Start to implement a built-in version of git add --interactive

This is hardly the first conversion of a Git command that is implemented
as a script to a built-in. So far, the most successful strategy for such
conversions has been to add a built-in helper and call that for more and
more functionality from the script, as more and more parts are
converted.

With the interactive add, we choose a different strategy. The sole
reason for this is that on Windows (where such a conversion has the most
benefits in terms of speed and robustness) we face the very specific
problem that a `system()` call in Perl seems to close `stdin` in the
parent process when the spawned process consumes even one character from
`stdin`. And that just does not work for us here, as it would stop the
main loop as soon as any interactive command was performed by the
helper. Which is almost all of the commands in `git add -i`.

It is almost as if Perl told us once again that it does not want us to
use it on Windows.

Instead, we follow the opposite route where we start with a bare-bones
version of the built-in interactive add, guarded by the new
`add.interactive.useBuiltin` config variable, and then add more and more
functionality to it, until it is feature complete.

At this point, the built-in version of `git add -i` only states that it
cannot do anything yet ;-)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/config/add.txt

@@ -5,3 +5,8 @@ add.ignore-errors (deprecated)::
 	option of linkgit:git-add[1].  `add.ignore-errors` is deprecated,
 	as it does not follow the usual naming convention for configuration
 	variables.
+
+add.interactive.useBuiltin::
+	[EXPERIMENTAL] Set to `true` to use the experimental built-in
+	implementation of the interactive version of linkgit:git-add[1]
+	instead of the Perl script version. Is `false` by default.

Makefile

@@ -823,6 +823,7 @@ LIB_H := $(sort $(patsubst ./%,%,$(shell git ls-files '*.h' ':!t/' ':!Documentat
 	-name '*.h' -print)))
 
 LIB_OBJS += abspath.o
+LIB_OBJS += add-interactive.o
 LIB_OBJS += advice.o
 LIB_OBJS += alias.o
 LIB_OBJS += alloc.o

add-interactive.c

@@ -0,0 +1,7 @@
+#include "cache.h"
+#include "add-interactive.h"
+
+int run_add_i(struct repository *r, const struct pathspec *ps)
+{
+	die(_("No commands are available in the built-in `git add -i` yet!"));
+}

add-interactive.h

@@ -0,0 +1,8 @@
+#ifndef ADD_INTERACTIVE_H
+#define ADD_INTERACTIVE_H
+
+struct repository;
+struct pathspec;
+int run_add_i(struct repository *r, const struct pathspec *ps);
+
+#endif

builtin/add.c

@@ -20,6 +20,7 @@
 #include "bulk-checkin.h"
 #include "argv-array.h"
 #include "submodule.h"
+#include "add-interactive.h"
 
 static const char * const builtin_add_usage[] = {
 	N_("git add [<options>] [--] <pathspec>..."),
@@ -185,6 +186,14 @@ int run_add_interactive(const char *revision, const char *patch_mode,
 {
 	int status, i;
 	struct argv_array argv = ARGV_ARRAY_INIT;
+	int use_builtin_add_i =
+		git_env_bool("GIT_TEST_ADD_I_USE_BUILTIN", -1);
+	if (use_builtin_add_i < 0)
+		git_config_get_bool("add.interactive.usebuiltin",
+				    &use_builtin_add_i);
+
+	if (use_builtin_add_i == 1 && !patch_mode)
+		return !!run_add_i(the_repository, pathspec);
 
 	argv_array_push(&argv, "add--interactive");
 	if (patch_mode)
@@ -319,6 +328,7 @@ static int add_config(const char *var, const char *value, void *cb)
 		ignore_add_errors = git_config_bool(var, value);
 		return 0;
 	}
+
 	return git_default_config(var, value, cb);
 }
 

t/README

@@ -397,6 +397,10 @@ GIT_TEST_STASH_USE_BUILTIN=<boolean>, when false, disables the
 built-in version of git-stash. See 'stash.useBuiltin' in
 git-config(1).
 
+GIT_TEST_ADD_I_USE_BUILTIN=<boolean>, when true, enables the
+built-in version of git add -i. See 'add.interactive.useBuiltin' in
+git-config(1).
+
 GIT_TEST_INDEX_THREADS=<n> enables exercising the multi-threaded loading
 of the index for the whole test suite by bypassing the default number of
 cache entries and thread minimums. Setting this to 1 will make the