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[PATCH v3 4/4] convert: add "status=delayed" to filter process protocol




Some `clean` / `smudge` filters might require a significant amount of
time to process a single blob. During this process the Git checkout
operation is blocked and Git needs to wait until the filter is done to
continue with the checkout.

Teach the filter process protocol (introduced in edcc858) to accept the
status "delayed" as response to a filter request. Upon this response Git
continues with the checkout operation. After the checkout operation Git
calls "finish_delayed_checkout" which queries the filter for remaining
blobs. If the filter is still working on the completion, then the filter
is expected to block. If the filter has completed all remaining blobs
then an empty response is expected.

Git has a multiple code paths that checkout a blob. Support delayed
checkouts only in `clone` (in unpack-trees.c) and `checkout` operations.

Signed-off-by: Lars Schneider <larsxschneider@xxxxxxxxx>
---
 Documentation/gitattributes.txt |  73 +++++++++++++-
 builtin/checkout.c              |   3 +
 cache.h                         |  37 ++++++-
 convert.c                       | 150 ++++++++++++++++++++++++----
 convert.h                       |   5 +
 entry.c                         | 124 +++++++++++++++++++++++-
 t/t0021-conversion.sh           |  73 ++++++++++++++
 t/t0021/rot13-filter.pl         | 210 ++++++++++++++++++++++++++++------------
 unpack-trees.c                  |   2 +
 9 files changed, 587 insertions(+), 90 deletions(-)

diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index e0b66c1220..329baa945f 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -425,8 +425,8 @@ packet:          git< capability=clean
 packet:          git< capability=smudge
 packet:          git< 0000
 ------------------------
-Supported filter capabilities in version 2 are "clean" and
-"smudge".
+Supported filter capabilities in version 2 are "clean", "smudge",
+and "delay".
 
 Afterwards Git sends a list of "key=value" pairs terminated with
 a flush packet. The list will contain at least the filter command
@@ -512,12 +512,77 @@ the protocol then Git will stop the filter process and restart it
 with the next file that needs to be processed. Depending on the
 `filter.<driver>.required` flag Git will interpret that as error.
 
-After the filter has processed a blob it is expected to wait for
-the next "key=value" list containing a command. Git will close
+After the filter has processed a command it is expected to wait for
+a "key=value" list containing the next command. Git will close
 the command pipe on exit. The filter is expected to detect EOF
 and exit gracefully on its own. Git will wait until the filter
 process has stopped.
 
+Delay
+^^^^^
+
+If the filter supports the "delay" capability, then Git can send the
+flag "delay-able" after the filter command and pathname. This flag
+denotes that the filter can delay filtering the current blob (e.g. to
+compensate network latencies) by responding with no content but with
+the status "delayed" and a flush packet. Git will answer with a
+"delay-id", a number that identifies the blob, and a flush packet. The
+filter acknowledges this number with a "success" status and a flush
+packet.
+------------------------
+packet:          git> command=smudge
+packet:          git> pathname=path/testfile.dat
+packet:          git> delay-able=1
+packet:          git> 0000
+packet:          git> CONTENT
+packet:          git> 0000
+packet:          git< status=delayed
+packet:          git< 0000
+packet:          git> delay-id=1
+packet:          git> 0000
+packet:          git< status=success
+packet:          git< 0000
+------------------------
+
+If the filter supports the "delay" capability then it must support the
+"list_available_blobs" command. If Git sends this command, then the
+filter is expected to return a list of "delay_ids" of blobs that are
+available. The list must be terminated with a flush packet followed
+by a "success" status that is also terminated with a flush packet. If
+no blobs for the delayed paths are available, yet, then the filter is
+expected to block the response until at least one blob becomes
+available. The filter can tell Git that it has no more delayed blobs
+by sending an empty list.
+------------------------
+packet:          git> command=list_available_blobs
+packet:          git> 0000
+packet:          git< 7
+packet:          git< 13
+packet:          git< 0000
+packet:          git< status=success
+packet:          git< 0000
+------------------------
+
+After Git received the "delay_ids", it will request the corresponding
+blobs again. These requests contain a "delay-id" and an empty content
+section. The filter is expected to respond with the smudged content
+in the usual way as explained above.
+------------------------
+packet:          git> command=smudge
+packet:          git> pathname=test-delay10.a
+packet:          git> delay-id=0
+packet:          git> 0000
+packet:          git> 0000  # empty content!
+packet:          git< status=success
+packet:          git< 0000
+packet:          git< SMUDGED_CONTENT
+packet:          git< 0000
+packet:          git< 0000
+------------------------
+
+Example
+^^^^^^^
+
 A long running filter demo implementation can be found in
 `contrib/long-running-filter/example.pl` located in the Git
 core repository. If you develop your own long running filter
diff --git a/builtin/checkout.c b/builtin/checkout.c
index f174f50303..e0a0bc92d4 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -355,6 +355,8 @@ static int checkout_paths(const struct checkout_opts *opts,
 	state.force = 1;
 	state.refresh_cache = 1;
 	state.istate = &the_index;
+
+	enable_delayed_checkout(&state);
 	for (pos = 0; pos < active_nr; pos++) {
 		struct cache_entry *ce = active_cache[pos];
 		if (ce->ce_flags & CE_MATCHED) {
@@ -369,6 +371,7 @@ static int checkout_paths(const struct checkout_opts *opts,
 			pos = skip_same_name(ce, pos) - 1;
 		}
 	}
+	errs |= finish_delayed_checkout(&state);
 
 	if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
 		die(_("unable to write new index file"));
diff --git a/cache.h b/cache.h
index 61fc86e6d7..46076279cf 100644
--- a/cache.h
+++ b/cache.h
@@ -1421,19 +1421,54 @@ const char *show_ident_date(const struct ident_split *id,
  */
 extern int ident_cmp(const struct ident_split *, const struct ident_split *);
 
+enum ce_delay_state {
+	CE_DELAY_DISABLED = 0,
+	CE_DELAY_AVAILABLE = 1,
+	CE_DELAY_APPLIED = 2,
+	CE_DELAY_RETRY = 3
+};
+
+struct delayed_checkout {
+	enum ce_delay_state state;
+	/* The value of "delay_id" has different meaning depending on the
+	 * "state" variable:
+	 *   - CE_DELAY_DISABLED  => "delay_id" not used.
+	 *   - CE_DELAY_AVAILABLE => "delay_id" is available to be presented
+	 *                           to the filter in case the filter wants to
+	 *                           delay the response of a blob.
+	 *   - CE_DELAY_APPLIED   => "delay_id" was presented to and applied by
+	 *                           the filter for a blob. The corresponding
+	 *                           cache entry in stored in the "entries"
+	 *                           array under the index "delay_id".
+	 *   - CE_DELAY_RETRY     => Git requests a blob from the filter that
+	 *                           was previously delayed using the "delay_id".
+	 */
+	int delay_id;
+	/* List of filter drivers that have delayed blobs. */
+	struct string_list filters;
+	/* Array of cache entries that have been delayed. */
+	struct cache_entry **entries;
+	int entries_nr;
+	int entries_alloc;
+};
+
 struct checkout {
 	struct index_state *istate;
 	const char *base_dir;
+	struct delayed_checkout *delayed_checkout;
 	int base_dir_len;
 	unsigned force:1,
 		 quiet:1,
 		 not_new:1,
 		 refresh_cache:1;
 };
-#define CHECKOUT_INIT { NULL, "" }
+#define CHECKOUT_INIT { NULL, "", NULL }
+
 
 #define TEMPORARY_FILENAME_LENGTH 25
 extern int checkout_entry(struct cache_entry *ce, const struct checkout *state, char *topath);
+extern void enable_delayed_checkout(struct checkout *state);
+extern int finish_delayed_checkout(struct checkout *state);
 
 struct cache_def {
 	struct strbuf path;
diff --git a/convert.c b/convert.c
index 4e17e45ed2..0d8fa0f833 100644
--- a/convert.c
+++ b/convert.c
@@ -495,6 +495,7 @@ static int apply_single_file_filter(const char *path, const char *src, size_t le
 
 #define CAP_CLEAN    (1u<<0)
 #define CAP_SMUDGE   (1u<<1)
+#define CAP_DELAY    (1u<<2)
 
 struct cmd2process {
 	struct hashmap_entry ent; /* must be the first member! */
@@ -632,7 +633,8 @@ static struct cmd2process *start_multi_file_filter(struct hashmap *hashmap, cons
 	if (err)
 		goto done;
 
-	err = packet_write_list(process->in, "capability=clean", "capability=smudge", NULL);
+	err = packet_write_list(process->in,
+		"capability=clean", "capability=smudge", "capability=delay", NULL);
 
 	for (;;) {
 		cap_buf = packet_read_line(process->out, NULL);
@@ -648,6 +650,8 @@ static struct cmd2process *start_multi_file_filter(struct hashmap *hashmap, cons
 			entry->supported_capabilities |= CAP_CLEAN;
 		} else if (!strcmp(cap_name, "smudge")) {
 			entry->supported_capabilities |= CAP_SMUDGE;
+		} else if (!strcmp(cap_name, "delay")) {
+			entry->supported_capabilities |= CAP_DELAY;
 		} else {
 			warning(
 				"external filter '%s' requested unsupported filter capability '%s'",
@@ -673,9 +677,11 @@ static struct cmd2process *start_multi_file_filter(struct hashmap *hashmap, cons
 
 static int apply_multi_file_filter(const char *path, const char *src, size_t len,
 				   int fd, struct strbuf *dst, const char *cmd,
-				   const unsigned int wanted_capability)
+				   const unsigned int wanted_capability,
+				   struct delayed_checkout *dco)
 {
 	int err;
+	int is_delay_available = 0;
 	struct cmd2process *entry;
 	struct child_process *process;
 	struct strbuf nbuf = STRBUF_INIT;
@@ -726,6 +732,24 @@ static int apply_multi_file_filter(const char *path, const char *src, size_t len
 	if (err)
 		goto done;
 
+	if (CAP_DELAY & entry->supported_capabilities && dco) {
+		switch (dco->state) {
+		case CE_DELAY_AVAILABLE:
+			is_delay_available = 1;
+			err = packet_write_fmt_gently(
+				process->in, "delay-able=1\n");
+			break;
+		case CE_DELAY_RETRY:
+			err = packet_write_fmt_gently(
+				process->in, "delay-id=%i\n", dco->delay_id);
+			break;
+		default:
+			break;
+		}
+		if (err)
+			goto done;
+	}
+
 	err = packet_flush_gently(process->in);
 	if (err)
 		goto done;
@@ -738,13 +762,27 @@ static int apply_multi_file_filter(const char *path, const char *src, size_t len
 		goto done;
 
 	read_multi_file_filter_status(process->out, &filter_status);
-	err = strcmp(filter_status.buf, "success");
-	if (err)
-		goto done;
+	if (is_delay_available && !strcmp(filter_status.buf, "delayed")) {
+		/* The filter wants to delay the response. Send it a delay id. */
+		err = packet_write_fmt_gently(
+			process->in, "delay-id=%i\n", dco->delay_id);
+		if (err)
+			goto done;
+		err = packet_flush_gently(process->in);
+		if (err)
+			goto done;
+		string_list_insert(&dco->filters, cmd);
+		dco->state = CE_DELAY_APPLIED;
+	} else {
+		/* The filter got the blob and wants to send us a response. */
+		err = strcmp(filter_status.buf, "success");
+		if (err)
+			goto done;
 
-	err = read_packetized_to_strbuf(process->out, &nbuf) < 0;
-	if (err)
-		goto done;
+		err = read_packetized_to_strbuf(process->out, &nbuf) < 0;
+		if (err)
+			goto done;
+	}
 
 	read_multi_file_filter_status(process->out, &filter_status);
 	err = strcmp(filter_status.buf, "success");
@@ -777,6 +815,74 @@ static int apply_multi_file_filter(const char *path, const char *src, size_t len
 	return !err;
 }
 
+
+int async_query_available_blobs(const char *cmd, unsigned long **delay_ids,
+				int *delay_ids_nr)
+{
+	int err;
+	char *line;
+	char *end;
+	struct cmd2process *entry;
+	struct child_process *process;
+	struct strbuf filter_status = STRBUF_INIT;
+	unsigned long delay_id;
+	int delay_ids_alloc = 0;
+	*delay_ids_nr = 0;
+
+	entry = find_multi_file_filter_entry(&cmd_process_map, cmd);
+	if (!entry) {
+		error("external filter '%s' is not available anymore although "
+		      "not all paths have been filtered", cmd);
+		return 0;
+	}
+	process = &entry->process;
+	sigchain_push(SIGPIPE, SIG_IGN);
+
+	err = packet_write_fmt_gently(
+		process->in, "command=list_available_blobs\n");
+	if (err)
+		goto done;
+
+	err = packet_flush_gently(process->in);
+	if (err)
+		goto done;
+
+	for (;;) {
+		line = packet_read_line(process->out, NULL);
+		if (!line)
+			break;
+		delay_id = strtoul(line, &end, 10);
+		err = (line == end);
+		if (err) {
+			error("invalid delay id '%s'", line);
+			goto done;
+		}
+		ALLOC_GROW(*delay_ids, *delay_ids_nr+1, delay_ids_alloc);
+		(*delay_ids)[(*delay_ids_nr)++] = delay_id;
+	}
+
+	read_multi_file_filter_status(process->out, &filter_status);
+	err = strcmp(filter_status.buf, "success");
+
+done:
+	sigchain_pop(SIGPIPE);
+
+	if (err || errno == EPIPE) {
+		if (!strcmp(filter_status.buf, "error")) {
+			/* The filter signaled a problem with the file. */
+		} else {
+			/*
+			 * Something went wrong with the protocol filter.
+			 * Force shutdown and restart if another blob requires
+			 * filtering.
+			 */
+			error("external filter '%s' failed", cmd);
+			kill_multi_file_filter(&cmd_process_map, entry);
+		}
+	}
+	return !err;
+}
+
 static struct convert_driver {
 	const char *name;
 	struct convert_driver *next;
@@ -788,7 +894,8 @@ static struct convert_driver {
 
 static int apply_filter(const char *path, const char *src, size_t len,
 			int fd, struct strbuf *dst, struct convert_driver *drv,
-			const unsigned int wanted_capability)
+			const unsigned int wanted_capability,
+			struct delayed_checkout *dco)
 {
 	const char *cmd = NULL;
 
@@ -806,7 +913,8 @@ static int apply_filter(const char *path, const char *src, size_t len,
 	if (cmd && *cmd)
 		return apply_single_file_filter(path, src, len, fd, dst, cmd);
 	else if (drv->process && *drv->process)
-		return apply_multi_file_filter(path, src, len, fd, dst, drv->process, wanted_capability);
+		return apply_multi_file_filter(path, src, len, fd, dst,
+			drv->process, wanted_capability, dco);
 
 	return 0;
 }
@@ -1152,7 +1260,7 @@ int would_convert_to_git_filter_fd(const char *path)
 	if (!ca.drv->required)
 		return 0;
 
-	return apply_filter(path, NULL, 0, -1, NULL, ca.drv, CAP_CLEAN);
+	return apply_filter(path, NULL, 0, -1, NULL, ca.drv, CAP_CLEAN, NULL);
 }
 
 const char *get_convert_attr_ascii(const char *path)
@@ -1189,7 +1297,7 @@ int convert_to_git(const char *path, const char *src, size_t len,
 
 	convert_attrs(&ca, path);
 
-	ret |= apply_filter(path, src, len, -1, dst, ca.drv, CAP_CLEAN);
+	ret |= apply_filter(path, src, len, -1, dst, ca.drv, CAP_CLEAN, NULL);
 	if (!ret && ca.drv && ca.drv->required)
 		die("%s: clean filter '%s' failed", path, ca.drv->name);
 
@@ -1214,7 +1322,7 @@ void convert_to_git_filter_fd(const char *path, int fd, struct strbuf *dst,
 	assert(ca.drv);
 	assert(ca.drv->clean || ca.drv->process);
 
-	if (!apply_filter(path, NULL, 0, fd, dst, ca.drv, CAP_CLEAN))
+	if (!apply_filter(path, NULL, 0, fd, dst, ca.drv, CAP_CLEAN, NULL))
 		die("%s: clean filter '%s' failed", path, ca.drv->name);
 
 	crlf_to_git(path, dst->buf, dst->len, dst, ca.crlf_action, checksafe);
@@ -1223,7 +1331,7 @@ void convert_to_git_filter_fd(const char *path, int fd, struct strbuf *dst,
 
 static int convert_to_working_tree_internal(const char *path, const char *src,
 					    size_t len, struct strbuf *dst,
-					    int normalizing)
+					    int normalizing, struct delayed_checkout *dco)
 {
 	int ret = 0, ret_filter = 0;
 	struct conv_attrs ca;
@@ -1248,21 +1356,29 @@ static int convert_to_working_tree_internal(const char *path, const char *src,
 		}
 	}
 
-	ret_filter = apply_filter(path, src, len, -1, dst, ca.drv, CAP_SMUDGE);
+	ret_filter = apply_filter(
+		path, src, len, -1, dst, ca.drv, CAP_SMUDGE, dco);
 	if (!ret_filter && ca.drv && ca.drv->required)
 		die("%s: smudge filter %s failed", path, ca.drv->name);
 
 	return ret | ret_filter;
 }
 
+int async_convert_to_working_tree(const char *path, const char *src,
+				  size_t len, struct strbuf *dst,
+				  void *dco)
+{
+	return convert_to_working_tree_internal(path, src, len, dst, 0, dco);
+}
+
 int convert_to_working_tree(const char *path, const char *src, size_t len, struct strbuf *dst)
 {
-	return convert_to_working_tree_internal(path, src, len, dst, 0);
+	return convert_to_working_tree_internal(path, src, len, dst, 0, NULL);
 }
 
 int renormalize_buffer(const char *path, const char *src, size_t len, struct strbuf *dst)
 {
-	int ret = convert_to_working_tree_internal(path, src, len, dst, 1);
+	int ret = convert_to_working_tree_internal(path, src, len, dst, 1, NULL);
 	if (ret) {
 		src = dst->buf;
 		len = dst->len;
diff --git a/convert.h b/convert.h
index 82871a11d5..da6c702090 100644
--- a/convert.h
+++ b/convert.h
@@ -42,6 +42,11 @@ extern int convert_to_git(const char *path, const char *src, size_t len,
 			  struct strbuf *dst, enum safe_crlf checksafe);
 extern int convert_to_working_tree(const char *path, const char *src,
 				   size_t len, struct strbuf *dst);
+extern int async_convert_to_working_tree(const char *path, const char *src,
+					 size_t len, struct strbuf *dst,
+					 void *dco);
+extern int async_query_available_blobs(const char *cmd, unsigned long **delay_ids,
+				       int *delay_ids_nr);
 extern int renormalize_buffer(const char *path, const char *src, size_t len,
 			      struct strbuf *dst);
 static inline int would_convert_to_git(const char *path)
diff --git a/entry.c b/entry.c
index c6eea240b6..963e94dee8 100644
--- a/entry.c
+++ b/entry.c
@@ -136,6 +136,86 @@ static int streaming_write_entry(const struct cache_entry *ce, char *path,
 	return result;
 }
 
+void enable_delayed_checkout(struct checkout *state)
+{
+	if (!state->delayed_checkout) {
+		state->delayed_checkout = xmalloc(sizeof(*state->delayed_checkout));
+		state->delayed_checkout->entries_nr = 0;
+		state->delayed_checkout->entries_alloc = 0;
+		state->delayed_checkout->delay_id = -1;
+		state->delayed_checkout->state = CE_DELAY_AVAILABLE;
+		ALLOC_ARRAY(state->delayed_checkout->entries, 0);
+		string_list_init(&state->delayed_checkout->filters, 0);
+	}
+}
+
+int finish_delayed_checkout(struct checkout *state)
+{
+	int errs = 0;
+	struct string_list_item *filter;
+	struct delayed_checkout *dco = state->delayed_checkout;
+
+	if (!state->delayed_checkout) {
+		return errs;
+	}
+
+	while (dco->entries_nr > 0 && dco->filters.nr > 0) {
+		for_each_string_list_item(filter, &dco->filters) {
+			int i;
+			int delay_ids_nr;
+			unsigned long *delay_ids;
+			ALLOC_ARRAY(delay_ids, 0);
+			if (!async_query_available_blobs(
+				filter->string, &delay_ids, &delay_ids_nr)) {
+				/* Filter reported an error */
+				errs = 1;
+				filter->string = "";
+				free(delay_ids);
+				continue;
+			}
+			if (delay_ids_nr <= 0) {
+				/* Filter responded with no entries. That means
+				   the filter is done and we can remove the
+				   filter from the list
+				   (see "string_list_remove_empty_items" call
+				   below).
+				*/
+				filter->string = "";
+				free(delay_ids);
+				continue;
+			}
+			for (i = 0; i < delay_ids_nr; i++) {
+				struct cache_entry* ce;
+				unsigned long delay_id = delay_ids[i];
+				assert(delay_id >= 0 && delay_id < dco->entries_nr);
+				ce = dco->entries[delay_id];
+				dco->entries[delay_id] = NULL;
+				dco->delay_id = delay_id;
+				dco->state = CE_DELAY_RETRY;
+				/* Shrink entries array as much as possible */
+				while (
+					dco->entries_nr > 0 &&
+					delay_id == dco->entries_nr - 1 &&
+					!dco->entries[i]
+				) {
+					delay_id--;
+					dco->entries_nr--;
+				}
+				errs |= (ce ? checkout_entry(ce, state, NULL) : 1);
+			}
+			free(delay_ids);
+		}
+		string_list_remove_empty_items(&dco->filters, 0);
+	}
+
+	string_list_clear(&dco->filters, 0);
+	free(dco->entries);
+	free(dco);
+	state->delayed_checkout = NULL;
+
+	return errs;
+}
+
 static int write_entry(struct cache_entry *ce,
 		       char *path, const struct checkout *state, int to_tempfile)
 {
@@ -177,11 +257,45 @@ static int write_entry(struct cache_entry *ce,
 		/*
 		 * Convert from git internal format to working tree format
 		 */
-		if (ce_mode_s_ifmt == S_IFREG &&
-		    convert_to_working_tree(ce->name, new, size, &buf)) {
-			free(new);
-			new = strbuf_detach(&buf, &newsize);
-			size = newsize;
+		if (ce_mode_s_ifmt == S_IFREG) {
+			struct delayed_checkout *dco = state->delayed_checkout;
+			if (dco && dco->state != CE_DELAY_DISABLED) {
+				switch (dco->state) {
+				case CE_DELAY_AVAILABLE:
+					dco->delay_id = dco->entries_nr; break;
+				case CE_DELAY_RETRY:
+					new = NULL; size = 0; break;
+				default: break;
+				}
+				assert(dco->delay_id >= 0);
+				ret = async_convert_to_working_tree(
+					ce->name, new, size, &buf, dco);
+				if (ret && dco->state == CE_DELAY_APPLIED) {
+					assert(dco->delay_id == dco->entries_nr);
+					free(new);
+					dco->entries_nr++;
+					ALLOC_GROW(
+						dco->entries, dco->entries_nr,
+						dco->entries_alloc);
+					dco->entries[dco->delay_id] = ce;
+					dco->state = CE_DELAY_AVAILABLE;
+					dco->delay_id = -1;
+					goto finish;
+				}
+			} else
+				ret = convert_to_working_tree(
+					ce->name, new, size, &buf);
+
+			if (ret) {
+				free(new);
+				new = strbuf_detach(&buf, &newsize);
+				size = newsize;
+			}
+			/*
+			 * No "else" here as errors from convert are OK at this
+			 * point. If the error would have been fatal (e.g.
+			 * filter is required), then we would have died already.
+			 */
 		}
 
 		fd = open_output_fd(path, ce, to_tempfile);
diff --git a/t/t0021-conversion.sh b/t/t0021-conversion.sh
index 0c04d346a1..21d4cd9453 100755
--- a/t/t0021-conversion.sh
+++ b/t/t0021-conversion.sh
@@ -701,4 +701,77 @@ test_expect_success PERL 'invalid process filter must fail (and not hang!)' '
 	)
 '
 
+test_expect_success PERL 'delayed checkout in process filter' '
+	test_config_global filter.a.process "rot13-filter.pl a.log clean smudge delay" &&
+	test_config_global filter.a.required true &&
+	test_config_global filter.b.process "rot13-filter.pl b.log clean smudge delay" &&
+	test_config_global filter.b.required true &&
+
+	rm -rf repo &&
+	mkdir repo &&
+	(
+		cd repo &&
+		git init &&
+		echo "*.a filter=a" >.gitattributes &&
+		echo "*.b filter=b" >>.gitattributes &&
+		cp "$TEST_ROOT/test.o" test.a &&
+		cp "$TEST_ROOT/test.o" test-delay10.a &&
+		cp "$TEST_ROOT/test.o" test-delay11.a &&
+		cp "$TEST_ROOT/test.o" test-delay20.a &&
+		cp "$TEST_ROOT/test.o" test-delay10.b &&
+		git add . &&
+		git commit -m "test commit 1"
+	) &&
+
+	S=$(file_size "$TEST_ROOT/test.o") &&
+	cat >a.exp <<-EOF &&
+		START
+		init handshake complete
+		IN: smudge test.a $S [OK] -- OUT: $S . [OK]
+		IN: smudge test-delay10.a $S [OK] -- [DELAYED]
+		IN: smudge test-delay11.a $S [OK] -- [DELAYED]
+		IN: smudge test-delay20.a $S [OK] -- [DELAYED]
+		IN: list_available_blobs test-delay10.a test-delay11.a [OK]
+		IN: smudge test-delay10.a 0 [OK] -- OUT: $S . [OK]
+		IN: smudge test-delay11.a 0 [OK] -- OUT: $S . [OK]
+		IN: list_available_blobs test-delay20.a [OK]
+		IN: smudge test-delay20.a 0 [OK] -- OUT: $S . [OK]
+		STOP
+	EOF
+	cat >b.exp <<-EOF &&
+		START
+		init handshake complete
+		IN: smudge test-delay10.b $S [OK] -- [DELAYED]
+		IN: list_available_blobs test-delay10.b [OK]
+		IN: smudge test-delay10.b 0 [OK] -- OUT: $S . [OK]
+		IN: list_available_blobs [OK]
+		STOP
+	EOF
+
+	rm -rf repo-cloned &&
+	filter_git clone repo repo-cloned &&
+	test_cmp_count a.exp repo-cloned/a.log &&
+	test_cmp_count b.exp repo-cloned/b.log &&
+
+	(
+		cd repo-cloned &&
+		test_cmp_committed_rot13 "$TEST_ROOT/test.o" test.a &&
+		test_cmp_committed_rot13 "$TEST_ROOT/test.o" test-delay10.a &&
+		test_cmp_committed_rot13 "$TEST_ROOT/test.o" test-delay11.a &&
+		test_cmp_committed_rot13 "$TEST_ROOT/test.o" test-delay20.a &&
+		test_cmp_committed_rot13 "$TEST_ROOT/test.o" test-delay10.b &&
+
+		rm *.a *.b &&
+		filter_git checkout . &&
+		test_cmp_count ../a.exp a.log &&
+		test_cmp_count ../b.exp b.log &&
+
+		test_cmp_committed_rot13 "$TEST_ROOT/test.o" test.a &&
+		test_cmp_committed_rot13 "$TEST_ROOT/test.o" test-delay10.a &&
+		test_cmp_committed_rot13 "$TEST_ROOT/test.o" test-delay11.a &&
+		test_cmp_committed_rot13 "$TEST_ROOT/test.o" test-delay20.a &&
+		test_cmp_committed_rot13 "$TEST_ROOT/test.o" test-delay10.b
+	)
+'
+
 test_done
diff --git a/t/t0021/rot13-filter.pl b/t/t0021/rot13-filter.pl
index 5e43faeec1..05024c6e1b 100644
--- a/t/t0021/rot13-filter.pl
+++ b/t/t0021/rot13-filter.pl
@@ -18,6 +18,11 @@
 #     operation then the filter signals that it cannot or does not want
 #     to process the file and any file after that is processed with the
 #     same command.
+# (5) If data with a pathname that is a key in the DELAY hash is
+#     requested (e.g. 'test-delay10.a') then the filter responds with
+#     a "delay" status and sets the "requested" field in the DELAY hash.
+#     The filter will signal the availability of this object after
+#     "count" (field in DELAY hash) "list_available_blobs" commands.
 #
 
 use strict;
@@ -30,6 +35,13 @@ my @capabilities            = @ARGV;
 
 open my $debug, ">>", $log_file or die "cannot open log file: $!";
 
+my %DELAY = (
+	'test-delay10.a' => { "requested" => 0, "count" => 1, "delay_id" => -1 },
+	'test-delay11.a' => { "requested" => 0, "count" => 1, "delay_id" => -1 },
+	'test-delay20.a' => { "requested" => 0, "count" => 2, "delay_id" => -1 },
+	'test-delay10.b' => { "requested" => 0, "count" => 1, "delay_id" => -1 },
+);
+
 sub rot13 {
 	my $str = shift;
 	$str =~ y/A-Za-z/N-ZA-Mn-za-m/;
@@ -66,7 +78,7 @@ sub packet_bin_read {
 
 sub packet_txt_read {
 	my ( $res, $buf ) = packet_bin_read();
-	unless ( $buf =~ s/\n$// ) {
+	unless ( $buf eq '' or $buf =~ s/\n$// ) {
 		die "A non-binary line MUST be terminated by an LF.";
 	}
 	return ( $res, $buf );
@@ -101,6 +113,7 @@ packet_flush();
 
 ( packet_txt_read() eq ( 0, "capability=clean" ) )  || die "bad capability";
 ( packet_txt_read() eq ( 0, "capability=smudge" ) ) || die "bad capability";
+( packet_txt_read() eq ( 0, "capability=delay" ) )  || die "bad capability";
 ( packet_bin_read() eq ( 1, "" ) )                  || die "bad capability end";
 
 foreach (@capabilities) {
@@ -115,84 +128,155 @@ while (1) {
 	print $debug "IN: $command";
 	$debug->flush();
 
-	my ($pathname) = packet_txt_read() =~ /^pathname=(.+)$/;
-	print $debug " $pathname";
-	$debug->flush();
+	if ( $command eq "list_available_blobs" ) {
+		# Flush
+		packet_bin_read();
 
-	if ( $pathname eq "" ) {
-		die "bad pathname '$pathname'";
-	}
+		foreach my $pathname (sort keys %DELAY) {
+			if ( $DELAY{$pathname}{"requested"} == 1 ) {
 
-	# Flush
-	packet_bin_read();
-
-	my $input = "";
-	{
-		binmode(STDIN);
-		my $buffer;
-		my $done = 0;
-		while ( !$done ) {
-			( $done, $buffer ) = packet_bin_read();
-			$input .= $buffer;
+				# die $pathname;
+				$DELAY{$pathname}{"count"} = $DELAY{$pathname}{"count"} - 1;
+				if ($DELAY{$pathname}{"count"} == 0 ) {
+					print $debug " $pathname";
+					# packet_txt_write($pathname);
+					packet_txt_write($DELAY{$pathname}{"delay_id"});
+				}
+			}
 		}
-		print $debug " " . length($input) . " [OK] -- ";
-		$debug->flush();
-	}
 
-	my $output;
-	if ( $pathname eq "error.r" or $pathname eq "abort.r" ) {
-		$output = "";
-	}
-	elsif ( $command eq "clean" and grep( /^clean$/, @capabilities ) ) {
-		$output = rot13($input);
-	}
-	elsif ( $command eq "smudge" and grep( /^smudge$/, @capabilities ) ) {
-		$output = rot13($input);
-	}
-	else {
-		die "bad command '$command'";
-	}
-
-	if ( $pathname eq "error.r" ) {
-		print $debug "[ERROR]\n";
-		$debug->flush();
-		packet_txt_write("status=error");
 		packet_flush();
-	}
-	elsif ( $pathname eq "abort.r" ) {
-		print $debug "[ABORT]\n";
+
+		print $debug " [OK]\n";
 		$debug->flush();
-		packet_txt_write("status=abort");
+		packet_txt_write("status=success");
 		packet_flush();
 	}
 	else {
-		packet_txt_write("status=success");
-		packet_flush();
+		my ($pathname) = packet_txt_read() =~ /^pathname=(.+)$/;
+		print $debug " $pathname";
+		$debug->flush();
+
+		if ( $pathname eq "" ) {
+			die "bad pathname '$pathname'";
+		}
+
+		# Read until flush
+		my ( $done, $buffer ) = packet_txt_read();
+		while ( $buffer ne '' ) {
+			if ( $buffer eq "delay-able=1" ) {
+				if ( exists $DELAY{$pathname} and $DELAY{$pathname}{"requested"} == 0 ) {
+					$DELAY{$pathname}{"requested"} = 1;
+				}
+			}
+			elsif ( $buffer =~ m/^delay-id=/ ) {
+				my ($delay_id) = $buffer =~ /^delay-id=(.+)$/;
+				if ( $DELAY{$pathname}{"delay_id"} != $delay_id ) {
+					die "unexpected delay-id for '$pathname'";
+				}
+			} else {
+				die "Unknown message '$buffer'";
+			}
+
+			( $done, $buffer ) = packet_txt_read();
+		}
 
-		if ( $pathname eq "${command}-write-fail.r" ) {
-			print $debug "[WRITE FAIL]\n";
+		my $input = "";
+		{
+			binmode(STDIN);
+			my $buffer;
+			my $done = 0;
+			while ( !$done ) {
+				( $done, $buffer ) = packet_bin_read();
+				$input .= $buffer;
+			}
+			print $debug " " . length($input) . " [OK] -- ";
 			$debug->flush();
-			die "${command} write error";
 		}
 
-		print $debug "OUT: " . length($output) . " ";
-		$debug->flush();
+		my $output;
+		if ( exists $DELAY{$pathname} and exists $DELAY{$pathname}{"output"} ) {
+			$output = $DELAY{$pathname}{"output"}
+		}
+		elsif ( $pathname eq "error.r" or $pathname eq "abort.r" ) {
+			$output = "";
+		}
+		elsif ( $command eq "clean" and grep( /^clean$/, @capabilities ) ) {
+			$output = rot13($input);
+		}
+		elsif ( $command eq "smudge" and grep( /^smudge$/, @capabilities ) ) {
+			$output = rot13($input);
+		}
+		else {
+			die "bad command '$command'";
+		}
+
+		if ( $pathname eq "error.r" ) {
+			print $debug "[ERROR]\n";
+			$debug->flush();
+			packet_txt_write("status=error");
+			packet_flush();
+		}
+		elsif ( $pathname eq "abort.r" ) {
+			print $debug "[ABORT]\n";
+			$debug->flush();
+			packet_txt_write("status=abort");
+			packet_flush();
+		}
+		elsif ( $command eq "smudge" and
+			exists $DELAY{$pathname} and
+			$DELAY{$pathname}{"requested"} == 1 and
+			$DELAY{$pathname}{"delay_id"} < 0
+		) {
+			print $debug "[DELAYED]\n";
+			$debug->flush();
+			packet_txt_write("status=delayed");
+			packet_flush();
+			my ( $done, $buffer ) = packet_txt_read();
+			if ( $buffer =~ m/^delay-id=/ ) {
+				my ($delay_id) = $buffer =~ /^delay-id=(.+)$/;
+				if ( $delay_id eq "" ) {
+					die "bad delay_id '$delay_id'";
+				}
+				$DELAY{$pathname}{"delay_id"} = $delay_id;
+				$DELAY{$pathname}{"output"} = $output;
+			}
 
-		while ( length($output) > 0 ) {
-			my $packet = substr( $output, 0, $MAX_PACKET_CONTENT_SIZE );
-			packet_bin_write($packet);
-			# dots represent the number of packets
-			print $debug ".";
-			if ( length($output) > $MAX_PACKET_CONTENT_SIZE ) {
-				$output = substr( $output, $MAX_PACKET_CONTENT_SIZE );
+			# Flush
+			packet_bin_read();
+
+			packet_txt_write("status=success");
+			packet_flush();
+		}
+		else {
+			packet_txt_write("status=success");
+			packet_flush();
+
+			if ( $pathname eq "${command}-write-fail.r" ) {
+				print $debug "[WRITE FAIL]\n";
+				$debug->flush();
+				die "${command} write error";
 			}
-			else {
-				$output = "";
+
+			print $debug "OUT: " . length($output) . " ";
+			$debug->flush();
+
+			while ( length($output) > 0 ) {
+				my $packet = substr( $output, 0, $MAX_PACKET_CONTENT_SIZE );
+				packet_bin_write($packet);
+				# dots represent the number of packets
+				print $debug ".";
+				if ( length($output) > $MAX_PACKET_CONTENT_SIZE ) {
+					$output = substr( $output, $MAX_PACKET_CONTENT_SIZE );
+				}
+				else {
+					$output = "";
+				}
 			}
+			packet_flush();
+			print $debug " [OK]\n";
+			$debug->flush();
+			packet_flush();
 		}
-		packet_flush();
-		print $debug " [OK]\n";
-		$debug->flush();
-		packet_flush();
 	}
 }
diff --git a/unpack-trees.c b/unpack-trees.c
index 3a8ee19fe8..9f50a417ec 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -301,6 +301,7 @@ static int check_updates(struct unpack_trees_options *o)
 	remove_marked_cache_entries(index);
 	remove_scheduled_dirs();
 
+	enable_delayed_checkout(&state);
 	for (i = 0; i < index->cache_nr; i++) {
 		struct cache_entry *ce = index->cache[i];
 
@@ -315,6 +316,7 @@ static int check_updates(struct unpack_trees_options *o)
 			}
 		}
 	}
+	errs |= finish_delayed_checkout(&state);
 	stop_progress(&progress);
 	if (o->update)
 		git_attr_set_direction(GIT_ATTR_CHECKIN, NULL);
-- 
2.12.2