Fix error: "fatal: '$GIT_DIR' too big" when checking out in a long path on

Windows with core.longpaths = true.

This is a fix for issue: #3372

Use case:
Allow git to checkout a repository or submodule in a directory with a long
path when core.longpaths = true.

Example:
> ./git.exe config --global core.longpaths true
> ./git.exe clone https://github.com/git/git.git --recurse-submodules \
/c/eval/git_test/loooooooooooooooooooooooooooooooooooooooooooooooooooooooo\
oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo\
oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo\
oooooooong

Context:
$ sh --version
	GNU bash, version 4.4.23(1)-release (x86_64-pc-msys)
$ ./git.exe --version --build-options
	git version 2.36.1.windows.1
	cpu: x86_64
	built from commit: e2ff68a
	sizeof-long: 4
	sizeof-size_t: 8
	shell-path: /bin/sh
	feature: fsmonitor--daemon

Error:
fatal: '$GIT_DIR' too big.

Problem analysis:
setup_explicit_git_dir in setup.c uses PATH_MAX to check if the git dir is
to long. On Windows PATH_MAX is set by limit.h to 260 and
setup_explicit_git_dir ignores core.longpaths.

Solution:
The implementation is based on the solution proposed by Johannes
Schindelin, see:
#3372 (comment)

* Refactor the part of trace2_initialize() that reads the config.
* Make tr2_sysenv_cb() a public function.
* No longer calling it from trace2_initialize(), but from a static callback
  function in common-main.c.
* Calling read_very_early_config() explicitly in main(), with that static
  callback function that calls into tr2_sysenv_cb().
* Extend the static callback function for Windows, and parse core.longPaths
  in that function.
* Extend startup_info struct in cache.h with 'int max_long_path' so we can
  use it in setup_explicit_git_dir instead of PATH_MAX.

Signed-off-by: Kevin Worm <kevin.worm_1@signify.com>

Author: kevin-worm

cache.h

@@ -1869,6 +1869,7 @@ void overlay_tree_on_index(struct index_state *istate,
 
 /* setup.c */
 struct startup_info {
+	int max_long_path;
 	int have_repository;
 	const char *prefix;
 	const char *original_cwd;

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 +25,21 @@ 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)
+{
+	tr2_sysenv_cb(key, value, d);
+
+#if defined(__MINGW32__)
+	if (!strcmp(key, "core.longpaths") && git_config_bool(key, value)) {
+		startup_info->max_long_path = MAX_LONG_PATH;
+	}
+#else
+	startup_info->max_long_path = PATH_MAX;
+#endif
+
+	return 0;
+}
+
 int main(int argc, const char **argv)
 {
 	int result;
@@ -47,6 +64,9 @@ int main(int argc, const char **argv)
 	attr_start();
 
 	trace2_initialize();
+
+	read_very_early_config(read_very_early_config_cb, NULL);
+
 	trace2_cmd_start(argv);
 	trace2_collect_process_info(TRACE2_PROCESS_INFO_STARTUP);
 

setup.c

@@ -875,7 +875,7 @@ static const char *setup_explicit_git_dir(const char *gitdirenv,
 	char *gitfile;
 	int offset;
 
-	if (PATH_MAX - 40 < strlen(gitdirenv))
+	if (startup_info->max_long_path - 40 < strlen(gitdirenv))
 		die(_("'$%s' too big"), GIT_DIR_ENVIRONMENT);
 
 	gitfile = (char*)read_gitfile(gitdirenv);

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);