[PATCH] log: allow users to follow a file

[john at keeping.me.uk (John Keeping)]

Teach the "log" UI to behave in the same way as "git log --follow", when
given a suitable instruction by the user.  The default behaviour remains
to show the log without following renames, but the follow behaviour can
be activated by following a link in the page header.

Follow is not the default because outputting merges in follow mode is
tricky ("git log --follow" will not show merges).  We also disable the
graph in follow mode because the commit graph is not simplified so we
end up with frequent gaps in the graph and many lines that do not
connect with any commits we're actually showing.

Since following renames requires running diff on every commit we
consider, I've added a knob to the configuration file to globally
enable/disable this feature.  Note that we may consider a large number
of commits the revision walking machinery no longer performs any path
limitation so we have to examine every commit until we find a page full
of commits that affect the target path or something related to it.

Suggested-by: Ren? Neumann <lists at necoro.eu>
Signed-off-by: John Keeping <john at keeping.me.uk>
---
 cgit.c        |  4 +++
 cgit.h        |  2 ++
 cgitrc.5.txt  |  4 +++
 ui-log.c      | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++++-------
 ui-refs.c     |  2 +-
 ui-repolist.c |  2 +-
 ui-shared.c   | 20 +++++++++++---
 ui-shared.h   |  2 +-
 ui-tree.c     |  2 +-
 9 files changed, 108 insertions(+), 17 deletions(-)

diff --git a/cgit.c b/cgit.c
index 6f44ef2..81312bb 100644
--- a/cgit.c
+++ b/cgit.c
@@ -171,6 +171,8 @@ static void config_cb(const char *name, const char *value)
 		ctx.cfg.snapshots = cgit_parse_snapshots_mask(value);
 	else if (!strcmp(name, "enable-filter-overrides"))
 		ctx.cfg.enable_filter_overrides = atoi(value);
+	else if (!strcmp(name, "enable-follow-links"))
+		ctx.cfg.enable_follow_links = atoi(value);
 	else if (!strcmp(name, "enable-http-clone"))
 		ctx.cfg.enable_http_clone = atoi(value);
 	else if (!strcmp(name, "enable-index-links"))
@@ -338,6 +340,8 @@ static void querystring_cb(const char *name, const char *value)
 		ctx.qry.context = atoi(value);
 	} else if (!strcmp(name, "ignorews")) {
 		ctx.qry.ignorews = atoi(value);
+	} else if (!strcmp(name, "follow")) {
+		ctx.qry.follow = atoi(value);
 	}
 }
 
diff --git a/cgit.h b/cgit.h
index 850b792..6c0c429 100644
--- a/cgit.h
+++ b/cgit.h
@@ -163,6 +163,7 @@ struct cgit_query {
 	int show_all;
 	int context;
 	int ignorews;
+	int follow;
 	char *vpath;
 };
 
@@ -203,6 +204,7 @@ struct cgit_config {
 	int case_sensitive_sort;
 	int embedded;
 	int enable_filter_overrides;
+	int enable_follow_links;
 	int enable_http_clone;
 	int enable_index_links;
 	int enable_index_owner;
diff --git a/cgitrc.5.txt b/cgitrc.5.txt
index 39b031e..48ef249 100644
--- a/cgitrc.5.txt
+++ b/cgitrc.5.txt
@@ -121,6 +121,10 @@ enable-filter-overrides::
 	Flag which, when set to "1", allows all filter settings to be
 	overridden in repository-specific cgitrc files. Default value: none.
 
+enable-follow-links::
+	Flag which, when set to "1", allows users to follow a file in the log
+	view.  Default value: "0".
+
 enable-http-clone::
 	If set to "1", cgit will act as an dumb HTTP endpoint for git clones.
 	If you use an alternate way of serving git repositories, you may wish
diff --git a/ui-log.c b/ui-log.c
index 2aa12c3..d008387 100644
--- a/ui-log.c
+++ b/ui-log.c
@@ -66,7 +66,7 @@ void show_commit_decorations(struct commit *commit)
 			strncpy(buf, deco->name + 11, sizeof(buf) - 1);
 			cgit_log_link(buf, NULL, "branch-deco", buf, NULL,
 				      ctx.qry.vpath, 0, NULL, NULL,
-				      ctx.qry.showmsg);
+				      ctx.qry.showmsg, 0);
 		}
 		else if (!prefixcmp(deco->name, "tag: refs/tags/")) {
 			strncpy(buf, deco->name + 15, sizeof(buf) - 1);
@@ -83,7 +83,7 @@ void show_commit_decorations(struct commit *commit)
 			cgit_log_link(buf, NULL, "remote-deco", NULL,
 				      sha1_to_hex(commit->object.sha1),
 				      ctx.qry.vpath, 0, NULL, NULL,
-				      ctx.qry.showmsg);
+				      ctx.qry.showmsg, 0);
 		}
 		else {
 			strncpy(buf, deco->name, sizeof(buf) - 1);
@@ -96,6 +96,50 @@ next:
 	}
 }
 
+static int show_commit(struct commit *commit, struct rev_info *revs)
+{
+	struct commit_list *parents = commit->parents;
+	struct commit *parent;
+	int found = 0, saved_fmt;
+	unsigned saved_flags = revs->diffopt.flags;
+
+
+	/* Always show if we're not in "follow" mode with a single file. */
+	if (!ctx.qry.follow)
+		return 1;
+
+	/*
+	 * In "follow" mode, we don't show merges.  This is consistent with
+	 * "git log --follow -- <file>".
+	 */
+	if (parents && parents->next)
+		return 0;
+
+	/*
+	 * If this is the root commit, do what rev_info tells us.
+	 */
+	if (!parents)
+		return revs->show_root_diff;
+
+	/* When we get here we have precisely one parent. */
+	parent = parents->item;
+	parse_commit(parent);
+	DIFF_OPT_SET(&revs->diffopt, RECURSIVE);
+	diff_tree_sha1(parent->tree->object.sha1,
+		       commit->tree->object.sha1,
+		       "", &revs->diffopt);
+	diffcore_std(&revs->diffopt);
+
+	found = !diff_queue_is_empty();
+	saved_fmt = revs->diffopt.output_format;
+	revs->diffopt.output_format = DIFF_FORMAT_NO_OUTPUT;
+	diff_flush(&revs->diffopt);
+	revs->diffopt.output_format = saved_fmt;
+	revs->diffopt.flags = saved_flags;
+
+	return found;
+}
+
 static void print_commit(struct commit *commit, struct rev_info *revs)
 {
 	struct commitinfo *info;
@@ -324,7 +368,17 @@ void cgit_print_log(const char *tip, int ofs, int cnt, char *grep, char *pattern
 			}
 		}
 	}
-	if (commit_graph) {
+
+	if (!path || !ctx.cfg.enable_follow_links) {
+		/*
+		 * If we don't have a path, "follow" is a no-op so make sure
+		 * the variable is set to false to avoid needing to check
+		 * both this and whether we have a path everywhere.
+		 */
+		ctx.qry.follow = 0;
+	}
+
+	if (commit_graph && !ctx.qry.follow) {
 		static const char *graph_arg = "--graph";
 		static const char *color_arg = "--color";
 		vector_push(&vec, &graph_arg, 0);
@@ -342,7 +396,10 @@ void cgit_print_log(const char *tip, int ofs, int cnt, char *grep, char *pattern
 	}
 
 	if (path) {
+		static const char *follow_arg = "--follow";
 		static const char *double_dash_arg = "--";
+		if (ctx.qry.follow)
+			vector_push(&vec, &follow_arg, 0);
 		vector_push(&vec, &double_dash_arg, 0);
 		vector_push(&vec, &path, 0);
 	}
@@ -356,6 +413,7 @@ void cgit_print_log(const char *tip, int ofs, int cnt, char *grep, char *pattern
 	rev.commit_format = CMIT_FMT_DEFAULT;
 	rev.verbose_header = 1;
 	rev.show_root_diff = 0;
+	rev.simplify_history = 1;
 	setup_revisions(vec.count, vec.data, &rev, NULL);
 	load_ref_decorations(DECORATE_FULL_REFS);
 	rev.show_decorations = 1;
@@ -377,7 +435,8 @@ void cgit_print_log(const char *tip, int ofs, int cnt, char *grep, char *pattern
 		cgit_log_link(ctx.qry.showmsg ? "Collapse" : "Expand", NULL,
 			      NULL, ctx.qry.head, ctx.qry.sha1,
 			      ctx.qry.vpath, ctx.qry.ofs, ctx.qry.grep,
-			      ctx.qry.search, ctx.qry.showmsg ? 0 : 1);
+			      ctx.qry.search, ctx.qry.showmsg ? 0 : 1,
+			      ctx.qry.follow);
 		html(")");
 	}
 	html("</th><th class='left'>Author</th>");
@@ -396,15 +455,20 @@ void cgit_print_log(const char *tip, int ofs, int cnt, char *grep, char *pattern
 	if (ofs<0)
 		ofs = 0;
 
-	for (i = 0; i < ofs && (commit = get_revision(&rev)) != NULL; i++) {
+	for (i = 0; i < ofs && (commit = get_revision(&rev)) != NULL;) {
+		if (show_commit(commit, &rev))
+			i++;
 		free(commit->buffer);
 		commit->buffer = NULL;
 		free_commit_list(commit->parents);
 		commit->parents = NULL;
 	}
 
-	for (i = 0; i < cnt && (commit = get_revision(&rev)) != NULL; i++) {
-		print_commit(commit, &rev);
+	for (i = 0; i < cnt && (commit = get_revision(&rev)) != NULL;) {
+		if (show_commit(commit, &rev)) {
+			i++;
+			print_commit(commit, &rev);
+		}
 		free(commit->buffer);
 		commit->buffer = NULL;
 		free_commit_list(commit->parents);
@@ -417,7 +481,8 @@ void cgit_print_log(const char *tip, int ofs, int cnt, char *grep, char *pattern
 			cgit_log_link("[prev]", NULL, NULL, ctx.qry.head,
 				      ctx.qry.sha1, ctx.qry.vpath,
 				      ofs - cnt, ctx.qry.grep,
-				      ctx.qry.search, ctx.qry.showmsg);
+				      ctx.qry.search, ctx.qry.showmsg,
+				      ctx.qry.follow);
 			html("</li>");
 		}
 		if ((commit = get_revision(&rev)) != NULL) {
@@ -425,14 +490,16 @@ void cgit_print_log(const char *tip, int ofs, int cnt, char *grep, char *pattern
 			cgit_log_link("[next]", NULL, NULL, ctx.qry.head,
 				      ctx.qry.sha1, ctx.qry.vpath,
 				      ofs + cnt, ctx.qry.grep,
-				      ctx.qry.search, ctx.qry.showmsg);
+				      ctx.qry.search, ctx.qry.showmsg,
+				      ctx.qry.follow);
 			html("</li>");
 		}
 		html("</ul>");
 	} else if ((commit = get_revision(&rev)) != NULL) {
 		htmlf("<tr class='nohover'><td colspan='%d'>", columns);
 		cgit_log_link("[...]", NULL, NULL, ctx.qry.head, NULL,
-			      ctx.qry.vpath, 0, NULL, NULL, ctx.qry.showmsg);
+			      ctx.qry.vpath, 0, NULL, NULL, ctx.qry.showmsg,
+			      ctx.qry.follow);
 		html("</td></tr>\n");
 	}
 
diff --git a/ui-refs.c b/ui-refs.c
index 0ae0612..f4eefd1 100644
--- a/ui-refs.c
+++ b/ui-refs.c
@@ -71,7 +71,7 @@ static int print_branch(struct refinfo *ref)
 		return 1;
 	html("<tr><td>");
 	cgit_log_link(name, NULL, NULL, name, NULL, NULL, 0, NULL, NULL,
-		      ctx.qry.showmsg);
+		      ctx.qry.showmsg, 0);
 	html("</td><td>");
 
 	if (ref->object->type == OBJ_COMMIT) {
diff --git a/ui-repolist.c b/ui-repolist.c
index 47ca997..d96f252 100644
--- a/ui-repolist.c
+++ b/ui-repolist.c
@@ -314,7 +314,7 @@ void cgit_print_repolist()
 			html("<td>");
 			cgit_summary_link("summary", NULL, "button", NULL);
 			cgit_log_link("log", NULL, "button", NULL, NULL, NULL,
-				      0, NULL, NULL, ctx.qry.showmsg);
+				      0, NULL, NULL, ctx.qry.showmsg, 0);
 			cgit_tree_link("tree", NULL, "button", NULL, NULL, NULL);
 			html("</td>");
 		}
diff --git a/ui-shared.c b/ui-shared.c
index 519eef7..24560ba 100644
--- a/ui-shared.c
+++ b/ui-shared.c
@@ -284,7 +284,8 @@ void cgit_plain_link(const char *name, const char *title, const char *class,
 
 void cgit_log_link(const char *name, const char *title, const char *class,
 		   const char *head, const char *rev, const char *path,
-		   int ofs, const char *grep, const char *pattern, int showmsg)
+		   int ofs, const char *grep, const char *pattern, int showmsg,
+		   int follow)
 {
 	char *delim;
 
@@ -313,6 +314,11 @@ void cgit_log_link(const char *name, const char *title, const char *class,
 	if (showmsg) {
 		html(delim);
 		html("showmsg=1");
+		delim = "&amp;";
+	}
+	if (follow) {
+		html(delim);
+		html("follow=1");
 	}
 	html("'>");
 	html_txt(name);
@@ -452,7 +458,7 @@ static void cgit_self_link(char *name, const char *title, const char *class,
 			      ctx->qry.has_sha1 ? ctx->qry.sha1 : NULL,
 			      ctx->qry.path, ctx->qry.ofs,
 			      ctx->qry.grep, ctx->qry.search,
-			      ctx->qry.showmsg);
+			      ctx->qry.showmsg, ctx->qry.follow);
 	else if (!strcmp(ctx->qry.page, "commit"))
 		cgit_commit_link(name, title, class, ctx->qry.head,
 				 ctx->qry.has_sha1 ? ctx->qry.sha1 : NULL,
@@ -854,7 +860,7 @@ void cgit_print_pageheader(struct cgit_context *ctx)
 			       ctx->qry.sha1, NULL);
 		cgit_log_link("log", NULL, hc(ctx, "log"), ctx->qry.head,
 			      NULL, ctx->qry.vpath, 0, NULL, NULL,
-			      ctx->qry.showmsg);
+			      ctx->qry.showmsg, ctx->qry.follow);
 		cgit_tree_link("tree", NULL, hc(ctx, "tree"), ctx->qry.head,
 			       ctx->qry.sha1, ctx->qry.vpath);
 		cgit_commit_link("commit", NULL, hc(ctx, "commit"),
@@ -906,6 +912,14 @@ void cgit_print_pageheader(struct cgit_context *ctx)
 		html("<div class='path'>");
 		html("path: ");
 		cgit_print_path_crumbs(ctx, ctx->qry.vpath);
+		if (ctx->cfg.enable_follow_links && !strcmp(ctx->qry.page, "log")) {
+			html(" (");
+			ctx->qry.follow = !ctx->qry.follow;
+			cgit_self_link(ctx->qry.follow ? "follow" : "unfollow",
+					NULL, NULL, ctx);
+			ctx->qry.follow = !ctx->qry.follow;
+			html(")");
+		}
 		html("</div>");
 	}
 	html("<div class='content'>");
diff --git a/ui-shared.h b/ui-shared.h
index 5987e77..3156846 100644
--- a/ui-shared.h
+++ b/ui-shared.h
@@ -26,7 +26,7 @@ extern void cgit_plain_link(const char *name, const char *title,
 extern void cgit_log_link(const char *name, const char *title,
 			  const char *class, const char *head, const char *rev,
 			  const char *path, int ofs, const char *grep,
-			  const char *pattern, int showmsg);
+			  const char *pattern, int showmsg, int follow);
 extern void cgit_commit_link(char *name, const char *title,
 			     const char *class, const char *head,
 			     const char *rev, const char *path,
diff --git a/ui-tree.c b/ui-tree.c
index aa5dee9..ebb3e9b 100644
--- a/ui-tree.c
+++ b/ui-tree.c
@@ -169,7 +169,7 @@ static int ls_item(const unsigned char *sha1, const char *base, int baselen,
 	html("<td>");
 	cgit_log_link("log", NULL, "button", ctx.qry.head,
 		      walk_tree_ctx->curr_rev, fullpath.buf, 0, NULL, NULL,
-		      ctx.qry.showmsg);
+		      ctx.qry.showmsg, 0);
 	if (ctx.repo->max_stats)
 		cgit_stats_link("stats", NULL, "button", ctx.qry.head,
 				fullpath.buf);
-- 
1.8.2.694.ga76e9c3.dirty