builtin/diff-pairs: allow explicit diff queue flush

The diffs queued from git-diff-pairs(1) are flushed when stdin is
closed. To enable greater flexibility, allow control over when the diff
queue is flushed by writing a single NUL byte on stdin between input
file pairs. Diff output between flushes is separated by a single NUL
byte.

Signed-off-by: Justin Tobler <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jltobler

Documentation/git-diff-pairs.adoc

@@ -17,6 +17,10 @@ in the NUL-terminated raw output format as generated by commands such as `git
 diff-tree -z -r --raw`. By default, the outputted diffs are computed and shown
 in the patch format when stdin closes.
 
+A single NUL byte may be written to stdin between raw input lines to compute
+file pair diffs up to that point instead of waiting for stdin to close. A NUL
+byte is also written to the output to delimit between these batches of diffs.
+
 Usage of this command enables the traditional diff pipeline to be broken up
 into separate stages where `diff-pairs` acts as the output phase. Other
 commands, such as `diff-tree`, may serve as a frontend to compute the raw

builtin/diff-pairs.c

@@ -57,6 +57,7 @@ int cmd_diff_pairs(int argc, const char **argv, const char *prefix,
 	show_usage_with_options_if_asked(argc, argv, builtin_diff_pairs_usage, parseopts);
 
 	repo_config(repo, git_diff_basic_config, NULL);
+	revs.diffopt.no_free = 1;
 	revs.disable_stdin = 1;
 	revs.abbrev = 0;
 	revs.diff = 1;
@@ -106,6 +107,18 @@ int cmd_diff_pairs(int argc, const char **argv, const char *prefix,
 			break;
 
 		p = meta.buf;
+		if (!*p) {
+			diffcore_std(&revs.diffopt);
+			diff_flush(&revs.diffopt);
+			/*
+			 * When the diff queue is explicitly flushed, append a
+			 * NUL byte to separate batches of diffs.
+			 */
+			fputc('\0', revs.diffopt.file);
+			fflush(revs.diffopt.file);
+			continue;
+		}
+
 		if (*p != ':')
 			die(_("invalid raw diff input"));
 		p++;
@@ -179,6 +192,7 @@ int cmd_diff_pairs(int argc, const char **argv, const char *prefix,
 		}
 	}
 
+	revs.diffopt.no_free = 0;
 	diffcore_std(&revs.diffopt);
 	diff_flush(&revs.diffopt);
 	ret = diff_result_code(&revs);

t/t4070-diff-pairs.sh

@@ -78,4 +78,13 @@ test_expect_success 'diff-pairs does not support pathspec arguments' '
 	test_cmp expect err
 '
 
+test_expect_success 'diff-pairs explicit queue flush' '
+	git diff-tree -r -M -C -C -z base new >expect &&
+	printf "\0" >>expect &&
+	git diff-tree -r -M -C -C -z base new >>expect &&
+
+	git diff-pairs --raw -z <expect >actual &&
+	test_cmp expect actual
+'
+
 test_done