[PATCH v2 21/22] pickaxe -G: terminate early on matching lines

[Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx>]

Solve a long-standing item for "git log -Grx" of us e.g. finding "+
str" in the diff context and noting that we had a "hit", but xdiff
diligently continuing to generate and spew the rest of the diff at us.

The TODO item has been there since "git log -G" was implemented. See
f506b8e8b5f (git log/diff: add -G<regexp> that greps in the patch
text, 2010-08-23).

Our xdiff interface also had the limitation of not being able to abort
early since the beginning, see d9ea73e0564 (combine-diff: refactor
built-in xdiff interface., 2006-04-05). Although at that time
"xdiff_emit_line_fn" was called "xdiff_emit_consume_fn", and
"xdiff_emit_hunk_fn" didn't exist yet.

But now with the support added in the preceding ""xdiff-interface:
allow early return from xdiff_emit_{line,hunk}_fn" commit we can
return early, and furthermore test the functionality of the new
early-exit xdiff-interface by having a BUG() call here to die if it
ever starts handing us needless work again.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx>
---
 diffcore-pickaxe.c | 31 ++++++++++++++++++++-----------
 xdiff-interface.h  |  5 +++++
 2 files changed, 25 insertions(+), 11 deletions(-)

diff --git a/diffcore-pickaxe.c b/diffcore-pickaxe.c
index 94601072bde..f11b38b7121 100644
--- a/diffcore-pickaxe.c
+++ b/diffcore-pickaxe.c
@@ -27,13 +27,12 @@ static int diffgrep_consume(void *priv, char *line, unsigned long len)
 	if (line[0] != '+' && line[0] != '-')
 		return 0;
 	if (data->hit)
-		/*
-		 * NEEDSWORK: we should have a way to terminate the
-		 * caller early.
-		 */
-		return 0;
-	data->hit = !regexec_buf(data->regexp, line + 1, len - 1, 1,
-				 &regmatch, 0);
+		BUG("Already matched in diffgrep_consume! Broken xdiff_emit_line_fn?");
+	if (!regexec_buf(data->regexp, line + 1, len - 1, 1,
+			 &regmatch, 0)) {
+		data->hit = 1;
+		return -1;
+	}
 	return 0;
 }
 
@@ -45,6 +44,7 @@ static int diff_grep(mmfile_t *one, mmfile_t *two,
 	struct diffgrep_cb ecbdata;
 	xpparam_t xpp;
 	xdemitconf_t xecfg;
+	int ret;
 
 	if (!one)
 		return !regexec_buf(regexp, two->ptr, two->size,
@@ -63,10 +63,19 @@ static int diff_grep(mmfile_t *one, mmfile_t *two,
 	ecbdata.hit = 0;
 	xecfg.ctxlen = o->context;
 	xecfg.interhunkctxlen = o->interhunkcontext;
-	if (xdi_diff_outf(one, two, discard_hunk_line, diffgrep_consume,
-			  &ecbdata, &xpp, &xecfg))
-		return 0;
-	return ecbdata.hit;
+
+	/*
+	 * An xdiff error might be our "data->hit" from above. See the
+	 * comment for xdiff_emit_{line,hunk}_fn in xdiff-interface.h
+	 * for why.
+	 */
+	ret = xdi_diff_outf(one, two, discard_hunk_line, diffgrep_consume,
+			    &ecbdata, &xpp, &xecfg);
+	if (ecbdata.hit)
+		return 1;
+	if (ret)
+		return ret;
+	return 0;
 }
 
 static unsigned int contains(mmfile_t *mf, regex_t *regexp, kwset_t kws,
diff --git a/xdiff-interface.h b/xdiff-interface.h
index 1b27d6104ce..347d8a4425f 100644
--- a/xdiff-interface.h
+++ b/xdiff-interface.h
@@ -25,6 +25,11 @@
  * granular return values, but for now use it carefully, or consider
  * e.g. using discard_hunk_line() if you say just don't care about
  * hunk headers.
+ *
+ * Note that just returning -1 will make your early return
+ * indistinguishable from an error internal to xdiff. See "diff_grep"
+ * in diffcore-pickaxe.c for a trick to work around this, i.e. using
+ * the "consume_callback_data" to note the desired early return.
  */
 typedef int (*xdiff_emit_line_fn)(void *, char *, unsigned long);
 typedef int (*xdiff_emit_hunk_fn)(void *data,
-- 
2.30.0.284.gd98b1dd5eaa7