[RFC PATCH v3 05/13] walken: configure rev_info and prepare for walk

[Emily Shaffer <emilyshaffer@google.com>]

`struct rev_info` is what's used by the walk itself.
`repo_init_revisions()` initializes the struct; then we need to set it
up for the walk we want to perform, which is done in
`final_rev_info_setup()`.

The most important step here is adding the first object we want to walk
to the pending array. Here, we take the easy road and use
`add_head_to_pending()`; there is also a way to do it with
`setup_revision_opt()` and `setup_revisions()` which we demonstrate but
do not use. If we were to forget this step, the walk would do nothing -
the pending queue would be checked, determined to be empty, and the walk
would terminate immediately.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Change-Id: I76754b740227cf17a449f3f536dbbe37031e6f9a
---
 builtin/walken.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/builtin/walken.c b/builtin/walken.c
index e53c42ea18..333d9ecc5e 100644
--- a/builtin/walken.c
+++ b/builtin/walken.c
@@ -5,6 +5,7 @@
  */
 
 #include "builtin.h"
+#include "revision.h"
 #include "config.h"
 #include "parse-options.h"
 
@@ -22,6 +23,40 @@ static void init_walken_defaults(void)
 	 */
 }
 
+/*
+ * Perform configuration for commit walk here. Within this function we set a
+ * starting point, and can customize our walk in various ways.
+ */
+static void final_rev_info_setup(int argc, const char **argv, const char *prefix,
+		struct rev_info *rev)
+{
+	/*
+	 * Optional:
+	 * setup_revision_opt is used to pass options to the setup_revisions()
+	 * call. It's got some special items for submodules and other types of
+	 * optimizations, but for now, we'll just point it to HEAD. First we
+	 * should make sure to reset it. This is useful for more complicated
+	 * stuff but a decent shortcut for the first pass is
+	 * add_head_to_pending().
+	 */
+
+	/*
+	 * struct setup_revision_opt opt;
+
+	 * memset(&opt, 0, sizeof(opt));
+	 * opt.def = "HEAD";
+	 * opt.revarg_opt = REVARG_COMMITTISH;
+	 * setup_revisions(argc, argv, rev, &opt);
+	 */
+
+	/* add the HEAD to pending so we can start */
+	add_head_to_pending(rev);
+
+	/* Let's force oneline format. */
+	get_commit_format("oneline", rev);
+	rev->verbose_header = 1;
+}
+
 /*
  * This method will be called back by git_config(). It is used to gather values
  * from the configuration files available to Git.
@@ -62,6 +97,8 @@ int cmd_walken(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	struct rev_info rev;
+
 	/*
 	 * parse_options() handles showing usage if incorrect options are
 	 * provided, or if '-h' is passed.
@@ -72,6 +109,19 @@ int cmd_walken(int argc, const char **argv, const char *prefix)
 
 	git_config(git_walken_config, NULL);
 
+	/*
+	 * Time to set up the walk. repo_init_revisions sets up rev_info with
+	 * the defaults, but then you need to make some configuration settings
+	 * to make it do what's special about your walk.
+	 */
+	repo_init_revisions(the_repository, &rev, prefix);
+
+	/*
+	 * Before we do the walk, we need to set a starting point by giving it
+	 * something to go in `pending` - that happens in here
+	 */
+	final_rev_info_setup(argc, argv, prefix, &rev);
+
 	/*
 	 * This line is "human-readable" and we are writing a plumbing command,
 	 * so we localize it and use the trace library to print only when
-- 
2.22.0.410.gd8fdbe21b5-goog