Currently if we have a config file like,
[foo]
        baz
        bar =

and we try something like, "git config --add foo.baz roll", Git will
segfault. Moreover, for "git config --add foo.bar roll", it will
overwrite the original value instead of appending after the existing
empty value.

The problem lies with the regexp used for simulating --add in
`git_config_set_multivar_in_file()`, "^$", which in ideal case should
not match with any string but is true for empty strings. Instead use a
regexp like "a^" which can not be true for any string, empty or not.

For removing the segfault add a check for NULL values in `matches()` in
config.c.

Signed-off-by: Tanay Abhra <tanay...@gmail.com>
---
 builtin/config.c        |  2 +-
 config.c                |  2 +-
 t/t1303-wacky-config.sh | 20 ++++++++++++++++++++
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/builtin/config.c b/builtin/config.c
index fcd8474..b9e7dce 100644
--- a/builtin/config.c
+++ b/builtin/config.c
@@ -586,7 +586,7 @@ int cmd_config(int argc, const char **argv, const char 
*prefix)
                check_argc(argc, 2, 2);
                value = normalize_value(argv[0], argv[1]);
                return git_config_set_multivar_in_file(given_config_source.file,
-                                                      argv[0], value, "^$", 0);
+                                                      argv[0], value, "a^", 0);
        }
        else if (actions == ACTION_REPLACE_ALL) {
                check_write();
diff --git a/config.c b/config.c
index 058505c..67a7729 100644
--- a/config.c
+++ b/config.c
@@ -1231,7 +1231,7 @@ static int matches(const char *key, const char *value)
        return !strcmp(key, store.key) &&
                (store.value_regex == NULL ||
                 (store.do_not_match ^
-                 !regexec(store.value_regex, value, 0, NULL, 0)));
+                 (value && !regexec(store.value_regex, value, 0, NULL, 0))));
 }
 
 static int store_aux(const char *key, const char *value, void *cb)
diff --git a/t/t1303-wacky-config.sh b/t/t1303-wacky-config.sh
index 3a2c819..3b92083 100755
--- a/t/t1303-wacky-config.sh
+++ b/t/t1303-wacky-config.sh
@@ -111,4 +111,24 @@ test_expect_success 'unset many entries' '
        test_must_fail git config section.key
 '
 
+test_expect_success '--add appends new value after existing empty value' '
+       cat >expect <<-\EOF &&
+
+
+       fool
+       roll
+       EOF
+       cp .git/config .git/config.old &&
+       test_when_finished "mv .git/config.old .git/config" &&
+       cat >.git/config <<-\EOF &&
+       [foo]
+               baz
+               baz =
+               baz = fool
+       EOF
+       git config --add foo.baz roll &&
+       git config --get-all foo.baz >output &&
+       test_cmp expect output
+'
+
 test_done
-- 
1.9.0.GIT

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