[PATCH 23/25] xdiff-interface: allow early return from xdiff_emit_{line,hunk}_fn

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Change the function prototype of xdiff_emit_{line,hunk}_fn to return
an int instead of void. This will allow for returning early from hunk
& diff consumers that want some of the data, but not all of it.

No behavior is being changed here, just replacing the equivalent of
"return" with "return 0", nothing acts on the changed return values
yet.

There was some work in this area of xdiff-interface.[ch] recently with
3b40a090fd4 (diff: avoid generating unused hunk header lines,
2018-11-02) and 7c61e25fbf1 (diff: use hunk callback for word-diff,
2018-11-02).

In combination those two changes allow us to not do any work on the
hunks and diff at all, but didn't change the status quo with regards
to consumers that e.g. want the diff lines, but might want to abort
early.

Whereas now we can abort e.g. on the first "-line" of a 1000 line diff
if that's all we needed.

This interface is rather scary as noted in the comment to
xdiff-interface.h being added here, but it will be useful for
diffcore-pickaxe.c in a subsequent commit. A future change could
e.g. add more exit codes, and hack xdl_emit_diff() and friends to
ignore or skip things more selectively as a result.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 combine-diff.c     |  9 ++++++---
 diff.c             | 39 +++++++++++++++++++++++----------------
 diffcore-pickaxe.c |  7 ++++---
 range-diff.c       |  8 +++++---
 xdiff-interface.c  | 10 ++++++----
 xdiff-interface.h  | 31 +++++++++++++++++++++++--------
 6 files changed, 67 insertions(+), 37 deletions(-)

diff --git a/combine-diff.c b/combine-diff.c
index 9228aebc16..6590c4b5fb 100644
--- a/combine-diff.c
+++ b/combine-diff.c
@@ -369,7 +369,7 @@ struct combine_diff_state {
 	struct sline *lost_bucket;
 };
 
-static void consume_hunk(void *state_,
+static int consume_hunk(void *state_,
 			 long ob, long on,
 			 long nb, long nn,
 			 const char *funcline, long funclen)
@@ -401,13 +401,15 @@ static void consume_hunk(void *state_,
 		state->sline[state->nb-1].p_lno =
 			xcalloc(state->num_parent, sizeof(unsigned long));
 	state->sline[state->nb-1].p_lno[state->n] = state->ob;
+
+	return 0;
 }
 
-static void consume_line(void *state_, char *line, unsigned long len)
+static int consume_line(void *state_, char *line, unsigned long len)
 {
 	struct combine_diff_state *state = state_;
 	if (!state->lost_bucket)
-		return; /* not in any hunk yet */
+		return 0; /* not in any hunk yet */
 	switch (line[0]) {
 	case '-':
 		append_lost(state->lost_bucket, state->n, line+1, len-1);
@@ -417,6 +419,7 @@ static void consume_line(void *state_, char *line, unsigned long len)
 		state->lno++;
 		break;
 	}
+	return 0;
 }
 
 static void combine_diff(struct repository *r,
diff --git a/diff.c b/diff.c
index 69e3bc00ed..bdedf9fdfa 100644
--- a/diff.c
+++ b/diff.c
@@ -1996,10 +1996,10 @@ static int color_words_output_graph_prefix(struct diff_words_data *diff_words)
 	}
 }
 
-static void fn_out_diff_words_aux(void *priv,
-				  long minus_first, long minus_len,
-				  long plus_first, long plus_len,
-				  const char *func, long funclen)
+static int fn_out_diff_words_aux(void *priv,
+				 long minus_first, long minus_len,
+				 long plus_first, long plus_len,
+				 const char *func, long funclen)
 {
 	struct diff_words_data *diff_words = priv;
 	struct diff_words_style *style = diff_words->style;
@@ -2047,6 +2047,8 @@ static void fn_out_diff_words_aux(void *priv,
 
 	diff_words->current_plus = plus_end;
 	diff_words->last_minus = minus_first;
+
+	return 0;
 }
 
 /* This function starts looking at *begin, and returns 0 iff a word was found. */
@@ -2338,7 +2340,7 @@ static void find_lno(const char *line, struct emit_callback *ecbdata)
 	ecbdata->lno_in_postimage = strtol(p + 1, NULL, 10);
 }
 
-static void fn_out_consume(void *priv, char *line, unsigned long len)
+static int fn_out_consume(void *priv, char *line, unsigned long len)
 {
 	struct emit_callback *ecbdata = priv;
 	struct diff_options *o = ecbdata->opt;
@@ -2374,7 +2376,7 @@ static void fn_out_consume(void *priv, char *line, unsigned long len)
 		len = sane_truncate_line(line, len);
 		find_lno(line, ecbdata);
 		emit_hunk_header(ecbdata, line, len);
-		return;
+		return 0;
 	}
 
 	if (ecbdata->diff_words) {
@@ -2384,11 +2386,11 @@ static void fn_out_consume(void *priv, char *line, unsigned long len)
 		if (line[0] == '-') {
 			diff_words_append(line, len,
 					  &ecbdata->diff_words->minus);
-			return;
+			return 0;
 		} else if (line[0] == '+') {
 			diff_words_append(line, len,
 					  &ecbdata->diff_words->plus);
-			return;
+			return 0;
 		} else if (starts_with(line, "\\ ")) {
 			/*
 			 * Eat the "no newline at eof" marker as if we
@@ -2397,11 +2399,11 @@ static void fn_out_consume(void *priv, char *line, unsigned long len)
 			 * defer processing. If this is the end of
 			 * preimage, more "+" lines may come after it.
 			 */
-			return;
+			return 0;
 		}
 		diff_words_flush(ecbdata);
 		emit_diff_symbol(o, s, line, len, 0);
-		return;
+		return 0;
 	}
 
 	switch (line[0]) {
@@ -2425,6 +2427,7 @@ static void fn_out_consume(void *priv, char *line, unsigned long len)
 				 line, len, 0);
 		break;
 	}
+	return 0;
 }
 
 static void pprint_rename(struct strbuf *name, const char *a, const char *b)
@@ -2524,7 +2527,7 @@ static struct diffstat_file *diffstat_add(struct diffstat_t *diffstat,
 	return x;
 }
 
-static void diffstat_consume(void *priv, char *line, unsigned long len)
+static int diffstat_consume(void *priv, char *line, unsigned long len)
 {
 	struct diffstat_t *diffstat = priv;
 	struct diffstat_file *x = diffstat->files[diffstat->nr - 1];
@@ -2533,6 +2536,7 @@ static void diffstat_consume(void *priv, char *line, unsigned long len)
 		x->added++;
 	else if (line[0] == '-')
 		x->deleted++;
+	return 0;
 }
 
 const char mime_boundary_leader[] = "------------";
@@ -3201,16 +3205,17 @@ static int is_conflict_marker(const char *line, int marker_size, unsigned long l
 	return 1;
 }
 
-static void checkdiff_consume_hunk(void *priv,
+static int checkdiff_consume_hunk(void *priv,
 				   long ob, long on, long nb, long nn,
 				   const char *func, long funclen)
 
 {
 	struct checkdiff_t *data = priv;
 	data->lineno = nb - 1;
+	return 0;
 }
 
-static void checkdiff_consume(void *priv, char *line, unsigned long len)
+static int checkdiff_consume(void *priv, char *line, unsigned long len)
 {
 	struct checkdiff_t *data = priv;
 	int marker_size = data->conflict_marker_size;
@@ -3234,7 +3239,7 @@ static void checkdiff_consume(void *priv, char *line, unsigned long len)
 		}
 		bad = ws_check(line + 1, len - 1, data->ws_rule);
 		if (!bad)
-			return;
+			return 0;
 		data->status |= bad;
 		err = whitespace_error_string(bad);
 		fprintf(data->o->file, "%s%s:%d: %s.\n",
@@ -3246,6 +3251,7 @@ static void checkdiff_consume(void *priv, char *line, unsigned long len)
 	} else if (line[0] == ' ') {
 		data->lineno++;
 	}
+	return 0;
 }
 
 static unsigned char *deflate_it(char *data,
@@ -6098,17 +6104,18 @@ void flush_one_hunk(struct object_id *result, git_hash_ctx *ctx)
 	}
 }
 
-static void patch_id_consume(void *priv, char *line, unsigned long len)
+static int patch_id_consume(void *priv, char *line, unsigned long len)
 {
 	struct patch_id_t *data = priv;
 	int new_len;
 
 	if (len > 12 && starts_with(line, "\\ "))
-		return;
+		return 0;
 	new_len = remove_space(line, len);
 
 	the_hash_algo->update_fn(data->ctx, line, new_len);
 	data->patchlen += new_len;
+	return 0;
 }
 
 static void patch_id_add_string(git_hash_ctx *ctx, const char *str)
diff --git a/diffcore-pickaxe.c b/diffcore-pickaxe.c
index 25ab1b2427..21f9d66b6a 100644
--- a/diffcore-pickaxe.c
+++ b/diffcore-pickaxe.c
@@ -19,7 +19,7 @@ struct diffgrep_cb {
 	int hit;
 };
 
-static void diffgrep_consume(void *priv, char *line, unsigned long len)
+static int diffgrep_consume(void *priv, char *line, unsigned long len)
 {
 	struct diffgrep_cb *data = priv;
 	regmatch_t regmatch;
@@ -27,14 +27,15 @@ static void diffgrep_consume(void *priv, char *line, unsigned long len)
 	struct grep_pat *grep_pat = grep_filter->pattern_list;
 
 	if (line[0] != '+' && line[0] != '-')
-		return;
+		return 0;
 	if (data->hit)
 		/*
 		 * NEEDSWORK: we should have a way to terminate the
 		 * caller early.
 		 */
-		return;
+		return 0;
 	data->hit = patmatch(grep_pat, line + 1, line + len + 1, &regmatch, 0);
+	return 0;
 }
 
 static int diff_grep(mmfile_t *one, mmfile_t *two,
diff --git a/range-diff.c b/range-diff.c
index b9950f10c8..cb40f67a26 100644
--- a/range-diff.c
+++ b/range-diff.c
@@ -267,15 +267,17 @@ static void find_exact_matches(struct string_list *a, struct string_list *b)
 	hashmap_clear(&map);
 }
 
-static void diffsize_consume(void *data, char *line, unsigned long len)
+static int diffsize_consume(void *data, char *line, unsigned long len)
 {
 	(*(int *)data)++;
+	return 0;
 }
 
-static void diffsize_hunk(void *data, long ob, long on, long nb, long nn,
-			  const char *funcline, long funclen)
+static int diffsize_hunk(void *data, long ob, long on, long nb, long nn,
+			 const char *funcline, long funclen)
 {
 	diffsize_consume(data, NULL, 0);
+	return 0;
 }
 
 static int diffsize(const char *a, const char *b)
diff --git a/xdiff-interface.c b/xdiff-interface.c
index 4d20069302..ef557dc4e6 100644
--- a/xdiff-interface.c
+++ b/xdiff-interface.c
@@ -31,7 +31,7 @@ static int xdiff_out_hunk(void *priv_,
 	return 0;
 }
 
-static void consume_one(void *priv_, char *s, unsigned long size)
+static int consume_one(void *priv_, char *s, unsigned long size)
 {
 	struct xdiff_emit_state *priv = priv_;
 	char *ep;
@@ -43,6 +43,7 @@ static void consume_one(void *priv_, char *s, unsigned long size)
 		size -= this_size;
 		s += this_size;
 	}
+	return 0;
 }
 
 static int xdiff_outf(void *priv_, mmbuffer_t *mb, int nbuf)
@@ -115,10 +116,11 @@ int xdi_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp, xdemitconf_t co
 	return xdl_diff(&a, &b, xpp, xecfg, xecb);
 }
 
-void discard_hunk_line(void *priv,
-		       long ob, long on, long nb, long nn,
-		       const char *func, long funclen)
+int discard_hunk_line(void *priv,
+		      long ob, long on, long nb, long nn,
+		      const char *func, long funclen)
 {
+	return 0;
 }
 
 int xdi_diff_outf(mmfile_t *mf1, mmfile_t *mf2,
diff --git a/xdiff-interface.h b/xdiff-interface.h
index 93df26900c..1b27d6104c 100644
--- a/xdiff-interface.h
+++ b/xdiff-interface.h
@@ -11,11 +11,26 @@
  */
 #define MAX_XDIFF_SIZE (1024UL * 1024 * 1023)
 
-typedef void (*xdiff_emit_line_fn)(void *, char *, unsigned long);
-typedef void (*xdiff_emit_hunk_fn)(void *data,
-				   long old_begin, long old_nr,
-				   long new_begin, long new_nr,
-				   const char *func, long funclen);
+/*
+ * The xdiff_emit_{line,hunk}_fn consumers can return -1 to abort
+ * early, or 0 to continue processing. Note that doing so is an
+ * all-or-nothing affair, as returning -1 will return all the way to
+ * the top-level, e.g. the xdi_diff_outf() call to generate the diff.
+ *
+ * Thus returning -1 from a hunk header callback means you won't be
+ * getting any more hunks, or diffs, and likewise returning from a
+ * line callback means you won't be getting anymore lines.
+ *
+ * We may extend the interface in the future to understand other more
+ * 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.
+ */
+typedef int (*xdiff_emit_line_fn)(void *, char *, unsigned long);
+typedef int (*xdiff_emit_hunk_fn)(void *data,
+				  long old_begin, long old_nr,
+				  long new_begin, long new_nr,
+				  const char *func, long funclen);
 
 int xdi_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp, xdemitconf_t const *xecfg, xdemitcb_t *ecb);
 int xdi_diff_outf(mmfile_t *mf1, mmfile_t *mf2,
@@ -36,9 +51,9 @@ extern int git_xmerge_style;
  * Can be used as a no-op hunk_fn for xdi_diff_outf(), since a NULL
  * one just sends the hunk line to the line_fn callback).
  */
-void discard_hunk_line(void *priv,
-		       long ob, long on, long nb, long nn,
-		       const char *func, long funclen);
+int discard_hunk_line(void *priv,
+		      long ob, long on, long nb, long nn,
+		      const char *func, long funclen);
 
 /*
  * Compare the strings l1 with l2 which are of size s1 and s2 respectively.
-- 
2.30.0.284.gd98b1dd5eaa7