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Re: [PATCH v2 4/4] pack-write: rename *.idx file into place last (really!)

From
Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date
Sep 8, 2021, 09:18 UTC
Message-ID
<87v93bidhn.fsf@evledraar.gmail.com>
In-Reply-To
<87lf47kgkh.fsf@evledraar.gmail.com>
On Wed, Sep 08 2021, Ævar Arnfjörð Bjarmason wrote:
Show 28 quoted lines
> On Wed, Sep 08 2021, Ævar Arnfjörð Bjarmason wrote:
>
>> Follow-up a preceding commit (pack-write.c: rename `.idx` file into
>> place last, 2021-08-16)[1] and rename the *.idx file in-place after we
>> write the *.bitmap. The preceding commit fixed the issue of *.idx
>> being written before *.rev files, but did not do so for *.idx files.
>>
>> See 7cc8f971085 (pack-objects: implement bitmap writing, 2013-12-21)
>> for commentary at the time when *.bitmap was implemented about how
>> those files are written out, nothing in that commit contradicts what's
>> being done here.
>>
>> Note that the referenced earlier commit[1] is overly optimistic about
>> "clos[ing the] race", i.e. yes we'll now write the files in the right
>> order, but we might still race due to our sloppy use of fsync(). See
>> the thread at [2] for a rabbit hole of various discussions about
>> filesystem races in the face of doing and not doing fsync() (and if
>> doing fsync(), not doing it properly).
>
> Actually I think it's a bit worse than that, we will unconditionally
> fsync() the *.pack we write out, but in stage_tmp_packfiles() (the
> behavior pre-dates both this series and its parent, I just think my
> stage_tmp_packfiles() is easier to follow) we'll not write the *.idx
> file with fsync() since we won't pass WRITE_IDX_VERIFY.
>
> The same goes for *.rev (which oddly makes its fsync() conditional on
> WRITE_IDX_VERIFY), but not *.bitmap, which fsyncs unconditionally just
> like *.pack does.

I was confused here, we use fsync() with *.bitmap because there's no --verify mode for it, i.e. we'll always fsync() the *.rev files if we're actually writing them "for real", the non-fsync() path is when we're doing the "noop-write" to /dev/null with "index-pack --verify", i.e. writing to an FD without an associated "real" filename.

That we use WRITE_IDX_VERIFY to decide on that fsync() for the *.rev files, as opposed to WRITE_REV_VERIFY is a bug in 8ef50d9958 (pack-write.c: prepare to write 'pack-*.rev' files, 2021-01-25).

In practice if WRITE_REV_VERIFY is true then WRITE_IDX_VERIFY is true as well. See e37d0b8730 (builtin/index-pack.c: write reverse indexes, 2021-01-25) and its interaction with 68be2fea50 (receive-pack, fetch-pack: reject bogus pack that records objects twice, 2011-11-16). I.e. both flags are set if --verify is there.

So it's not a bug as far as the behavior of the program goes, it just makes for some very confusing code. Why should *.rev fsyncing be contingent on whether or not we want to fsync *.idx files on writes? It shouldn't, but it turns out those two flags always go hand in hand if we're writing *.rev at all.

But re the thread started at <patch-1.1-366ba928bd-20210908T010743Z-avarab@gmail.com> about how exactly we inspect these flags I think we should just take that change as-is for now, and refactor & fix this properly later.

Once write_rev_file{_order}() don't pretend to take the trie-state bitflags of WRITE_REV|WRITE_REV_VERIFY|WRITE_IDX_VERIFY but just take a boolean "int write_file" the whole callchain & confusion around "do we have more than 1 flag?" goes away.

I.e. something like the end-state shown by this patch, which I'm not submitting now because it conflicts with the other in-flight topic (and it's on top of some trivial but not-included whitespace changes in pack.h):

diff --git a/builtin/fast-import.c b/builtin/fast-import.c
index 20406f6775..69301fc044 100644
--- a/builtin/fast-import.c
+++ b/builtin/fast-import.c
@@ -778,7 +778,7 @@ static const char *create_index(void)
 		die("internal consistency error creating the index");
 
 	tmpfile = write_idx_file(NULL, idx, object_count, &pack_idx_opts,
-				 pack_data->hash);
+				 pack_data->hash, 0);
 	free(idx);
 	return tmpfile;
 }
diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 8336466865..b9c2a21216 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -1590,12 +1590,9 @@ static int git_index_pack_config(const char *k, const char *v, void *cb)
 		}
 		return 0;
 	}
-	if (!strcmp(k, "pack.writereverseindex")) {
-		if (git_config_bool(k, v))
-			opts->flags |= WRITE_REV;
-		else
-			opts->flags &= ~WRITE_REV;
-	}
+	if (!strcmp(k, "pack.writereverseindex"))
+		opts->write_rev = git_config_bool(k, v);
+
 	return git_default_config(k, v, cb);
 }
 
@@ -1713,7 +1710,7 @@ static void show_pack_info(int stat_only)
 
 int cmd_index_pack(int argc, const char **argv, const char *prefix)
 {
-	int i, fix_thin_pack = 0, verify = 0, stat_only = 0, rev_index;
+	int i, fix_thin_pack = 0, verify = 0, stat_only = 0;
 	const char *curr_index;
 	const char *curr_rev_index = NULL;
 	const char *index_name = NULL, *pack_name = NULL, *rev_index_name = NULL;
@@ -1747,10 +1744,8 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 	if (prefix && chdir(prefix))
 		die(_("Cannot come back to cwd"));
 
-	if (git_env_bool(GIT_TEST_WRITE_REV_INDEX, 0))
-		rev_index = 1;
-	else
-		rev_index = !!(opts.flags & (WRITE_REV_VERIFY | WRITE_REV));
+	opts.write_rev = git_env_bool(GIT_TEST_WRITE_REV_INDEX,
+				       opts.write_rev);
 
 	for (i = 1; i < argc; i++) {
 		const char *arg = argv[i];
@@ -1831,9 +1826,9 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 					die(_("unknown hash algorithm '%s'"), arg);
 				repo_set_hash_algo(the_repository, hash_algo);
 			} else if (!strcmp(arg, "--rev-index")) {
-				rev_index = 1;
+				opts.write_rev = 1;
 			} else if (!strcmp(arg, "--no-rev-index")) {
-				rev_index = 0;
+				opts.write_rev = 0;
 			} else
 				usage(index_pack_usage);
 			continue;
@@ -1855,9 +1850,7 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 	if (!index_name && pack_name)
 		index_name = derive_filename(pack_name, "pack", "idx", &index_name_buf);
 
-	opts.flags &= ~(WRITE_REV | WRITE_REV_VERIFY);
-	if (rev_index) {
-		opts.flags |= verify ? WRITE_REV_VERIFY : WRITE_REV;
+	if (opts.write_rev) {
 		if (index_name)
 			rev_index_name = derive_filename(index_name,
 							 "idx", "rev",
@@ -1868,10 +1861,9 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 		if (!index_name)
 			die(_("--verify with no packfile name given"));
 		read_idx_option(&opts, index_name);
-		opts.flags |= WRITE_IDX_VERIFY | WRITE_IDX_STRICT;
 	}
 	if (strict)
-		opts.flags |= WRITE_IDX_STRICT;
+		opts.write_idx_strict = 1;
 
 	if (HAVE_THREADS && !nr_threads) {
 		nr_threads = online_cpus();
@@ -1915,11 +1907,12 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 	ALLOC_ARRAY(idx_objects, nr_objects);
 	for (i = 0; i < nr_objects; i++)
 		idx_objects[i] = &objects[i].idx;
-	curr_index = write_idx_file(index_name, idx_objects, nr_objects, &opts, pack_hash);
-	if (rev_index)
+	curr_index = write_idx_file(index_name, idx_objects, nr_objects, &opts,
+				    pack_hash, verify);
+	if (opts.write_rev)
 		curr_rev_index = write_rev_file(rev_index_name, idx_objects,
 						nr_objects, pack_hash,
-						opts.flags);
+						verify);
 	free(idx_objects);
 
 	if (!verify)
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index df49f656b9..1dae01800f 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -3139,10 +3139,7 @@ static int git_pack_config(const char *k, const char *v, void *cb)
 		return 0;
 	}
 	if (!strcmp(k, "pack.writereverseindex")) {
-		if (git_config_bool(k, v))
-			pack_idx_opts.flags |= WRITE_REV;
-		else
-			pack_idx_opts.flags &= ~WRITE_REV;
+		pack_idx_opts.write_rev = git_config_bool(k, v);
 		return 0;
 	}
 	if (!strcmp(k, "uploadpack.blobpackfileuri")) {
@@ -4030,8 +4027,8 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 
 	reset_pack_idx_option(&pack_idx_opts);
 	git_config(git_pack_config, NULL);
-	if (git_env_bool(GIT_TEST_WRITE_REV_INDEX, 0))
-		pack_idx_opts.flags |= WRITE_REV;
+	pack_idx_opts.write_rev = git_env_bool(GIT_TEST_WRITE_REV_INDEX,
+					       pack_idx_opts.write_rev);
 
 	progress = isatty(2);
 	argc = parse_options(argc, argv, prefix, pack_objects_options,
diff --git a/midx.c b/midx.c
index 321c6fdd2f..63da740cd4 100644
--- a/midx.c
+++ b/midx.c
@@ -874,7 +874,7 @@ static void write_midx_reverse_index(char *midx_name, unsigned char *midx_hash,
 	strbuf_addf(&buf, "%s-%s.rev", midx_name, hash_to_hex(midx_hash));
 
 	tmp_file = write_rev_file_order(NULL, ctx->pack_order, ctx->entries_nr,
-					midx_hash, WRITE_REV);
+					midx_hash, 0);
 
 	if (finalize_object_file(tmp_file, buf.buf))
 		die(_("cannot store reverse index file"));
diff --git a/pack-write.c b/pack-write.c
index 1883848e7c..4f54191499 100644
--- a/pack-write.c
+++ b/pack-write.c
@@ -44,7 +44,7 @@ static int need_large_offset(off_t offset, const struct pack_idx_option *opts)
  */
 const char *write_idx_file(const char *index_name, struct pack_idx_entry **objects,
 			   int nr_objects, const struct pack_idx_option *opts,
-			   const unsigned char *sha1)
+			   const unsigned char *sha1, int verify)
 {
 	struct hashfile *f;
 	struct pack_idx_entry **sorted_by_sha, **list, **last;
@@ -65,7 +65,7 @@ const char *write_idx_file(const char *index_name, struct pack_idx_entry **objec
 	else
 		sorted_by_sha = list = last = NULL;
 
-	if (opts->flags & WRITE_IDX_VERIFY) {
+	if (verify) {
 		assert(index_name);
 		f = hashfd_check(index_name);
 	} else {
@@ -119,7 +119,7 @@ const char *write_idx_file(const char *index_name, struct pack_idx_entry **objec
 		if (index_version < 2)
 			hashwrite_be32(f, obj->offset);
 		hashwrite(f, obj->oid.hash, the_hash_algo->rawsz);
-		if ((opts->flags & WRITE_IDX_STRICT) &&
+		if (opts->write_idx_strict &&
 		    (i && oideq(&list[-2]->oid, &obj->oid)))
 			die("The same object %s appears twice in the pack",
 			    oid_to_hex(&obj->oid));
@@ -162,8 +162,7 @@ const char *write_idx_file(const char *index_name, struct pack_idx_entry **objec
 
 	hashwrite(f, sha1, the_hash_algo->rawsz);
 	finalize_hashfile(f, NULL, CSUM_HASH_IN_STREAM | CSUM_CLOSE |
-				    ((opts->flags & WRITE_IDX_VERIFY)
-				    ? 0 : CSUM_FSYNC));
+			  (verify ? 0 : CSUM_FSYNC));
 	return index_name;
 }
 
@@ -218,22 +217,19 @@ const char *write_rev_file(const char *rev_name,
 			   struct pack_idx_entry **objects,
 			   uint32_t nr_objects,
 			   const unsigned char *hash,
-			   unsigned flags)
+			   int verify)
 {
 	uint32_t *pack_order;
 	uint32_t i;
 	const char *ret;
 
-	if (!(flags & WRITE_REV) && !(flags & WRITE_REV_VERIFY))
-		return NULL;
-
 	ALLOC_ARRAY(pack_order, nr_objects);
 	for (i = 0; i < nr_objects; i++)
 		pack_order[i] = i;
 	QSORT_S(pack_order, nr_objects, pack_order_cmp, objects);
 
 	ret = write_rev_file_order(rev_name, pack_order, nr_objects, hash,
-				   flags);
+				   verify);
 
 	free(pack_order);
 
@@ -244,15 +240,12 @@ const char *write_rev_file_order(const char *rev_name,
 				 uint32_t *pack_order,
 				 uint32_t nr_objects,
 				 const unsigned char *hash,
-				 unsigned flags)
+				 int verify)
 {
 	struct hashfile *f;
 	int fd;
 
-	if ((flags & WRITE_REV) && (flags & WRITE_REV_VERIFY))
-		die(_("cannot both write and verify reverse index"));
-
-	if (flags & WRITE_REV) {
+	if (!verify) {
 		if (!rev_name) {
 			struct strbuf tmp_file = STRBUF_INIT;
 			fd = odb_mkstemp(&tmp_file, "pack/tmp_rev_XXXXXX");
@@ -264,7 +257,7 @@ const char *write_rev_file_order(const char *rev_name,
 				die_errno("unable to create '%s'", rev_name);
 		}
 		f = hashfd(fd, rev_name);
-	} else if (flags & WRITE_REV_VERIFY) {
+	} else {
 		struct stat statbuf;
 		if (stat(rev_name, &statbuf)) {
 			if (errno == ENOENT) {
@@ -274,8 +267,7 @@ const char *write_rev_file_order(const char *rev_name,
 				die_errno(_("could not stat: %s"), rev_name);
 		}
 		f = hashfd_check(rev_name);
-	} else
-		return NULL;
+	}
 
 	write_rev_header(f);
 
@@ -286,7 +278,7 @@ const char *write_rev_file_order(const char *rev_name,
 		die(_("failed to make %s readable"), rev_name);
 
 	finalize_hashfile(f, NULL, CSUM_HASH_IN_STREAM | CSUM_CLOSE |
-				    ((flags & WRITE_IDX_VERIFY) ? 0 : CSUM_FSYNC));
+			  (verify ? 0 : CSUM_FSYNC));
 
 	return rev_name;
 }
@@ -479,12 +471,13 @@ void finish_tmp_packfile(struct strbuf *name_buffer,
 		die_errno("unable to make temporary pack file readable");
 
 	idx_tmp_name = write_idx_file(NULL, written_list, nr_written,
-				      pack_idx_opts, hash);
+				      pack_idx_opts, hash, 0);
 	if (adjust_shared_perm(idx_tmp_name))
 		die_errno("unable to make temporary index file readable");
 
-	rev_tmp_name = write_rev_file(NULL, written_list, nr_written, hash,
-				      pack_idx_opts->flags);
+	if (pack_idx_opts->write_rev)
+		rev_tmp_name = write_rev_file(NULL, written_list, nr_written, hash,
+					      0);
 
 	strbuf_addf(name_buffer, "%s.pack", hash_to_hex(hash));
 
diff --git a/pack.h b/pack.h
index aa23be74df..e096dea4c9 100644
--- a/pack.h
+++ b/pack.h
@@ -38,12 +38,8 @@ struct pack_header {
 #define PACK_IDX_SIGNATURE 0xff744f63	/* "\377tOc" */
 
 struct pack_idx_option {
-	unsigned flags;
-	/* flag bits */
-#define WRITE_IDX_VERIFY 01 /* verify only, do not write the idx file */
-#define WRITE_IDX_STRICT 02
-#define WRITE_REV 04
-#define WRITE_REV_VERIFY 010
+	unsigned int write_idx_strict:1,
+		     write_rev:1;
 
 	uint32_t version;
 	uint32_t off32_limit;
@@ -84,7 +80,8 @@ typedef int (*verify_fn)(const struct object_id *, enum object_type, unsigned lo
 const char *write_idx_file(const char *index_name,
 			   struct pack_idx_entry **objects, int nr_objects,
 			   const struct pack_idx_option *opts,
-			   const unsigned char *sha1);
+			   const unsigned char *sha1,
+			   int verify);
 int check_pack_crc(struct packed_git *p, struct pack_window **w_curs, off_t offset, off_t len, unsigned int nr);
 int verify_pack_index(struct packed_git *);
 int verify_pack(struct repository *, struct packed_git *, verify_fn fn, struct progress *, uint32_t);
@@ -99,11 +96,11 @@ void write_promisor_file(const char *promisor_name, struct ref **sought, int nr_
 const char *write_rev_file(const char *rev_name,
 			   struct pack_idx_entry **objects,
 			   uint32_t nr_objects,
-			   const unsigned char *hash, unsigned flags);
+			   const unsigned char *hash, int verify);
 const char *write_rev_file_order(const char *rev_name, uint32_t *pack_order,
 				 uint32_t nr_objects,
 				 const unsigned char *hash,
-				 unsigned flags);
+				 int verify);
 
 /*
  * The "hdr" output buffer should be at least this big, which will handle sizes
Previous: Ævar Arnfjörð BjarmasonNext: Taylor Blau
Message 28 of 81 in “pack-write,repack: prevent opening packs too early”
  1. 0/2 pack-write,repack: prevent opening packs too earlyTaylor Blau, Sep 1, 2021
  2. 1/2 pack-write.c: rename `.idx` file into place lastTaylor Blau, Sep 1, 2021
  3. 2/2 builtin/repack.c: move `.idx` files into place lastTaylor Blau, Sep 1, 2021
  4. Jeff KingSep 1, 2021
  5. Taylor BlauSep 1, 2021
  6. Jeff KingSep 1, 2021
  7. Taylor BlauSep 1, 2021
  8. Jeff KingSep 1, 2021
  9. Taylor BlauSep 1, 2021
  10. Jeff KingSep 7, 2021
  11. 0/3 rename *.idx file into place last (also after *.bitmap)Ævar Arnfjörð Bjarmason, Sep 7, 2021
  12. 1/3 pack-write: use more idiomatic strbuf usage for packname constructionÆvar Arnfjörð Bjarmason, Sep 7, 2021
  13. Taylor BlauSep 7, 2021
  14. Ævar Arnfjörð BjarmasonSep 7, 2021
  15. 2/3 pack-write: split up finish_tmp_packfile() functionÆvar Arnfjörð Bjarmason, Sep 7, 2021
  16. Taylor BlauSep 7, 2021
  17. 3/3 pack-write: rename *.idx file into place last (really!)Ævar Arnfjörð Bjarmason, Sep 7, 2021
  18. Taylor BlauSep 7, 2021
  19. Taylor BlauSep 7, 2021
  20. Ævar Arnfjörð BjarmasonSep 7, 2021
  21. 0/4 rename *.idx file into place last (also after *.bitmap)Ævar Arnfjörð Bjarmason, Sep 8, 2021
  22. 1/4 pack.h: line-wrap the definition of finish_tmp_packfile()Ævar Arnfjörð Bjarmason, Sep 8, 2021
  23. 2/4 pack-write: refactor renaming in finish_tmp_packfile()Ævar Arnfjörð Bjarmason, Sep 8, 2021
  24. Taylor BlauSep 8, 2021
  25. 3/4 pack-write: split up finish_tmp_packfile() functionÆvar Arnfjörð Bjarmason, Sep 8, 2021
  26. 4/4 pack-write: rename *.idx file into place last (really!)Ævar Arnfjörð Bjarmason, Sep 8, 2021
  27. Ævar Arnfjörð BjarmasonSep 8, 2021
  28. Ævar Arnfjörð BjarmasonSep 8, 2021
  29. Taylor BlauSep 8, 2021
  30. 0/3 prevent opening packs too earlyTaylor Blau, Sep 8, 2021
  31. 1/3 pack-write.c: rename `.idx` files into place lastTaylor Blau, Sep 8, 2021
  32. 2/3 builtin/repack.c: move `.idx` files into place lastTaylor Blau, Sep 8, 2021
  33. 3/3 builtin/index-pack.c: move `.idx` files into place lastTaylor Blau, Sep 8, 2021
  34. Ævar Arnfjörð BjarmasonSep 8, 2021
  35. Ævar Arnfjörð BjarmasonSep 9, 2021
  36. Taylor BlauSep 9, 2021
  37. Ævar Arnfjörð BjarmasonSep 9, 2021
  38. Ævar Arnfjörð BjarmasonSep 9, 2021
  39. Taylor BlauSep 9, 2021
  40. 0/9 packfile: avoid .idx rename racesTaylor Blau, Sep 9, 2021
  41. 1/9 pack.h: line-wrap the definition of finish_tmp_packfile()Taylor Blau, Sep 9, 2021
  42. 2/9 bulk-checkin.c: store checksum directlyTaylor Blau, Sep 9, 2021
  43. Ævar Arnfjörð BjarmasonSep 9, 2021
  44. 3/9 pack-write: refactor renaming in finish_tmp_packfile()Taylor Blau, Sep 9, 2021
  45. Junio C HamanoSep 9, 2021
  46. Taylor BlauSep 9, 2021
  47. Junio C HamanoSep 9, 2021
  48. Taylor BlauSep 9, 2021
  49. Junio C HamanoSep 10, 2021
  50. 4/9 pack-write.c: rename `.idx` files after `*.rev`Taylor Blau, Sep 9, 2021
  51. Ævar Arnfjörð BjarmasonSep 9, 2021
  52. Taylor BlauSep 9, 2021
  53. Junio C HamanoSep 9, 2021
  54. 5/9 builtin/repack.c: move `.idx` files into place lastTaylor Blau, Sep 9, 2021
  55. Junio C HamanoSep 9, 2021
  56. Taylor BlauSep 9, 2021
  57. 6/9 index-pack: refactor renaming in final()Taylor Blau, Sep 9, 2021
  58. Junio C HamanoSep 9, 2021
  59. Taylor BlauSep 9, 2021
  60. 8/9 pack-write: split up finish_tmp_packfile() functionTaylor Blau, Sep 9, 2021
  61. 7/9 builtin/index-pack.c: move `.idx` files into place lastTaylor Blau, Sep 9, 2021
  62. Ævar Arnfjörð BjarmasonSep 9, 2021
  63. Junio C HamanoSep 9, 2021
  64. 9/9 pack-objects: rename .idx files into place after .bitmap filesTaylor Blau, Sep 9, 2021
  65. Ævar Arnfjörð BjarmasonSep 9, 2021
  66. Junio C HamanoSep 9, 2021
  67. Taylor BlauSep 9, 2021
  68. Ævar Arnfjörð BjarmasonSep 9, 2021
  69. Taylor BlauSep 9, 2021
  70. Junio C HamanoSep 9, 2021
  71. 0/9 packfile: avoid .idx rename racesTaylor Blau, Sep 9, 2021
  72. 1/9 pack.h: line-wrap the definition of finish_tmp_packfile()Taylor Blau, Sep 9, 2021
  73. 2/9 bulk-checkin.c: store checksum directlyTaylor Blau, Sep 9, 2021
  74. 3/9 pack-write: refactor renaming in finish_tmp_packfile()Taylor Blau, Sep 9, 2021
  75. 4/9 pack-write.c: rename `.idx` files after `*.rev`Taylor Blau, Sep 9, 2021
  76. 5/9 builtin/repack.c: move `.idx` files into place lastTaylor Blau, Sep 9, 2021
  77. 6/9 index-pack: refactor renaming in final()Taylor Blau, Sep 9, 2021
  78. 8/9 pack-write: split up finish_tmp_packfile() functionTaylor Blau, Sep 9, 2021
  79. 7/9 builtin/index-pack.c: move `.idx` files into place lastTaylor Blau, Sep 9, 2021
  80. 9/9 pack-objects: rename .idx files into place after .bitmap filesTaylor Blau, Sep 9, 2021
  81. Junio C HamanoSep 10, 2021

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