[PATCH 05/15] pretty: extend pretty_print_context with callback

From: Nguyễn Thái Ngọc Duy <redacted>

The struct pretty_print_context contains the context in which the
placeholders in format_commit_one() should be parsed.  Although
format_commit_one() primarily acts as a parser, there is no way for a
caller to plug in custom callbacks.  Now, callers can:

1. Parse a custom placeholder that is not supported by
   format_commit_one(), and act on it independently of the pretty
   machinery.

2. Parse a custom placeholder to substitute the custom placeholder with
   a placeholder that format_commit_one() understands.  This is
   especially useful for supporting %>(*), where * is substituted with a
   length computed by the caller.

To support these two usecases, the interface for the function looks
like:

typedef size_t (*format_message_fn)(struct strbuf *sb,
				const char *placeholder,
				void *format_context,
				void *user_data,
				struct strbuf *placeholder_subst)

It is exactly like format_commit_one(), except that there are two
additional fields: user_data (to pass custom data to the callback), and
placeholder_subst (to set the substitution).  The callback should return
the length of the original string parsed, and optionally set
placeholder_subst.

[rr: commit message, minor modifications]

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Ramkumar Ramachandra <redacted>
---
 commit.h |  8 ++++++++
 pretty.c | 25 +++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/commit.h b/commit.h
index 4d452dc..ced7100 100644
--- a/commit.h
+++ b/commit.h

@@ -78,6 +78,12 @@ enum cmit_fmt {
 	CMIT_FMT_UNSPECIFIED
 };
 
+typedef size_t (*format_message_fn)(struct strbuf *sb,
+				const char *placeholder,
+				void *format_context,
+				void *user_data,
+				struct strbuf *placeholder_subst);
+
 struct pretty_print_context {
 	enum cmit_fmt fmt;
 	int abbrev;

@@ -92,6 +98,8 @@ struct pretty_print_context {
 	const char *output_encoding;
 	struct string_list *mailmap;
 	int color;
+	format_message_fn format;
+	void *user_data;
 };
 
 struct userformat_want {

diff --git a/pretty.c b/pretty.c
index 9e43154..095e5ba 100644
--- a/pretty.c
+++ b/pretty.c

@@ -1069,6 +1069,31 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 	struct commit_list *p;
 	int h1, h2;
 
+	if (c->pretty_ctx->format) {
+		struct strbuf subst = STRBUF_INIT;
+		int ret = c->pretty_ctx->format(sb, placeholder, context,
+						c->pretty_ctx->user_data,
+						&subst);
+		if (ret && subst.len) {
+			/*
+			 * Something was parsed by format(), and a
+			 * placeholder-substitution was set.
+			 * Recursion is required to override the
+			 * return value of format_commit_one() with
+			 * ret: the length of the original string
+			 * before substitution.
+			 */
+			ret = format_commit_one(sb, subst.buf, context) ? ret : 0;
+			strbuf_release(&subst);
+			return ret;
+		} else if (ret)
+			/*
+			 * Something was parsed by format(), but
+			 * no placeholder-substitution was set.
+			 */
+			return ret;
+	}
+
 	/* these are independent of the commit */
 	switch (placeholder[0]) {
 	case 'C':

-- 
1.8.3.2.736.g869de25