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
sentinel value CONFIG_REGEX_NONE to say "we do not want to replace any
existing entry" and use it in the implementation of "git config --add".

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

Helped-by: Junio C Hamano <gits...@pobox.com>
Signed-off-by: Tanay Abhra <tanay...@gmail.com>
---
 builtin/config.c        |  2 +-
 cache.h                 |  2 ++
 config.c                | 21 +++++++++++++++++----
 t/t1303-wacky-config.sh |  2 +-
 4 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/builtin/config.c b/builtin/config.c
index aba7135..195664b 100644
--- a/builtin/config.c
+++ b/builtin/config.c
@@ -611,7 +611,7 @@ 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, 
CONFIG_REGEX_NONE, 0);
        }
        else if (actions == ACTION_REPLACE_ALL) {
                check_write();
diff --git a/cache.h b/cache.h
index dfa1a56..a09217d 100644
--- a/cache.h
+++ b/cache.h
@@ -1284,6 +1284,8 @@ extern int update_server_info(int);
 #define CONFIG_INVALID_PATTERN 6
 #define CONFIG_GENERIC_ERROR 7

+extern const char CONFIG_REGEX_NONE[];
+
 struct git_config_source {
        unsigned int use_stdin:1;
        const char *file;
diff --git a/config.c b/config.c
index 83c913a..20476e0 100644
--- a/config.c
+++ b/config.c
@@ -46,6 +46,8 @@ static int zlib_compression_seen;
  */
 static struct config_set the_config_set;

+const char CONFIG_REGEX_NONE[] = "a^";
+
 static int config_file_fgetc(struct config_source *conf)
 {
        return fgetc(conf->u.file);
@@ -1607,14 +1609,20 @@ static struct {
        unsigned int offset_alloc;
        enum { START, SECTION_SEEN, SECTION_END_SEEN, KEY_SEEN } state;
        int seen;
+       unsigned value_never_matches:1;
 } store;

 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)));
+       if (strcmp(key, store.key))
+               return 0; /* not ours */
+       if (!store.value_regex)
+               return 1; /* always matches */
+       if (store.value_never_matches)
+               return 0; /* never matches */
+
+       return store.do_not_match ^
+       (value && !regexec(store.value_regex, value, 0, NULL, 0));
 }

 static int store_aux(const char *key, const char *value, void *cb)
@@ -1876,6 +1884,8 @@ out_free_ret_1:
 /*
  * If value==NULL, unset in (remove from) config,
  * if value_regex!=NULL, disregard key/value pairs where value does not match.
+ * if value_regex==CONFIG_REGEX_NONE, do not match any existing values
+ * (only add a new one)
  * if multi_replace==0, nothing, or only one matching key/value is replaced,
  *     else all matching key/values (regardless how many) are removed,
  *     before the new pair is written.
@@ -1966,6 +1976,9 @@ int git_config_set_multivar_in_file(const char 
*config_filename,
                        } else
                                store.do_not_match = 0;

+                       if (value_regex == CONFIG_REGEX_NONE)
+                               store.value_never_matches = 1;
+
                        store.value_regex = (regex_t*)xmalloc(sizeof(regex_t));
                        if (regcomp(store.value_regex, value_regex,
                                        REG_EXTENDED)) {
diff --git a/t/t1303-wacky-config.sh b/t/t1303-wacky-config.sh
index e5c0f07..3b92083 100755
--- a/t/t1303-wacky-config.sh
+++ b/t/t1303-wacky-config.sh
@@ -111,7 +111,7 @@ test_expect_success 'unset many entries' '
        test_must_fail git config section.key
 '

-test_expect_failure '--add appends new value after existing empty value' '
+test_expect_success '--add appends new value after existing empty value' '
        cat >expect <<-\EOF &&


-- 
1.9.0.GIT
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