diffcore: add a filter to find a specific blobsb/diff-blobfind

Sometimes users are given a hash of an object and they want to
identify it further (ex.: Use verify-pack to find the largest blobs,
but what are these? or [1])

One might be tempted to extend git-describe to also work with blobs,
such that `git describe <blob-id>` gives a description as
'<commit-ish>:<path>'.  This was implemented at [2]; as seen by the sheer
number of responses (>110), it turns out this is tricky to get right.
The hard part to get right is picking the correct 'commit-ish' as that
could be the commit that (re-)introduced the blob or the blob that
removed the blob; the blob could exist in different branches.

Junio hinted at a different approach of solving this problem, which this
patch implements. Teach the diff machinery another flag for restricting
the information to what is shown. For example:

  $ ./git log --oneline --find-object=v2.0.0:Makefile
  b2feb64309 Revert the whole "ask curl-config" topic for now
  47fbfded53 i18n: only extract comments marked with "TRANSLATORS:"

we observe that the Makefile as shipped with 2.0 was appeared in
v1.9.2-471-g47fbfded53 and in v2.0.0-rc1-5-gb2feb6430b.  The
reason why these commits both occur prior to v2.0.0 are evil
merges that are not found using this new mechanism.

[1] https://stackoverflow.com/questions/223678/which-commit-has-this-blob
[2] https://public-inbox.org/git/20171028004419.10139-1-sbeller@google.com/

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 42 insertions, 0 deletions

diff --git a/diffcore-objfind.c b/diffcore-objfind.c
new file mode 100644
index 0000000000..676bbfff00
--- /dev/null
+++ b/diffcore-objfind.c
@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2017 Google Inc.
+ */
+#include "cache.h"
+#include "diff.h"
+#include "diffcore.h"
+
+static void diffcore_filter_blobs(struct diff_queue_struct *q,
+				  struct diff_options *options)
+{
+	int src, dst;
+
+	if (!options->objfind)
+		BUG("objfind oidset not initialized???");
+
+	for (src = dst = 0; src < q->nr; src++) {
+		struct diff_filepair *p = q->queue[src];
+
+		if (!DIFF_PAIR_UNMERGED(p) &&
+		    ((DIFF_FILE_VALID(p->one) &&
+		     oidset_contains(options->objfind, &p->one->oid)) ||
+		    (DIFF_FILE_VALID(p->two) &&
+		     oidset_contains(options->objfind, &p->two->oid)))) {
+			q->queue[dst] = p;
+			dst++;
+		} else {
+			diff_free_filepair(p);
+		}
+	}
+
+	if (!dst) {
+		free(q->queue);
+		DIFF_QUEUE_CLEAR(q);
+	} else {
+		q->nr = dst;
+	}
+}
+
+void diffcore_objfind(struct diff_options *options)
+{
+	diffcore_filter_blobs(&diff_queued_diff, options);
+}