blame: consult diff process for no-hunk detection

When a diff process is configured via diff.<driver>.process,
consult it during blame's per-commit diffing.  If the process
returns no hunks for a commit's changes to a file, treat the
commit as having no changes, causing blame to attribute lines
to earlier commits.

The consultation happens at the pass_blame_to_parent() callsite
using diff_process_fill_hunks(), matching how builtin_diff() in
diff.c uses the same function.  A new diff_hunks_xpp() variant
accepts a pre-populated xpparam_t so callers can pass external
hunks, while the existing diff_hunks() retains its original
signature and behavior.  The copy-detection callsite is
unaffected since it does not use the diff process.

The subprocess is long-running (one startup cost amortized
across the blame traversal), but each commit in the file's
history incurs a round-trip to the tool.

Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>

Author: mmontalbo

blame.c

@@ -19,6 +19,8 @@
 #include "tag.h"
 #include "trace2.h"
 #include "blame.h"
+#include "diff-process.h"
+#include "xdiff-interface.h"
 #include "alloc.h"
 #include "commit-slab.h"
 #include "bloom.h"
@@ -314,17 +316,25 @@ static struct commit *fake_working_tree_commit(struct repository *r,
 
 
 
-static int diff_hunks(mmfile_t *file_a, mmfile_t *file_b,
-		      xdl_emit_hunk_consume_func_t hunk_func, void *cb_data, int xdl_opts)
+static int diff_hunks_xpp(mmfile_t *file_a, mmfile_t *file_b,
+			  xdl_emit_hunk_consume_func_t hunk_func,
+			  void *cb_data, xpparam_t *xpp)
 {
-	xpparam_t xpp = {0};
 	xdemitconf_t xecfg = {0};
 	xdemitcb_t ecb = {NULL};
 
-	xpp.flags = xdl_opts;
 	xecfg.hunk_func = hunk_func;
 	ecb.priv = cb_data;
-	return xdi_diff(file_a, file_b, &xpp, &xecfg, &ecb);
+	return xdi_diff(file_a, file_b, xpp, &xecfg, &ecb);
+}
+
+static int diff_hunks(mmfile_t *file_a, mmfile_t *file_b,
+		      xdl_emit_hunk_consume_func_t hunk_func, void *cb_data, int xdl_opts)
+{
+	xpparam_t xpp = {0};
+
+	xpp.flags = xdl_opts;
+	return diff_hunks_xpp(file_a, file_b, hunk_func, cb_data, &xpp);
 }
 
 static const char *get_next_line(const char *start, const char *end)
@@ -1943,6 +1953,7 @@ static void pass_blame_to_parent(struct blame_scoreboard *sb,
 				 struct blame_origin *parent, int ignore_diffs)
 {
 	mmfile_t file_p, file_o;
+	xpparam_t xpp = {0};
 	struct blame_chunk_cb_data d;
 	struct blame_entry *newdest = NULL;
 
@@ -1961,10 +1972,21 @@ static void pass_blame_to_parent(struct blame_scoreboard *sb,
 			 &sb->num_read_blob, ignore_diffs);
 	sb->num_get_patch++;
 
-	if (diff_hunks(&file_p, &file_o, blame_chunk_cb, &d, sb->xdl_opts))
-		die("unable to generate diff (%s -> %s)",
-		    oid_to_hex(&parent->commit->object.oid),
-		    oid_to_hex(&target->commit->object.oid));
+	xpp.flags = sb->xdl_opts;
+	/*
+	 * If the diff process considers the files equivalent,
+	 * skip the diff so blame looks past this commit.
+	 */
+	if (diff_process_fill_hunks(&sb->revs->diffopt, target->path,
+				    &file_p, &file_o, &xpp)
+	    != DIFF_PROCESS_EQUIVALENT) {
+		if (diff_hunks_xpp(&file_p, &file_o, blame_chunk_cb,
+				   &d, &xpp))
+			die("unable to generate diff (%s -> %s)",
+			    oid_to_hex(&parent->commit->object.oid),
+			    oid_to_hex(&target->commit->object.oid));
+	}
+	free(xpp.external_hunks);
 	/* The rest are the same as the parent */
 	blame_chunk(&d.dstq, &d.srcq, INT_MAX, d.offset, INT_MAX, 0,
 		    parent, target, 0);

t/t4080-diff-process.sh

@@ -447,21 +447,6 @@ test_expect_success 'diff process skipped when tool omits capability' '
 	test_must_be_empty stderr
 '
 
-test_expect_success 'diff process fallback on malformed hunk line' '
-	git -c diff.cdiff.process="$BACKEND --mode=bad-parse" \
-		diff boundary.c >actual 2>stderr &&
-	test_grep "^-OLD5" actual &&
-	test_grep "^+NEW5" actual
-'
-
-test_expect_success 'diff process skipped when tool omits capability' '
-	git -c diff.cdiff.process="$BACKEND --mode=no-cap" \
-		diff boundary.c >actual 2>stderr &&
-	test_grep "^-OLD5" actual &&
-	test_grep "^+NEW5" actual &&
-	test_must_be_empty stderr
-'
-
 #
 # Blame integration.
 #
@@ -566,4 +551,5 @@ test_expect_success 'blame --no-ext-diff uses builtin hunks' '
 	test_grep "$CHANGE" line9 &&
 	test_path_is_missing backend.log
 '
+
 test_done