-cache.c     |  2 +-
>  t/t3903-stash.sh | 18 ++++++------------
>  2 files changed, 7 insertions(+), 13 deletions(-)
>
> diff --git a/read-cache.c b/read-cache.c
> index c0bb760ad..50e842bfa 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -1456,7 +1456,7 @@ int repo_refresh_and_write_index(struct repository *repo,
>  	struct lock_file lock_file = LOCK_INIT;
>  	int fd, ret = 0;
>  
> -	fd = repo_hold_locked_index(repo, &lock_file, 0);
> +	fd = repo_hold_locked_index(repo, &lock_file, gentle ? 0 : LOCK_REPORT_ON_ERROR);