diff: factor out --follow pathspec check

In --follow mode, we require exactly one pathspec. We check this
condition in two places:

  - in diff_setup_done(), we complain if --follow is used with an
    inapropriate pathspec

  - in git-log's revision "tweak" function, we enable log.follow only if
    the pathspec allows it

The duplication isn't a big deal right now, since the logic is so
simple. But in preparation for it becoming more complex, let's pull it
into a shared function.

Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

builtin/log.c

@@ -856,7 +856,7 @@ static void log_setup_revisions_tweak(struct rev_info *rev,
 				      struct setup_revision_opt *opt)
 {
 	if (rev->diffopt.flags.default_follow_renames &&
-	    rev->prune_data.nr == 1)
+	    diff_check_follow_pathspec(&rev->prune_data, 0))
 		rev->diffopt.flags.follow_renames = 1;
 
 	if (rev->first_parent_only)

diff.c

@@ -4721,6 +4721,16 @@ unsigned diff_filter_bit(char status)
 	return filter_bit[(int) status];
 }
 
+int diff_check_follow_pathspec(struct pathspec *ps, int die_on_error)
+{
+	if (ps->nr != 1) {
+		if (die_on_error)
+			die(_("--follow requires exactly one pathspec"));
+		return 0;
+	}
+	return 1;
+}
+
 void diff_setup_done(struct diff_options *options)
 {
 	unsigned check_mask = DIFF_FORMAT_NAME |
@@ -4828,8 +4838,8 @@ void diff_setup_done(struct diff_options *options)
 
 	options->diff_path_counter = 0;
 
-	if (options->flags.follow_renames && options->pathspec.nr != 1)
-		die(_("--follow requires exactly one pathspec"));
+	if (options->flags.follow_renames)
+		diff_check_follow_pathspec(&options->pathspec, 1);
 
 	if (!options->use_color || external_diff())
 		options->color_moved = 0;

diff.h

@@ -542,6 +542,13 @@ void repo_diff_setup(struct repository *, struct diff_options *);
 struct option *add_diff_options(const struct option *, struct diff_options *);
 int diff_opt_parse(struct diff_options *, const char **, int, const char *);
 void diff_setup_done(struct diff_options *);
+
+/*
+ * Returns true if the pathspec can work with --follow mode. If die_on_error is
+ * set, die() with a specific error message rather than returning false.
+ */
+int diff_check_follow_pathspec(struct pathspec *ps, int die_on_error);
+
 int git_config_rename(const char *var, const char *value);
 
 #define DIFF_DETECT_RENAME	1