config.c: do sanity checks before xmemdupz()

Adjust the code added in 1ff21c0 (config: store "git -c" variables
using more robust format, 2021-01-12) to avoid allocating a "key"
until after we've done the sanity checks on it.

There reason for allocating it this in the function is because the
"env_name" pointer was about to be incremented, let's instead note our
position at that point. The key length is the number of characters
before the first "=".

As a result of this early allocation we'd have a memory leak as
reported by valgrind, just not the "still reachable" kind we usually
care about[1] with SANITIZE=leak:

	$ valgrind --leak-check=full --show-leak-kinds=all ./git --config-env=foo.flag= config --bool foo.flag
        [...]
	==6540== 13 bytes in 1 blocks are still reachable in loss record 3 of 17
	==6540==    at 0x48437B4: malloc (vg_replace_malloc.c:381)
	==6540==    by 0x40278B: do_xmalloc (wrapper.c:51)
	==6540==    by 0x402884: do_xmallocz (wrapper.c:85)
	==6540==    by 0x4028C0: xmallocz (wrapper.c:93)
	==6540==    by 0x4028FD: xmemdupz (wrapper.c:109)
	==6540==    by 0x402962: xstrndup (wrapper.c:115)
	==6540==    by 0x342F53: strip_path_suffix (path.c:1300)
	==6540==    by 0x2B4C89: system_prefix (exec-cmd.c:50)
	==6540==    by 0x2B5057: system_path (exec-cmd.c:268)
	==6540==    by 0x2C5E17: git_setup_gettext (gettext.c:109)
	==6540==    by 0x21DC57: main (common-main.c:44)

Since the "key" was reachable when the "die()" ran the semantics of
SANITIZE=leak wouldn't show it, but "clang" at higher optimization
levels would optimize this to the point of thinking the variable
wasn't reachable, e.g. with Debian clang 14.0.6-2 with CFLAGS="-O3
-g":

	$ ./git --config-env=foo.flag= config --bool foo.flag
	fatal: missing environment variable name for configuration 'foo.flag'

	=================================================================
	==18018==ERROR: LeakSanitizer: detected memory leaks

	Direct leak of 9 byte(s) in 1 object(s) allocated from:
	    #0 0x55e40aad7cd2 in __interceptor_malloc (/home/avar/g/git/git+0x78cd2) (BuildId: b89fa8f797ccb02ef1148beed300071a5f9b9ab1)
	    #1 0x55e40ad41071 in do_xmalloc /home/avar/g/git/wrapper.c:51:8
	    #2 0x55e40ad41071 in do_xmallocz /home/avar/g/git/wrapper.c:85:8
	    #3 0x55e40ad41071 in xmallocz /home/avar/g/git/wrapper.c:93:9
	    #4 0x55e40ad41071 in xmemdupz /home/avar/g/git/wrapper.c:109:16
	    #5 0x55e40abec960 in git_config_push_env /home/avar/g/git/config.c:521:8
	    git#6 0x55e40aadb8b9 in handle_options /home/avar/g/git/git.c:268:4
	    #7 0x55e40aada68d in cmd_main /home/avar/g/git/git.c:896:2
	    git#8 0x55e40abae219 in main /home/avar/g/git/common-main.c:57:11
	    git#9 0x7fbb5287e209 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
	    git#10 0x7fbb5287e2bb in __libc_start_main csu/../csu/libc-start.c:389:3
	    git#11 0x55e40aaac130 in _start (/home/avar/g/git/git+0x4d130) (BuildId: b89fa8f797ccb02ef1148beed300071a5f9b9ab1)

	SUMMARY: LeakSanitizer: 9 byte(s) leaked in 1 allocation(s).
	Aborted

Whatever clang has to say about it it makes sense to save ourselves
the xmemdupz() if we can help it, but it's worth noting why this came
up.

The actual meaningful fix here is that we don't need to do this
allocation at all. The only reason it's needed is because there hasn't
been a variant of "sq_quote_buf()" in quote.c that takes a "len"
parameter.

In previous commits we added one, and will move to using it in the
subsequent commit, but let's first make this smaller change.

1. https://lore.kernel.org/git/[email protected]/

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>

Author: avar

config.c

@@ -571,13 +571,14 @@ void git_config_push_parameter(const char *text)
 void git_config_push_env(const char *spec)
 {
 	char *key;
+	size_t len;
 	const char *env_name;
 	const char *env_value;
 
 	env_name = strrchr(spec, '=');
 	if (!env_name)
 		die(_("invalid config format: %s"), spec);
-	key = xmemdupz(spec, env_name - spec);
+	len = env_name - spec;
 	env_name++;
 	if (!*env_name)
 		die(_("missing environment variable name for configuration '%.*s'"),
@@ -588,6 +589,7 @@ void git_config_push_env(const char *spec)
 		die(_("missing environment variable '%s' for configuration '%.*s'"),
 		    env_name, (int)(env_name - spec - 1), spec);
 
+	key = xmemdupz(spec, len);
 	git_config_push_split_parameter(key, env_value);
 	free(key);
 }