[PATCH 2/6] config.c: don't assign to "cf" directly

["Glen Choo via GitGitGadget" <gitgitgadget@gmail.com>]

From: Glen Choo <chooglen@google.com>

To make "cf" easier to remove, replace all direct assignments to it with
function calls. This refactor has an additional maintainability benefit:
all of these functions were manually implementing stack pop/push
semantics on "struct config_source", so replacing them with function
calls allows us to only implement this logic once.

In this process, perform some now-obvious clean ups:

- Drop some unnecessary "cf" assignments in populate_remote_urls().
  Since it was introduced in 399b198489 (config: include file if remote
  URL matches a glob, 2022-01-18), it has stored and restored the value
  of "cf" to ensure that it doesn't get accidentally mutated. However,
  this was never necessary since "do_config_from()" already pushes/pops
  "cf" further down the call chain.

- Zero out every "struct config_source" with a dedicated initializer.
  This matters because the "struct config_source" is assigned to "cf"
  and we later 'pop the stack' by assigning "cf = cf->prev", but
  "cf->prev" could be pointing to uninitialized garbage.

  Fortunately, this has never bothered us since we never try to read
  "cf" except while iterating through config, in which case, "cf" is
  either set to a sensible value (when parsing a file), or it is ignored
  (when iterating a configset). Later in the series, zero-ing out memory
  will also let us enforce the constraint that "cf" and
  "current_config_kvi" are never non-NULL together.

Signed-off-by: Glen Choo <chooglen@google.com>
---
 config.c | 40 ++++++++++++++++++++++++++--------------
 1 file changed, 26 insertions(+), 14 deletions(-)

diff --git a/config.c b/config.c
index 84ae97741ac..1f89addc771 100644
--- a/config.c
+++ b/config.c
@@ -49,6 +49,7 @@ struct config_source {
 	int (*do_ungetc)(int c, struct config_source *conf);
 	long (*do_ftell)(struct config_source *c);
 };
+#define CONFIG_SOURCE_INIT { 0 }
 
 /*
  * These variables record the "current" config source, which
@@ -78,6 +79,23 @@ static struct key_value_info *current_config_kvi;
  */
 static enum config_scope current_parsing_scope;
 
+static inline void config_state_push_source(struct config_source *top)
+{
+	if (cf)
+		top->prev = cf;
+	cf = top;
+}
+
+static inline struct config_source *config_state_pop_source()
+{
+	struct config_source *ret;
+	if (!cf)
+		BUG("tried to pop config source, but we weren't reading config");
+	ret = cf;
+	cf = cf->prev;
+	return ret;
+}
+
 static int pack_compression_seen;
 static int zlib_compression_seen;
 
@@ -345,14 +363,12 @@ static void populate_remote_urls(struct config_include_data *inc)
 {
 	struct config_options opts;
 
-	struct config_source *store_cf = cf;
 	struct key_value_info *store_kvi = current_config_kvi;
 	enum config_scope store_scope = current_parsing_scope;
 
 	opts = *inc->opts;
 	opts.unconditional_remote_url = 1;
 
-	cf = NULL;
 	current_config_kvi = NULL;
 	current_parsing_scope = 0;
 
@@ -360,7 +376,6 @@ static void populate_remote_urls(struct config_include_data *inc)
 	string_list_init_dup(inc->remote_urls);
 	config_with_options(add_remote_url, inc->remote_urls, inc->config_source, &opts);
 
-	cf = store_cf;
 	current_config_kvi = store_kvi;
 	current_parsing_scope = store_scope;
 }
@@ -714,12 +729,10 @@ int git_config_from_parameters(config_fn_t fn, void *data)
 	struct strvec to_free = STRVEC_INIT;
 	int ret = 0;
 	char *envw = NULL;
-	struct config_source source;
+	struct config_source source = CONFIG_SOURCE_INIT;
 
-	memset(&source, 0, sizeof(source));
-	source.prev = cf;
 	source.origin_type = CONFIG_ORIGIN_CMDLINE;
-	cf = &source;
+	config_state_push_source(&source);
 
 	env = getenv(CONFIG_COUNT_ENVIRONMENT);
 	if (env) {
@@ -777,7 +790,7 @@ out:
 	strbuf_release(&envvar);
 	strvec_clear(&to_free);
 	free(envw);
-	cf = source.prev;
+	config_state_pop_source();
 	return ret;
 }
 
@@ -1948,20 +1961,19 @@ static int do_config_from(struct config_source *top, config_fn_t fn, void *data,
 	int ret;
 
 	/* push config-file parsing state stack */
-	top->prev = cf;
 	top->linenr = 1;
 	top->eof = 0;
 	top->total_len = 0;
 	strbuf_init(&top->value, 1024);
 	strbuf_init(&top->var, 1024);
-	cf = top;
+	config_state_push_source(top);
 
-	ret = git_parse_source(cf, fn, data, opts);
+	ret = git_parse_source(top, fn, data, opts);
 
 	/* pop config-file parsing state stack */
 	strbuf_release(&top->value);
 	strbuf_release(&top->var);
-	cf = top->prev;
+	config_state_pop_source();
 
 	return ret;
 }
@@ -1971,7 +1983,7 @@ static int do_config_from_file(config_fn_t fn,
 		const char *name, const char *path, FILE *f,
 		void *data, const struct config_options *opts)
 {
-	struct config_source top;
+	struct config_source top = CONFIG_SOURCE_INIT;
 	int ret;
 
 	top.u.file = f;
@@ -2023,7 +2035,7 @@ int git_config_from_mem(config_fn_t fn,
 			const char *name, const char *buf, size_t len,
 			void *data, const struct config_options *opts)
 {
-	struct config_source top;
+	struct config_source top = CONFIG_SOURCE_INIT;
 
 	top.u.buf.buf = buf;
 	top.u.buf.len = len;
-- 
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