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Comparing IO-AIO/AIO.pm (file contents):
Revision 1.208 by root, Mon Sep 26 20:19:08 2011 UTC vs.
Revision 1.216 by root, Sun Oct 9 08:24:49 2011 UTC

… …
168use common::sense; 168use common::sense;
169 169
170use base 'Exporter'; 170use base 'Exporter';
171 171
172BEGIN { 172BEGIN {
173 our $VERSION = '4.0'; 173 our $VERSION = '4.1';
174 174
175 our @AIO_REQ = qw(aio_sendfile aio_read aio_write aio_open aio_close 175 our @AIO_REQ = qw(aio_sendfile aio_read aio_write aio_open aio_close
176 aio_stat aio_lstat aio_unlink aio_rmdir aio_readdir aio_readdirx 176 aio_stat aio_lstat aio_unlink aio_rmdir aio_readdir aio_readdirx
177 aio_scandir aio_symlink aio_readlink aio_realpath aio_sync 177 aio_scandir aio_symlink aio_readlink aio_realpath aio_sync
178 aio_fsync aio_syncfs aio_fdatasync aio_sync_file_range aio_fallocate 178 aio_fsync aio_syncfs aio_fdatasync aio_sync_file_range aio_fallocate
… …
221 aio_utime $fh_or_path, $atime, $mtime, $callback->($status) 221 aio_utime $fh_or_path, $atime, $mtime, $callback->($status)
222 aio_chown $fh_or_path, $uid, $gid, $callback->($status) 222 aio_chown $fh_or_path, $uid, $gid, $callback->($status)
223 aio_truncate $fh_or_path, $offset, $callback->($status) 223 aio_truncate $fh_or_path, $offset, $callback->($status)
224 aio_chmod $fh_or_path, $mode, $callback->($status) 224 aio_chmod $fh_or_path, $mode, $callback->($status)
225 aio_unlink $pathname, $callback->($status) 225 aio_unlink $pathname, $callback->($status)
226 aio_mknod $path, $mode, $dev, $callback->($status) 226 aio_mknod $pathname, $mode, $dev, $callback->($status)
227 aio_link $srcpath, $dstpath, $callback->($status) 227 aio_link $srcpath, $dstpath, $callback->($status)
228 aio_symlink $srcpath, $dstpath, $callback->($status) 228 aio_symlink $srcpath, $dstpath, $callback->($status)
229 aio_readlink $path, $callback->($link) 229 aio_readlink $pathname, $callback->($link)
230 aio_realpath $path, $callback->($link) 230 aio_realpath $pathname, $callback->($link)
231 aio_rename $srcpath, $dstpath, $callback->($status) 231 aio_rename $srcpath, $dstpath, $callback->($status)
232 aio_mkdir $pathname, $mode, $callback->($status) 232 aio_mkdir $pathname, $mode, $callback->($status)
233 aio_rmdir $pathname, $callback->($status) 233 aio_rmdir $pathname, $callback->($status)
234 aio_readdir $pathname, $callback->($entries) 234 aio_readdir $pathname, $callback->($entries)
235 aio_readdirx $pathname, $flags, $callback->($entries, $flags) 235 aio_readdirx $pathname, $flags, $callback->($entries, $flags)
236 IO::AIO::READDIR_DENTS IO::AIO::READDIR_DIRS_FIRST 236 IO::AIO::READDIR_DENTS IO::AIO::READDIR_DIRS_FIRST
237 IO::AIO::READDIR_STAT_ORDER IO::AIO::READDIR_FOUND_UNKNOWN 237 IO::AIO::READDIR_STAT_ORDER IO::AIO::READDIR_FOUND_UNKNOWN
238 aio_scandir $pathname, $maxreq, $callback->($dirs, $nondirs)
238 aio_load $path, $data, $callback->($status) 239 aio_load $pathname, $data, $callback->($status)
239 aio_copy $srcpath, $dstpath, $callback->($status) 240 aio_copy $srcpath, $dstpath, $callback->($status)
240 aio_move $srcpath, $dstpath, $callback->($status) 241 aio_move $srcpath, $dstpath, $callback->($status)
241 aio_scandir $path, $maxreq, $callback->($dirs, $nondirs)
242 aio_rmtree $path, $callback->($status) 242 aio_rmtree $pathname, $callback->($status)
243 aio_sync $callback->($status) 243 aio_sync $callback->($status)
244 aio_syncfs $fh, $callback->($status) 244 aio_syncfs $fh, $callback->($status)
245 aio_fsync $fh, $callback->($status) 245 aio_fsync $fh, $callback->($status)
246 aio_fdatasync $fh, $callback->($status) 246 aio_fdatasync $fh, $callback->($status)
247 aio_sync_file_range $fh, $offset, $nbytes, $flags, $callback->($status) 247 aio_sync_file_range $fh, $offset, $nbytes, $flags, $callback->($status)
248 aio_pathsync $path, $callback->($status) 248 aio_pathsync $pathname, $callback->($status)
249 aio_msync $scalar, $offset = 0, $length = undef, flags = 0, $callback->($status) 249 aio_msync $scalar, $offset = 0, $length = undef, flags = 0, $callback->($status)
250 aio_mtouch $scalar, $offset = 0, $length = undef, flags = 0, $callback->($status) 250 aio_mtouch $scalar, $offset = 0, $length = undef, flags = 0, $callback->($status)
251 aio_mlock $scalar, $offset = 0, $length = undef, $callback->($status) 251 aio_mlock $scalar, $offset = 0, $length = undef, $callback->($status)
252 aio_mlockall $flags, $callback->($status) 252 aio_mlockall $flags, $callback->($status)
253 aio_group $callback->(...) 253 aio_group $callback->(...)
… …
281=head2 AIO REQUEST FUNCTIONS 281=head2 AIO REQUEST FUNCTIONS
282 282
283All the C<aio_*> calls are more or less thin wrappers around the syscall 283All the C<aio_*> calls are more or less thin wrappers around the syscall
284with the same name (sans C<aio_>). The arguments are similar or identical, 284with the same name (sans C<aio_>). The arguments are similar or identical,
285and they all accept an additional (and optional) C<$callback> argument 285and they all accept an additional (and optional) C<$callback> argument
286which must be a code reference. This code reference will get called with 286which must be a code reference. This code reference will be called after
287the syscall return code (e.g. most syscalls return C<-1> on error, unlike 287the syscall has been executed in an asynchronous fashion. The results
288perl, which usually delivers "false") as its sole argument after the given 288of the request will be passed as arguments to the callback (and, if an
289syscall has been executed asynchronously. 289error occured, in C<$!>) - for most requests the syscall return code (e.g.
290most syscalls return C<-1> on error, unlike perl, which usually delivers
291"false").
292
293Some requests (such as C<aio_readdir>) pass the actual results and
294communicate failures by passing C<undef>.
290 295
291All functions expecting a filehandle keep a copy of the filehandle 296All functions expecting a filehandle keep a copy of the filehandle
292internally until the request has finished. 297internally until the request has finished.
293 298
294All functions return request objects of type L<IO::AIO::REQ> that allow 299All functions return request objects of type L<IO::AIO::REQ> that allow
295further manipulation of those requests while they are in-flight. 300further manipulation of those requests while they are in-flight.
296 301
297The pathnames you pass to these routines I<must> be absolute and 302The pathnames you pass to these routines I<should> be absolute. The
298encoded as octets. The reason for the former is that at the time the 303reason for this is that at the time the request is being executed, the
299request is being executed, the current working directory could have 304current working directory could have changed. Alternatively, you can
300changed. Alternatively, you can make sure that you never change the 305make sure that you never change the current working directory anywhere
301current working directory anywhere in the program and then use relative 306in the program and then use relative paths. You can also take advantage
302paths. 307of IO::AIOs working directory abstraction, that lets you specify paths
308relative to some previously-opened "working directory object" - see the
309description of the C<IO::AIO::WD> class later in this document.
303 310
304To encode pathnames as octets, either make sure you either: a) always pass 311To encode pathnames as octets, either make sure you either: a) always pass
305in filenames you got from outside (command line, readdir etc.) without 312in filenames you got from outside (command line, readdir etc.) without
306tinkering, b) are ASCII or ISO 8859-1, c) use the Encode module and encode 313tinkering, b) are in your native filesystem encoding, c) use the Encode
307your pathnames to the locale (or other) encoding in effect in the user 314module and encode your pathnames to the locale (or other) encoding in
308environment, d) use Glib::filename_from_unicode on unicode filenames or e) 315effect in the user environment, d) use Glib::filename_from_unicode on
309use something else to ensure your scalar has the correct contents. 316unicode filenames or e) use something else to ensure your scalar has the
317correct contents.
310 318
311This works, btw. independent of the internal UTF-8 bit, which IO::AIO 319This works, btw. independent of the internal UTF-8 bit, which IO::AIO
312handles correctly whether it is set or not. 320handles correctly whether it is set or not.
313 321
314=over 4 322=over 4
… …
616 624
617Asynchronously unlink (delete) a file and call the callback with the 625Asynchronously unlink (delete) a file and call the callback with the
618result code. 626result code.
619 627
620 628
621=item aio_mknod $path, $mode, $dev, $callback->($status) 629=item aio_mknod $pathname, $mode, $dev, $callback->($status)
622 630
623[EXPERIMENTAL] 631[EXPERIMENTAL]
624 632
625Asynchronously create a device node (or fifo). See mknod(2). 633Asynchronously create a device node (or fifo). See mknod(2).
626 634
627The only (POSIX-) portable way of calling this function is: 635The only (POSIX-) portable way of calling this function is:
628 636
629 aio_mknod $path, IO::AIO::S_IFIFO | $mode, 0, sub { ... 637 aio_mknod $pathname, IO::AIO::S_IFIFO | $mode, 0, sub { ...
630 638
631See C<aio_stat> for info about some potentially helpful extra constants 639See C<aio_stat> for info about some potentially helpful extra constants
632and functions. 640and functions.
633 641
634=item aio_link $srcpath, $dstpath, $callback->($status) 642=item aio_link $srcpath, $dstpath, $callback->($status)
… …
641 649
642Asynchronously create a new symbolic link to the existing object at C<$srcpath> at 650Asynchronously create a new symbolic link to the existing object at C<$srcpath> at
643the path C<$dstpath> and call the callback with the result code. 651the path C<$dstpath> and call the callback with the result code.
644 652
645 653
646=item aio_readlink $path, $callback->($link) 654=item aio_readlink $pathname, $callback->($link)
647 655
648Asynchronously read the symlink specified by C<$path> and pass it to 656Asynchronously read the symlink specified by C<$path> and pass it to
649the callback. If an error occurs, nothing or undef gets passed to the 657the callback. If an error occurs, nothing or undef gets passed to the
650callback. 658callback.
651 659
652 660
653=item aio_realpath $path, $callback->($path) 661=item aio_realpath $pathname, $callback->($path)
654 662
655Asynchronously make the path absolute and resolve any symlinks in 663Asynchronously make the path absolute and resolve any symlinks in
656C<$path>. The resulting path only consists of directories (Same as 664C<$path>. The resulting path only consists of directories (Same as
657L<Cwd::realpath>). 665L<Cwd::realpath>).
658 666
… …
753C<$type>'s are known, which can be used to speed up some algorithms. 761C<$type>'s are known, which can be used to speed up some algorithms.
754 762
755=back 763=back
756 764
757 765
758=item aio_load $path, $data, $callback->($status) 766=item aio_load $pathname, $data, $callback->($status)
759 767
760This is a composite request that tries to fully load the given file into 768This is a composite request that tries to fully load the given file into
761memory. Status is the same as with aio_read. 769memory. Status is the same as with aio_read.
762 770
763=cut 771=cut
… …
898 }; 906 };
899 907
900 $grp 908 $grp
901} 909}
902 910
903=item aio_scandir $path, $maxreq, $callback->($dirs, $nondirs) 911=item aio_scandir $pathname, $maxreq, $callback->($dirs, $nondirs)
904 912
905Scans a directory (similar to C<aio_readdir>) but additionally tries to 913Scans a directory (similar to C<aio_readdir>) but additionally tries to
906efficiently separate the entries of directory C<$path> into two sets of 914efficiently separate the entries of directory C<$path> into two sets of
907names, directories you can recurse into (directories), and ones you cannot 915names, directories you can recurse into (directories), and ones you cannot
908recurse into (everything else, including symlinks to directories). 916recurse into (everything else, including symlinks to directories).
… …
965 973
966 my $grp = aio_group $cb; 974 my $grp = aio_group $cb;
967 975
968 $maxreq = 4 if $maxreq <= 0; 976 $maxreq = 4 if $maxreq <= 0;
969 977
970 # stat once 978 # get a wd object
971 aioreq_pri $pri; 979 aioreq_pri $pri;
972 add $grp aio_stat $path, sub { 980 add $grp aio_wd $path, sub {
981 $_[0]
973 return $grp->result () if $_[0]; 982 or return $grp->result ();
974 my $now = time;
975 my $hash1 = join ":", (stat _)[0,1,3,7,9];
976 983
977 # read the directory entries 984 my $wd = [shift, "."];
985
986 # stat once
978 aioreq_pri $pri; 987 aioreq_pri $pri;
979 add $grp aio_readdirx $path, READDIR_DIRS_FIRST, sub { 988 add $grp aio_stat $wd, sub {
980 my $entries = shift
981 or return $grp->result (); 989 return $grp->result () if $_[0];
990 my $now = time;
991 my $hash1 = join ":", (stat _)[0,1,3,7,9];
982 992
983 # stat the dir another time 993 # read the directory entries
984 aioreq_pri $pri; 994 aioreq_pri $pri;
995 add $grp aio_readdirx $wd, READDIR_DIRS_FIRST, sub {
996 my $entries = shift
997 or return $grp->result ();
998
999 # stat the dir another time
1000 aioreq_pri $pri;
985 add $grp aio_stat $path, sub { 1001 add $grp aio_stat $wd, sub {
986 my $hash2 = join ":", (stat _)[0,1,3,7,9]; 1002 my $hash2 = join ":", (stat _)[0,1,3,7,9];
987 1003
988 my $ndirs; 1004 my $ndirs;
989 1005
990 # take the slow route if anything looks fishy 1006 # take the slow route if anything looks fishy
991 if ($hash1 ne $hash2 or (stat _)[9] == $now) { 1007 if ($hash1 ne $hash2 or (stat _)[9] == $now) {
992 $ndirs = -1; 1008 $ndirs = -1;
993 } else { 1009 } else {
994 # if nlink == 2, we are finished 1010 # if nlink == 2, we are finished
995 # for non-posix-fs's, we rely on nlink < 2 1011 # for non-posix-fs's, we rely on nlink < 2
996 $ndirs = (stat _)[3] - 2 1012 $ndirs = (stat _)[3] - 2
997 or return $grp->result ([], $entries); 1013 or return $grp->result ([], $entries);
998 } 1014 }
999 1015
1000 my (@dirs, @nondirs); 1016 my (@dirs, @nondirs);
1001 1017
1002 my $statgrp = add $grp aio_group sub { 1018 my $statgrp = add $grp aio_group sub {
1003 $grp->result (\@dirs, \@nondirs); 1019 $grp->result (\@dirs, \@nondirs);
1004 }; 1020 };
1005 1021
1006 limit $statgrp $maxreq; 1022 limit $statgrp $maxreq;
1007 feed $statgrp sub { 1023 feed $statgrp sub {
1008 return unless @$entries; 1024 return unless @$entries;
1009 my $entry = shift @$entries; 1025 my $entry = shift @$entries;
1010 1026
1011 aioreq_pri $pri; 1027 aioreq_pri $pri;
1028 $wd->[1] = "$entry/.";
1012 add $statgrp aio_stat "$path/$entry/.", sub { 1029 add $statgrp aio_stat $wd, sub {
1013 if ($_[0] < 0) { 1030 if ($_[0] < 0) {
1014 push @nondirs, $entry; 1031 push @nondirs, $entry;
1015 } else { 1032 } else {
1016 # need to check for real directory 1033 # need to check for real directory
1017 aioreq_pri $pri; 1034 aioreq_pri $pri;
1035 $wd->[1] = $entry;
1018 add $statgrp aio_lstat "$path/$entry", sub { 1036 add $statgrp aio_lstat $wd, sub {
1019 if (-d _) { 1037 if (-d _) {
1020 push @dirs, $entry; 1038 push @dirs, $entry;
1021 1039
1022 unless (--$ndirs) { 1040 unless (--$ndirs) {
1023 push @nondirs, @$entries; 1041 push @nondirs, @$entries;
1024 feed $statgrp; 1042 feed $statgrp;
1043 }
1044 } else {
1045 push @nondirs, $entry;
1025 } 1046 }
1026 } else {
1027 push @nondirs, $entry;
1028 } 1047 }
1029 } 1048 }
1030 } 1049 };
1031 }; 1050 };
1032 }; 1051 };
1033 }; 1052 };
1034 }; 1053 };
1035 }; 1054 };
1036 1055
1037 $grp 1056 $grp
1038} 1057}
1039 1058
1040=item aio_rmtree $path, $callback->($status) 1059=item aio_rmtree $pathname, $callback->($status)
1041 1060
1042Delete a directory tree starting (and including) C<$path>, return the 1061Delete a directory tree starting (and including) C<$path>, return the
1043status of the final C<rmdir> only. This is a composite request that 1062status of the final C<rmdir> only. This is a composite request that
1044uses C<aio_scandir> to recurse into and rmdir directories, and unlink 1063uses C<aio_scandir> to recurse into and rmdir directories, and unlink
1045everything else. 1064everything else.
… …
1106C<$flags> can be a combination of C<IO::AIO::SYNC_FILE_RANGE_WAIT_BEFORE>, 1125C<$flags> can be a combination of C<IO::AIO::SYNC_FILE_RANGE_WAIT_BEFORE>,
1107C<IO::AIO::SYNC_FILE_RANGE_WRITE> and 1126C<IO::AIO::SYNC_FILE_RANGE_WRITE> and
1108C<IO::AIO::SYNC_FILE_RANGE_WAIT_AFTER>: refer to the sync_file_range 1127C<IO::AIO::SYNC_FILE_RANGE_WAIT_AFTER>: refer to the sync_file_range
1109manpage for details. 1128manpage for details.
1110 1129
1111=item aio_pathsync $path, $callback->($status) 1130=item aio_pathsync $pathname, $callback->($status)
1112 1131
1113This request tries to open, fsync and close the given path. This is a 1132This request tries to open, fsync and close the given path. This is a
1114composite request intended to sync directories after directory operations 1133composite request intended to sync directories after directory operations
1115(E.g. rename). This might not work on all operating systems or have any 1134(E.g. rename). This might not work on all operating systems or have any
1116specific effect, but usually it makes sure that directory changes get 1135specific effect, but usually it makes sure that directory changes get
… …
1259immense (it blocks a thread for a long time) so do not use this function 1278immense (it blocks a thread for a long time) so do not use this function
1260except to put your application under artificial I/O pressure. 1279except to put your application under artificial I/O pressure.
1261 1280
1262=back 1281=back
1263 1282
1283
1284=head2 IO::AIO::WD - multiple working directories
1285
1286Your process only has one current working directory, which is used by all
1287threads. This makes it hard to use relative paths (some other component
1288could call C<chdir> at any time, and it is hard to control when the path
1289will be used by IO::AIO).
1290
1291One solution for this is to always use absolute paths. This usually works,
1292but can be quite slow (the kernel has to walk the whole path on every
1293access), and can also be a hassle to implement.
1294
1295Newer POSIX systems have a number of functions (openat, fdopendir,
1296futimensat and so on) that make it possible to specify working directories
1297per operation.
1298
1299For portability, and because the clowns who "designed", or shall I write,
1300perpetrated this new interface were obviously half-drunk, this abstraction
1301cannot be perfect, though.
1302
1303IO::AIO allows you to convert directory paths into a so-called IO::AIO::WD
1304object. This object stores the canonicalised, absolute version of the
1305path, and on systems that allow it, also a directory file descriptor.
1306
1307Everywhere where a pathname is accepted by IO::AIO (e.g. in C<aio_stat>
1308or C<aio_unlink>), one can specify an array reference with an IO::AIO::WD
1309object and a pathname instead (or the IO::AIO::WD object alone, which
1310gets interpreted as C<[$wd, "."]>). If the pathname is absolute, the
1311IO::AIO::WD object is ignored, otherwise the pathname is resolved relative
1312to that IO::AIO::WD object.
1313
1314For example, to get a wd object for F</etc> and then stat F<passwd>
1315inside, you would write:
1316
1317 aio_wd "/etc", sub {
1318 my $etcdir = shift;
1319
1320 # although $etcdir can be undef on error, there is generally no reason
1321 # to check for errors here, as aio_stat will fail with ENOENT
1322 # when $etcdir is undef.
1323
1324 aio_stat [$etcdir, "passwd"], sub {
1325 # yay
1326 };
1327 };
1328
1329That C<aio_wd> is a request and not a normal function shows that creating
1330an IO::AIO::WD object is itself a potentially blocking operation, which is
1331why it is done asynchronously.
1332
1333To stat the directory obtained with C<aio_wd> above, one could write
1334either of the following three request calls:
1335
1336 aio_lstat "/etc" , sub { ... # pathname as normal string
1337 aio_lstat [$wd, "."], sub { ... # "." relative to $wd (i.e. $wd itself)
1338 aio_lstat $wd , sub { ... # shorthand for the previous
1339
1340As with normal pathnames, IO::AIO keeps a copy of the working directory
1341object and the pathname string, so you could write the following without
1342causing any issues due to C<$path> getting reused:
1343
1344 my $path = [$wd, undef];
1345
1346 for my $name (qw(abc def ghi)) {
1347 $path->[1] = $name;
1348 aio_stat $path, sub {
1349 # ...
1350 };
1351 }
1352
1353There are some caveats: when directories get renamed (or deleted), the
1354pathname string doesn't change, so will point to the new directory (or
1355nowhere at all), while the directory fd, if available on the system,
1356will still point to the original directory. Most functions accepting a
1357pathname will use the directory fd on newer systems, and the string on
1358older systems. Some functions (such as realpath) will always rely on the
1359string form of the pathname.
1360
1361So this fucntionality is mainly useful to get some protection against
1362C<chdir>, to easily get an absolute path out of a relative path for future
1363reference, and to speed up doing many operations in the same directory
1364(e.g. when stat'ing all files in a directory).
1365
1366The following functions implement this working directory abstraction:
1367
1368=over 4
1369
1370=item aio_wd $pathname, $callback->($wd)
1371
1372Asynchonously canonicalise the given pathname and convert it to an
1373IO::AIO::WD object representing it. If possible and supported on the
1374system, also open a directory fd to speed up pathname resolution relative
1375to this working directory.
1376
1377If something goes wrong, then C<undef> is passwd to the callback instead
1378of a working directory object and C<$!> is set appropriately. Since
1379passing C<undef> as working directory component of a pathname fails the
1380request with C<ENOENT>, there is often no need for error checking in the
1381C<aio_wd> callback, as future requests using the value will fail in the
1382expected way.
1383
1384If this call isn't available because your OS lacks it or it couldn't be
1385detected, it will be emulated by calling C<fsync> instead.
1386
1387=item IO::AIO::CWD
1388
1389This is a compiletime constant (object) that represents the process
1390current working directory.
1391
1392Specifying this object as working directory object for a pathname is as
1393if the pathname would be specified directly, without a directory object,
1394e.g., these calls are functionally identical:
1395
1396 aio_stat "somefile", sub { ... };
1397 aio_stat [IO::AIO::CWD, "somefile"], sub { ... };
1398
1399=back
1400
1401
1264=head2 IO::AIO::REQ CLASS 1402=head2 IO::AIO::REQ CLASS
1265 1403
1266All non-aggregate C<aio_*> functions return an object of this class when 1404All non-aggregate C<aio_*> functions return an object of this class when
1267called in non-void context. 1405called in non-void context.
1268 1406
… …
1385 1523
1386Sets a feeder/generator on this group: every group can have an attached 1524Sets a feeder/generator on this group: every group can have an attached
1387generator that generates requests if idle. The idea behind this is that, 1525generator that generates requests if idle. The idea behind this is that,
1388although you could just queue as many requests as you want in a group, 1526although you could just queue as many requests as you want in a group,
1389this might starve other requests for a potentially long time. For example, 1527this might starve other requests for a potentially long time. For example,
1390C<aio_scandir> might generate hundreds of thousands C<aio_stat> requests, 1528C<aio_scandir> might generate hundreds of thousands of C<aio_stat>
1391delaying any later requests for a long time. 1529requests, delaying any later requests for a long time.
1392 1530
1393To avoid this, and allow incremental generation of requests, you can 1531To avoid this, and allow incremental generation of requests, you can
1394instead a group and set a feeder on it that generates those requests. The 1532instead a group and set a feeder on it that generates those requests. The
1395feed callback will be called whenever there are few enough (see C<limit>, 1533feed callback will be called whenever there are few enough (see C<limit>,
1396below) requests active in the group itself and is expected to queue more 1534below) requests active in the group itself and is expected to queue more

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