[PATCH/WIP v2 03/19] am: implement skeletal builtin am

[Paul Tan <pyokagan@gmail.com>]

For the purpose of rewriting git-am.sh into a C builtin, implement a
skeletal builtin/am.c that redirects to $GIT_EXEC_PATH/git-am if the
environment variable _GIT_USE_BUILTIN_AM is not defined. Since in the
Makefile git-am.sh takes precedence over builtin/am.c,
$GIT_EXEC_PATH/git-am will contain the shell script git-am.sh, and thus
this allows us to fall back on the functional git-am.sh when running the
test suite for tests that depend on a working git-am implementation.

This redirection will be removed when all the features of git-am.sh have
been re-implemented in builtin/am.c.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
---

Notes:
    v2
    
    * Declare struct am_state state as static.

 Makefile     |  1 +
 builtin.h    |  1 +
 builtin/am.c | 20 ++++++++++++++++++++
 git.c        |  1 +
 4 files changed, 23 insertions(+)
 create mode 100644 builtin/am.c

diff --git a/Makefile b/Makefile
index 54ec511..b82ba0e 100644
--- a/Makefile
+++ b/Makefile
@@ -812,6 +812,7 @@ LIB_OBJS += xdiff-interface.o
 LIB_OBJS += zlib.o
 
 BUILTIN_OBJS += builtin/add.o
+BUILTIN_OBJS += builtin/am.o
 BUILTIN_OBJS += builtin/annotate.o
 BUILTIN_OBJS += builtin/apply.o
 BUILTIN_OBJS += builtin/archive.o
diff --git a/builtin.h b/builtin.h
index b87df70..d50c9d1 100644
--- a/builtin.h
+++ b/builtin.h
@@ -30,6 +30,7 @@ extern int textconv_object(const char *path, unsigned mode, const unsigned char
 extern int is_builtin(const char *s);
 
 extern int cmd_add(int argc, const char **argv, const char *prefix);
+extern int cmd_am(int argc, const char **argv, const char *prefix);
 extern int cmd_annotate(int argc, const char **argv, const char *prefix);
 extern int cmd_apply(int argc, const char **argv, const char *prefix);
 extern int cmd_archive(int argc, const char **argv, const char *prefix);
diff --git a/builtin/am.c b/builtin/am.c
new file mode 100644
index 0000000..0ccbe33
--- /dev/null
+++ b/builtin/am.c
@@ -0,0 +1,20 @@
+/*
+ * Builtin "git am"
+ *
+ * Based on git-am.sh by Junio C Hamano.
+ */
+#include "cache.h"
+#include "builtin.h"
+#include "exec_cmd.h"
+
+int cmd_am(int argc, const char **argv, const char *prefix)
+{
+	if (!getenv("_GIT_USE_BUILTIN_AM")) {
+		const char *path = mkpath("%s/git-am", git_exec_path());
+
+		if (sane_execvp(path, (char**) argv) < 0)
+			die_errno("could not exec %s", path);
+	}
+
+	return 0;
+}
diff --git a/git.c b/git.c
index 44374b1..42328ed 100644
--- a/git.c
+++ b/git.c
@@ -370,6 +370,7 @@ static int run_builtin(struct cmd_struct *p, int argc, const char **argv)
 
 static struct cmd_struct commands[] = {
 	{ "add", cmd_add, RUN_SETUP | NEED_WORK_TREE },
+	{ "am", cmd_am, RUN_SETUP | NEED_WORK_TREE },
 	{ "annotate", cmd_annotate, RUN_SETUP },
 	{ "apply", cmd_apply, RUN_SETUP_GENTLY },
 	{ "archive", cmd_archive },
-- 
2.1.4