log: allow users to follow a file

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.

We also teach the "diff" and "commit" UIs to respect the follow flag on
URLs, causing the single-file version of these UIs to detect renames.
This feature is needed only for commits that rename the path we're
interested in.

For commits before the file has been renamed (i.e. that appear later in
the log list) we change the file path in the links from the log to point
to the old name; this means that links to commits always limit by the
path known to that commit.  If we didn't do this we would need to walk
down the log diff'ing every commit whenever we want to show a commit.
The drawback is that the "Log" link in the top bar of such a page links
to the log limited by the old name, so it will only show pre-rename
commits.  I consider this a reasonable trade-off since the "Back" button
still works and the log matches the path displayed in the top bar.

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: René Neumann <necoro@necoro.eu>
Signed-off-by: John Keeping <john@keeping.me.uk>

1 files changed, 35 insertions, 0 deletions

diff --git a/ui-diff.c b/ui-diff.c
index 1cf2ce0..caebd5d 100644
--- a/ui-diff.c
+++ b/ui-diff.c
@@ -36,6 +36,7 @@ static struct fileinfo {
 
 static int use_ssdiff = 0;
 static struct diff_filepair *current_filepair;
+static const char *current_prefix;
 
 struct diff_filespec *cgit_get_current_old_file(void)
 {
@@ -132,11 +133,30 @@ static void count_diff_lines(char *line, int len)
         }
 }
 
+static int show_filepair(struct diff_filepair *pair)
+{
+        /* Always show if we have no limiting prefix. */
+        if (!current_prefix)
+                return 1;
+
+        /* Show if either path in the pair begins with the prefix. */
+        if (starts_with(pair->one->path, current_prefix) ||
+            starts_with(pair->two->path, current_prefix))
+                return 1;
+
+        /* Otherwise we don't want to show this filepair. */
+        return 0;
+}
+
 static void inspect_filepair(struct diff_filepair *pair)
 {
         int binary = 0;
         unsigned long old_size = 0;
         unsigned long new_size = 0;
+
+        if (!show_filepair(pair))
+                return;
+
         files++;
         lines_added = 0;
         lines_removed = 0;
@@ -279,6 +299,9 @@ static void filepair_cb(struct diff_filepair *pair)
         int binary = 0;
         linediff_fn print_line_fn = print_line;
 
+        if (!show_filepair(pair))
+                return;
+
         current_filepair = pair;
         if (use_ssdiff) {
                 cgit_ssdiff_header_begin();
@@ -365,6 +388,18 @@ void cgit_print_diff(const char *new_rev, const char *old_rev,
         const unsigned char *old_tree_sha1, *new_tree_sha1;
         diff_type difftype;
 
+        /*
+         * If "follow" is set then the diff machinery needs to examine the
+         * entire commit to detect renames so we must limit the paths in our
+         * own callbacks and not pass the prefix to the diff machinery.
+         */
+        if (ctx.qry.follow && ctx.cfg.enable_follow_links) {
+                current_prefix = prefix;
+                prefix = "";
+        } else {
+                current_prefix = NULL;
+        }
+
         if (!new_rev)
                 new_rev = ctx.qry.head;
         if (get_sha1(new_rev, new_rev_sha1)) {