Refactor early config parsing

The Trace2 machinery wishes to parse the system and the global config
early. To this end, it taps into the `common-main` framework to run
first thing.

There are more Git features that could benefit from such a handling,
most notably the Windows-specific `core.longPaths` setting: If a user
has worktrees whose path already requires long paths support, we cannot
wait until we parse the the config settings in the usual way because
the gitdir discovery needs to happen first and would fail because any
`core.longPaths` setting in, say, `~/.gitconfig` would not have been
parsed yet.

To that end, let's refactor Trace2's early config parsing so that other
users can tap into it, too.

Signed-off-by: Kevin Worm <kevin.worm_1@signify.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: kevin-worm

common-main.c

@@ -1,6 +1,8 @@
 #include "cache.h"
+#include "config.h"
 #include "exec-cmd.h"
 #include "attr.h"
+#include "trace2/tr2_sysenv.h"
 
 /*
  * Many parts of Git have subprograms communicate via pipe, expect the
@@ -23,6 +25,11 @@ static void restore_sigpipe_to_default(void)
 	signal(SIGPIPE, SIG_DFL);
 }
 
+static int read_very_early_config_cb(const char *key, const char *value, void *d)
+{
+	return tr2_sysenv_cb(key, value, d);
+}
+
 int main(int argc, const char **argv)
 {
 	int result;
@@ -46,7 +53,10 @@ int main(int argc, const char **argv)
 
 	attr_start();
 
+	read_very_early_config(read_very_early_config_cb, NULL);
+
 	trace2_initialize();
+
 	trace2_cmd_start(argv);
 	trace2_collect_process_info(TRACE2_PROCESS_INFO_STARTUP);
 

trace2.c

@@ -158,7 +158,7 @@ void trace2_initialize_fl(const char *file, int line)
 	if (trace2_enabled)
 		return;
 
-	tr2_sysenv_load();
+	tr2_sysenv_check_size();
 
 	if (!tr2_tgt_want_builtins())
 		return;

trace2/tr2_sysenv.c

@@ -57,7 +57,14 @@ static struct tr2_sysenv_entry tr2_sysenv_settings[] = {
 };
 /* clang-format on */
 
-static int tr2_sysenv_cb(const char *key, const char *value, void *d)
+/*
+ * Load Trace2 settings from the system config (usually "/etc/gitconfig"
+ * unless we were built with a runtime-prefix).  These are intended to
+ * define the default values for Trace2 as requested by the administrator.
+ *
+ * Then override with the Trace2 settings from the global config.
+ */
+int tr2_sysenv_cb(const char *key, const char *value, void *d)
 {
 	int k;
 
@@ -75,19 +82,10 @@ static int tr2_sysenv_cb(const char *key, const char *value, void *d)
 	return 0;
 }
 
-/*
- * Load Trace2 settings from the system config (usually "/etc/gitconfig"
- * unless we were built with a runtime-prefix).  These are intended to
- * define the default values for Trace2 as requested by the administrator.
- *
- * Then override with the Trace2 settings from the global config.
- */
-void tr2_sysenv_load(void)
+void tr2_sysenv_check_size(void)
 {
 	if (ARRAY_SIZE(tr2_sysenv_settings) != TR2_SYSENV_MUST_BE_LAST)
 		BUG("tr2_sysenv_settings size is wrong");
-
-	read_very_early_config(tr2_sysenv_cb, NULL);
 }
 
 /*

trace2/tr2_sysenv.h

@@ -30,7 +30,8 @@ enum tr2_sysenv_variable {
 	TR2_SYSENV_MUST_BE_LAST
 };
 
-void tr2_sysenv_load(void);
+int tr2_sysenv_cb(const char *key, const char *value, void *d);
+void tr2_sysenv_check_size(void);
 
 const char *tr2_sysenv_get(enum tr2_sysenv_variable);
 const char *tr2_sysenv_display_name(enum tr2_sysenv_variable var);