[RFC] diff: add diff.<driver>.process for external hunk providers

Documentation/config/diff.adoc

@@ -218,6 +218,11 @@ endif::git-diff[]
 	Set this option to `true` to make the diff driver cache the text
 	conversion outputs.  See linkgit:gitattributes[5] for details.
 
+`diff.<driver>.process`::
+	The command to run as a long-running diff process that
+	provides hunks to Git's diff pipeline.
+	See linkgit:gitattributes[5] for details.
+
 `diff.indentHeuristic`::
 	Set this option to `false` to disable the default heuristics
 	that shift diff hunk boundaries to make patches easier to read.

Documentation/diff-algorithm-option.adoc

@@ -18,3 +18,6 @@
 For instance, if you configured the `diff.algorithm` variable to a
 non-default value and want to use the default one, then you
 have to use `--diff-algorithm=default` option.
++
+If you explicitly choose a diff algorithm, it also bypasses
+`diff.<driver>.process` (see linkgit:gitattributes[5]).

Documentation/diff-options.adoc

@@ -825,7 +825,9 @@ endif::git-format-patch[]
 	to use this option with linkgit:git-log[1] and friends.
 
 `--no-ext-diff`::
-	Disallow external diff drivers.
+	Disallow external diff helpers, including
+	`diff.<driver>.command` and `diff.<driver>.process`
+	(see linkgit:gitattributes[5]).
 
 `--textconv`::
 `--no-textconv`::

Documentation/gitattributes.adoc

@@ -821,6 +821,145 @@ NOTE: If `diff.<name>.command` is defined for path with the
 (see above), and adding `diff.<name>.algorithm` has no effect, as the
 algorithm is not passed to the external diff driver.
 
+Using an external diff process
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If `diff.<name>.process` is defined, Git sends the old and new file
+content to an external tool and receives back a list of changed
+regions (pairs of line ranges in the old and new file).  Git uses
+these instead of its builtin diff algorithm, but still controls
+all output formatting, so features like word diff, function context,
+color, and blame work normally.  This is achieved by using the
+long-running process protocol (described in
+Documentation/technical/long-running-process-protocol.adoc).
+Unlike `diff.<name>.command`, which replaces Git's output entirely,
+the diff process feeds results back into the standard pipeline.
+
+First, in `.gitattributes`, assign the `diff` attribute for paths.
+
+------------------------
+*.c diff=cdiff
+------------------------
+
+Then, define a "diff.<name>.process" configuration to specify
+the diff process command.
+
+----------------------------------------------------------------
+[diff "cdiff"]
+  process = /path/to/diff-process-tool
+----------------------------------------------------------------
+
+When Git encounters the first file that needs to be diffed, it starts
+the process and performs the handshake.  In the handshake, the welcome
+message sent by Git is "git-diff-client", only version 1 is supported,
+and the supported capability is "hunks" (the changed regions
+described below).
+
+For each file, Git sends a list of "key=value" pairs terminated with
+a flush packet, followed by the old and new file content as packetized
+data, each terminated with a flush packet.  The pathname is relative
+to the repository root.  When `diff.<name>.textconv` is also set,
+the tool receives the textconv-transformed content rather than the
+raw blob.  Git does not send binary files to the diff process.
+
+-----------------------
+packet:          git> command=hunks
+packet:          git> pathname=path/file.c
+packet:          git> 0000
+packet:          git> OLD_CONTENT
+packet:          git> 0000
+packet:          git> NEW_CONTENT
+packet:          git> 0000
+-----------------------
+
+The tool is expected to respond with zero or more hunk lines,
+a flush packet, and a status packet terminated with a flush packet.
+Each hunk line has the form:
+
+  `hunk <old_start> <old_count> <new_start> <new_count>`
+
+where `<old_start>` and `<old_count>` identify a range of lines in
+the old file, and `<new_start>` and `<new_count>` identify the
+replacement range in the new file.  Start values are 1-based and
+counts are non-negative.  Ranges must not extend beyond the end of
+the file.  For example, `hunk 3 2 3 4` means that 2 lines starting
+at line 3 in the old file were replaced by 4 lines starting at
+line 3 in the new file.  An `<old_count>` of 0 means no lines were
+removed (pure insertion); a `<new_count>` of 0 means no lines were
+added (pure deletion).
+
+Lines are delimited by newlines.  A file `"foo\nbar\n"` and a
+file `"foo\nbar"` both have 2 lines.
+
+Hunks must be listed in order and must not overlap.  Any line
+not covered by a hunk is treated as unchanged, so the total
+number of unchanged lines must be the same on both sides.
+For example, if the old file has 10 lines and the hunks cover
+4 of them (`old_count` values summing to 4), then 6 old lines
+are unchanged.  The new file must also have exactly 6 lines
+not covered by hunks, so the `new_count` values must sum to
+`new_file_lines - 6`.
+
+-----------------------
+packet:          git< hunk 1 3 1 5
+packet:          git< hunk 10 2 12 2
+packet:          git< 0000
+packet:          git< status=success
+packet:          git< 0000
+-----------------------
+
+If the tool responds with hunks and "success", Git marks those lines
+as changed and feeds them into the standard diff pipeline.  Patch
+output features (word diff, function context, color) work normally.
+Note that `--stat` and other summary formats use their own diff path
+and are not affected by the diff process.
+
+If no hunk lines precede the flush, followed by "success", Git
+treats the files as having no changes: `git diff` produces no output
+and `git blame` skips the commit, attributing lines to earlier commits.
+
+-----------------------
+packet:          git< 0000
+packet:          git< status=success
+packet:          git< 0000
+-----------------------
+
+If the tool returns invalid hunks (out of bounds, overlapping), Git
+silently falls back to the builtin diff algorithm.
+
+In case the tool cannot or does not want to process the content,
+it is expected to respond with an "error" status.  Git warns and
+falls back to the builtin diff algorithm for this file.  The tool
+remains available for subsequent files.
+
+-----------------------
+packet:          git< 0000
+packet:          git< status=error
+packet:          git< 0000
+-----------------------
+
+In case the tool cannot or does not want to process the content as
+well as any future content for the lifetime of the Git process, it
+is expected to respond with an "abort" status.  Git silently falls
+back to the builtin diff algorithm for this file and does not send
+further requests to the tool.
+
+-----------------------
+packet:          git< 0000
+packet:          git< status=abort
+packet:          git< 0000
+-----------------------
+
+If the tool dies during the communication or does not adhere to the
+protocol then Git will stop the process and fall back to the builtin
+diff algorithm.  Git warns once and does not restart the process for
+subsequent files.
+
+Tools should ignore unknown keys in the per-file request to remain
+forward-compatible.  Future versions of Git may send additional
+`command=` values; tools that receive an unrecognized command should
+respond with `status=error` rather than terminating.
+
 Defining a custom hunk-header
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

Makefile

@@ -815,6 +815,7 @@ TEST_BUILTINS_OBJS += test-csprng.o
 TEST_BUILTINS_OBJS += test-date.o
 TEST_BUILTINS_OBJS += test-delete-gpgsig.o
 TEST_BUILTINS_OBJS += test-delta.o
+TEST_BUILTINS_OBJS += test-diff-process-backend.o
 TEST_BUILTINS_OBJS += test-dir-iterator.o
 TEST_BUILTINS_OBJS += test-drop-caches.o
 TEST_BUILTINS_OBJS += test-dump-cache-tree.o
@@ -1142,6 +1143,7 @@ LIB_OBJS += diff-delta.o
 LIB_OBJS += diff-merges.o
 LIB_OBJS += diff-lib.o
 LIB_OBJS += diff-no-index.o
+LIB_OBJS += diff-process.o
 LIB_OBJS += diff.o
 LIB_OBJS += diffcore-break.o
 LIB_OBJS += diffcore-delta.o

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);

builtin/log.c

@@ -2213,6 +2213,13 @@ int cmd_format_patch(int argc,
 	if (argc > 1)
 		die(_("unrecognized argument: %s"), argv[1]);
 
+	/*
+	 * Disable diff.<driver>.process so that patches generated by
+	 * format-patch are always based on the builtin diff algorithm
+	 * and can be applied reliably.
+	 */
+	rev.diffopt.flags.no_diff_process = 1;
+
 	if (rev.diffopt.output_format & DIFF_FORMAT_NAME)
 		die(_("--name-only does not make sense"));
 	if (rev.diffopt.output_format & DIFF_FORMAT_NAME_STATUS)