[PATCH v2 05/10] git.c: provide setup_auto_pager()

["Martin Ågren" <martin.agren@gmail.com>]

The previous patch introduced a way for builtins to declare that they
will take responsibility for handling the `pager.foo`-config item. (See
the commit message of that patch for why that could be useful.)

Provide setup_auto_pager(), which builtins can call in order to handle
`pager.<cmd>`, including possibly starting the pager. Make this function
don't do anything if a pager has already been started, as indicated by
use_pager or pager_in_use().

Whenever this function is called from a builtin, git.c will already have
called commit_pager_choice(). Since commit_pager_choice() treats the
special value -1 as "punt" or "not yet decided", it is not a problem
that we might end up calling commit_pager_choice() once in git.c and
once (or more) in the builtin. Make the new function use -1 in the same
way and document it as "punt".

Don't add any users of setup_auto_pager just yet, one will follow in
a later patch.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
---
 builtin.h |  9 +++++++++
 git.c     | 10 ++++++++++
 2 files changed, 19 insertions(+)

diff --git a/builtin.h b/builtin.h
index 4186635de..3ca4a53a8 100644
--- a/builtin.h
+++ b/builtin.h
@@ -113,6 +113,15 @@ struct fmt_merge_msg_opts {
 extern int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 			 struct fmt_merge_msg_opts *);
 
+/**
+ * If a builtin has DELAY_PAGER_CONFIG set, the builtin should call this early
+ * when it wishes to respect the `pager.foo`-config. The `cmd` is the name of
+ * the builtin, e.g., "foo". If a paging-choice has already been setup, this
+ * does nothing. The default in `def` should be 0 for "pager off", 1 for "pager
+ * on" or -1 for "punt".
+ */
+extern void setup_auto_pager(const char *cmd, int def);
+
 extern int is_builtin(const char *s);
 
 extern int cmd_add(int argc, const char **argv, const char *prefix);
diff --git a/git.c b/git.c
index 79195ebbd..66832f232 100644
--- a/git.c
+++ b/git.c
@@ -33,6 +33,16 @@ static void commit_pager_choice(void) {
 	}
 }
 
+void setup_auto_pager(const char *cmd, int def)
+{
+	if (use_pager != -1 || pager_in_use())
+		return;
+	use_pager = check_pager_config(cmd);
+	if (use_pager == -1)
+		use_pager = def;
+	commit_pager_choice();
+}
+
 static int handle_options(const char ***argv, int *argc, int *envchanged)
 {
 	const char **orig_argv = *argv;
-- 
2.14.0.rc0