[PATCH v2] config: fix several access(NULL) calls

[Matthieu Moy <Matthieu.Moy@imag.fr>]

When $HOME is unset, home_config_paths fails and returns NULL pointers
for user_config and xdg_config. Valgrind complains with Syscall param
access(pathname) points to unaddressable byte(s).

Don't call blindly access() on these variables, but test them for
NULL-ness before.

The when the XDG configuration file can be found but not $HOME/.gitconfig
requires a bit of attention. We chose to error out in "git config --set"
if $HOME is unset anyway.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
---

Before I forget about it, here's the patch assuming people do want to
error out when $HOME is unset. It should be functionally equivalent to
the previous one, but the code should be clearer.

 builtin/config.c | 15 +++++++++++----
 config.c         |  4 ++--
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/builtin/config.c b/builtin/config.c
index e8e1c0a..f064d6e 100644
--- a/builtin/config.c
+++ b/builtin/config.c
@@ -387,12 +387,19 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 
 		home_config_paths(&user_config, &xdg_config, "config");
 
-		if (access(user_config, R_OK) && !access(xdg_config, R_OK))
+		if (!user_config)
+			/*
+			 * Don't even try to access the xdg_config, as
+			 * unset $HOME means something is really
+			 * broken and should be fixed. Silently
+			 * writing to xdg_config may be confusing.
+			 */
+			die("$HOME not set");
+		else if (access(user_config, R_OK) &&
+			 xdg_config && !access(xdg_config, R_OK))
 			given_config_file = xdg_config;
-		else if (user_config)
-			given_config_file = user_config;
 		else
-			die("$HOME not set");
+			given_config_file = user_config;
 	}
 	else if (use_system_config)
 		given_config_file = git_etc_gitconfig();
diff --git a/config.c b/config.c
index d28a499..6b97503 100644
--- a/config.c
+++ b/config.c
@@ -940,12 +940,12 @@ int git_config_early(config_fn_t fn, void *data, const char *repo_config)
 		found += 1;
 	}
 
-	if (!access(xdg_config, R_OK)) {
+	if (xdg_config && !access(xdg_config, R_OK)) {
 		ret += git_config_from_file(fn, xdg_config, data);
 		found += 1;
 	}
 
-	if (!access(user_config, R_OK)) {
+	if (user_config && !access(user_config, R_OK)) {
 		ret += git_config_from_file(fn, user_config, data);
 		found += 1;
 	}
-- 
1.7.11.1.30.g7e1baf9.dirty