* [PATCH 0/6] process pipe improvements
@ 2021-01-02 9:13 Eric Wong
2021-01-02 9:13 ` [PATCH 1/6] processpipe: allow synchronous close to set $? Eric Wong
` (5 more replies)
0 siblings, 6 replies; 7+ messages in thread
From: Eric Wong @ 2021-01-02 9:13 UTC (permalink / raw)
To: meta
The DS::in_loop clobbering in MboxReader annoyed me and drove
this series.
I wanted to do it for the bidirectional pipes with
"git cat-file --batch", too, but there seems to be some
lifetime management issues which get hard-to-control
with things like the the waitpid(..,WNOHANG) call
in GitAsyncCat. Maybe switching --batch + DS to use UNIX
sockets can be done to save FDs (or I'm too brain-damaged
to figure this out).
But right now, all of our codebase is robust against children
attempting to reap siblings (or PIDs of former siblings)
Eric Wong (6):
processpipe: allow synchronous close to set $?
processpipe: lazy-require PublicInbox::DS for dwaitpid
git: qx: waitpid synchronously via ProcessPipe->CLOSE
import: switch to using ProcessPipe
git: manifest_entry: use ProcessPipe via popen_rd
qspawn: switch to ProcessPipe via popen_rd
lib/PublicInbox/Git.pm | 20 +++++++++-----
lib/PublicInbox/Import.pm | 23 +++++++---------
lib/PublicInbox/MboxReader.pm | 3 ---
lib/PublicInbox/ProcessPipe.pm | 49 +++++++++++++++++++++-------------
lib/PublicInbox/Qspawn.pm | 15 +++++------
lib/PublicInbox/Spawn.pm | 10 +++----
t/mbox_reader.t | 17 ++++++++++++
t/spawn.t | 38 ++++++++++++++++++++++++++
8 files changed, 119 insertions(+), 56 deletions(-)
^ permalink raw reply [flat|nested] 7+ messages in thread
* [PATCH 1/6] processpipe: allow synchronous close to set $?
2021-01-02 9:13 [PATCH 0/6] process pipe improvements Eric Wong
@ 2021-01-02 9:13 ` Eric Wong
2021-01-02 9:13 ` [PATCH 2/6] processpipe: lazy-require PublicInbox::DS for dwaitpid Eric Wong
` (4 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: Eric Wong @ 2021-01-02 9:13 UTC (permalink / raw)
To: meta
To get rid of the ugly $PublicInbox::DS::in_loop localization
in MboxReader, we'll distinguish between ->CLOSE and ->DESTROY
with ProcessPipe.
If we end up closing via ->DESTROY, we'll assume the caller will
want to deal with $? asynchronously via the event loop (or not
even care about $?).
If we hit ->CLOSE directly, we'll assume the caller called
close() and wants to check $? synchronously.
Note: wantarray doesn't seem to propagate into tied methods,
otherwise I'd be relying on that.
---
lib/PublicInbox/MboxReader.pm | 3 ---
lib/PublicInbox/ProcessPipe.pm | 45 +++++++++++++++++++++-------------
lib/PublicInbox/Spawn.pm | 10 ++++----
t/mbox_reader.t | 17 +++++++++++++
t/spawn.t | 38 ++++++++++++++++++++++++++++
5 files changed, 88 insertions(+), 25 deletions(-)
diff --git a/lib/PublicInbox/MboxReader.pm b/lib/PublicInbox/MboxReader.pm
index 1894756d..59ce4fb6 100644
--- a/lib/PublicInbox/MboxReader.pm
+++ b/lib/PublicInbox/MboxReader.pm
@@ -5,7 +5,6 @@
package PublicInbox::MboxReader;
use strict;
use v5.10.1;
-use PublicInbox::DS (); # localize $in_loop for error detection :<
use Data::Dumper;
$Data::Dumper::Useqq = 1; # should've been the default, for bad data
@@ -14,7 +13,6 @@ my $from_strict =
sub _mbox_from {
my ($mbfh, $from_re, $eml_cb, @arg) = @_;
- local $PublicInbox::DS::in_loop; # disable dwaitpid
my $buf = '';
my @raw;
while (defined(my $r = read($mbfh, $buf, 65536, length($buf)))) {
@@ -75,7 +73,6 @@ sub _extract_hdr {
sub _mbox_cl ($$$;@) {
my ($mbfh, $uxs_from, $eml_cb, @arg) = @_;
- local $PublicInbox::DS::in_loop; # disable dwaitpid
my $buf = '';
while (defined(my $r = read($mbfh, $buf, 65536, length($buf)))) {
if ($r == 0) { # detect "curl --fail"
diff --git a/lib/PublicInbox/ProcessPipe.pm b/lib/PublicInbox/ProcessPipe.pm
index 336d5ac4..400a22f3 100644
--- a/lib/PublicInbox/ProcessPipe.pm
+++ b/lib/PublicInbox/ProcessPipe.pm
@@ -6,10 +6,12 @@ package PublicInbox::ProcessPipe;
use strict;
use v5.10.1;
use PublicInbox::DS qw(dwaitpid);
+use Carp qw(carp);
sub TIEHANDLE {
my ($class, $pid, $fh, $cb, $arg) = @_;
- bless { pid => $pid, fh => $fh, cb => $cb, arg => $arg }, $class;
+ bless { pid => $pid, fh => $fh, ppid => $$, cb => $cb, arg => $arg },
+ $class;
}
sub READ { read($_[0]->{fh}, $_[1], $_[2], $_[3] || 0) }
@@ -26,32 +28,41 @@ sub PRINT {
print { $self->{fh} } @_;
}
-sub adjust_ret { # dwaitpid callback
- my ($retref, $pid) = @_;
- $$retref = '' if $?
-}
+sub FILENO { fileno($_[0]->{fh}) }
-sub CLOSE {
- my $fh = delete($_[0]->{fh});
- my $ret = defined $fh ? close($fh) : '';
- my ($pid, $cb, $arg) = delete @{$_[0]}{qw(pid cb arg)};
- if (defined $pid) {
- unless ($cb) {
- $cb = \&adjust_ret;
- $arg = \$ret;
+sub _close ($;$) {
+ my ($self, $wait) = @_;
+ my $fh = delete $self->{fh};
+ my $ret = defined($fh) ? close($fh) : '';
+ my ($pid, $cb, $arg) = delete @$self{qw(pid cb arg)};
+ return $ret unless defined($pid) && $self->{ppid} == $$;
+ if ($wait) { # caller cares about the exit status:
+ my $wp = waitpid($pid, 0);
+ if ($wp == $pid) {
+ $ret = '' if $?;
+ if ($cb) {
+ eval { $cb->($arg, $pid) };
+ carp "E: cb(arg, $pid): $@" if $@;
+ }
+ } else {
+ carp "waitpid($pid, 0) = $wp, \$!=$!, \$?=$?";
}
+ } else { # caller just undef-ed it, let event loop deal with it
dwaitpid $pid, $cb, $arg;
}
$ret;
}
-sub FILENO { fileno($_[0]->{fh}) }
+# if caller uses close(), assume they want to check $? immediately so
+# we'll waitpid() synchronously. n.b. wantarray doesn't seem to
+# propagate `undef' down to tied methods, otherwise I'd rely on that.
+sub CLOSE { _close($_[0], 1) }
+# if relying on DESTROY, assume the caller doesn't care about $? and
+# we can let the event loop call waitpid() whenever it gets SIGCHLD
sub DESTROY {
- CLOSE(@_);
+ _close($_[0]);
undef;
}
-sub pid { $_[0]->{pid} }
-
1;
diff --git a/lib/PublicInbox/Spawn.pm b/lib/PublicInbox/Spawn.pm
index 1ee40503..762a0549 100644
--- a/lib/PublicInbox/Spawn.pm
+++ b/lib/PublicInbox/Spawn.pm
@@ -295,14 +295,14 @@ sub spawn ($;$$) {
}
sub popen_rd {
- my ($cmd, $env, $opts) = @_;
+ my ($cmd, $env, $opt) = @_;
pipe(my ($r, $w)) or die "pipe: $!\n";
- $opts ||= {};
- $opts->{1} = fileno($w);
- my $pid = spawn($cmd, $env, $opts);
+ $opt ||= {};
+ $opt->{1} = fileno($w);
+ my $pid = spawn($cmd, $env, $opt);
return ($r, $pid) if wantarray;
my $ret = gensym;
- tie *$ret, 'PublicInbox::ProcessPipe', $pid, $r;
+ tie *$ret, 'PublicInbox::ProcessPipe', $pid, $r, @$opt{qw(cb arg)};
$ret;
}
diff --git a/t/mbox_reader.t b/t/mbox_reader.t
index 53458ec2..4ea2ae29 100644
--- a/t/mbox_reader.t
+++ b/t/mbox_reader.t
@@ -72,4 +72,21 @@ for my $fmt (@mbox) { $check_fmt->($fmt) }
s/\n/\r\n/sg for (values %raw);
for my $fmt (@mbox) { $check_fmt->($fmt) }
+SKIP: {
+ use PublicInbox::Spawn qw(popen_rd);
+ use Time::HiRes qw(alarm);
+ my $fh = popen_rd([ $^X, '-E', <<'' ]);
+say "From x@y Fri Oct 2 00:00:00 1993";
+print "a: b\n\n", "x" x 70000, "\n\n";
+say "From x@y Fri Oct 2 00:00:00 2010";
+print "Final: bit\n\n", "Incomplete\n\n";
+exit 1
+
+ my @x;
+ eval { $reader->mboxrd($fh, sub { push @x, shift->as_string }) };
+ like($@, qr/error closing mbox/, 'detects error reading from pipe');
+ is(scalar(@x), 1, 'only saw one message');
+ is(scalar(grep(/Final/, @x)), 0, 'no incomplete bit');
+}
+
done_testing;
diff --git a/t/spawn.t b/t/spawn.t
index 552bba33..d97e13a6 100644
--- a/t/spawn.t
+++ b/t/spawn.t
@@ -98,6 +98,44 @@ EOF
isnt($?, 0, '$? set properly: '.$?);
}
+{ # ->CLOSE vs ->DESTROY waitpid caller distinction
+ my @c;
+ my $fh = popen_rd(['true'], undef, { cb => sub { @c = caller } });
+ ok(close($fh), '->CLOSE fired and successful');
+ ok(scalar(@c), 'callback fired by ->CLOSE');
+ ok(grep(!m[/PublicInbox/DS\.pm\z], @c), 'callback not invoked by DS');
+
+ @c = ();
+ $fh = popen_rd(['true'], undef, { cb => sub { @c = caller } });
+ undef $fh; # ->DESTROY
+ ok(scalar(@c), 'callback fired by ->DESTROY');
+ ok(grep(!m[/PublicInbox/ProcessPipe\.pm\z], @c),
+ 'callback not invoked by ProcessPipe');
+}
+
+{ # children don't wait on siblings
+ use POSIX qw(_exit);
+ pipe(my ($r, $w)) or BAIL_OUT $!;
+ my $cb = sub { warn "x=$$\n" };
+ my $fh = popen_rd(['cat'], undef, { 0 => $r, cb => $cb });
+ my $pp = tied *$fh;
+ my $pid = fork // BAIL_OUT $!;
+ local $SIG{__WARN__} = sub { _exit(1) };
+ if ($pid == 0) {
+ local $SIG{__DIE__} = sub { _exit(2) };
+ undef $fh;
+ _exit(0);
+ }
+ waitpid($pid, 0);
+ is($?, 0, 'forked process exited');
+ my @w;
+ local $SIG{__WARN__} = sub { push @w, @_ };
+ close $w;
+ close $fh;
+ is($?, 0, 'cat exited');
+ is_deeply(\@w, [ "x=$$\n" ], 'callback fired from owner');
+}
+
SKIP: {
eval {
require BSD::Resource;
^ permalink raw reply related [flat|nested] 7+ messages in thread
* [PATCH 2/6] processpipe: lazy-require PublicInbox::DS for dwaitpid
2021-01-02 9:13 [PATCH 0/6] process pipe improvements Eric Wong
2021-01-02 9:13 ` [PATCH 1/6] processpipe: allow synchronous close to set $? Eric Wong
@ 2021-01-02 9:13 ` Eric Wong
2021-01-02 9:13 ` [PATCH 3/6] git: qx: waitpid synchronously via ProcessPipe->CLOSE Eric Wong
` (3 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: Eric Wong @ 2021-01-02 9:13 UTC (permalink / raw)
To: meta
This saves over 20ms with scripts that only use PublicInbox::Spawn.
---
lib/PublicInbox/ProcessPipe.pm | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/lib/PublicInbox/ProcessPipe.pm b/lib/PublicInbox/ProcessPipe.pm
index 400a22f3..e540dc22 100644
--- a/lib/PublicInbox/ProcessPipe.pm
+++ b/lib/PublicInbox/ProcessPipe.pm
@@ -5,7 +5,6 @@
package PublicInbox::ProcessPipe;
use strict;
use v5.10.1;
-use PublicInbox::DS qw(dwaitpid);
use Carp qw(carp);
sub TIEHANDLE {
@@ -48,7 +47,8 @@ sub _close ($;$) {
carp "waitpid($pid, 0) = $wp, \$!=$!, \$?=$?";
}
} else { # caller just undef-ed it, let event loop deal with it
- dwaitpid $pid, $cb, $arg;
+ require PublicInbox::DS;
+ PublicInbox::DS::dwaitpid($pid, $cb, $arg);
}
$ret;
}
^ permalink raw reply related [flat|nested] 7+ messages in thread
* [PATCH 3/6] git: qx: waitpid synchronously via ProcessPipe->CLOSE
2021-01-02 9:13 [PATCH 0/6] process pipe improvements Eric Wong
2021-01-02 9:13 ` [PATCH 1/6] processpipe: allow synchronous close to set $? Eric Wong
2021-01-02 9:13 ` [PATCH 2/6] processpipe: lazy-require PublicInbox::DS for dwaitpid Eric Wong
@ 2021-01-02 9:13 ` Eric Wong
2021-01-02 9:13 ` [PATCH 4/6] import: switch to using ProcessPipe Eric Wong
` (2 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: Eric Wong @ 2021-01-02 9:13 UTC (permalink / raw)
To: meta
If we're using ->qx, we're operating synchronously anyways,
so there's little point in relying on the event loop for
waitpid.
---
lib/PublicInbox/Git.pm | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)
diff --git a/lib/PublicInbox/Git.pm b/lib/PublicInbox/Git.pm
index f7332bb6..cdd2b400 100644
--- a/lib/PublicInbox/Git.pm
+++ b/lib/PublicInbox/Git.pm
@@ -365,8 +365,17 @@ sub popen {
sub qx {
my $self = shift;
my $fh = $self->popen(@_);
- local $/ = wantarray ? "\n" : undef;
- <$fh>;
+ if (wantarray) {
+ local $/ = "\n";
+ my @ret = <$fh>;
+ close $fh; # caller should check $?
+ @ret;
+ } else {
+ local $/;
+ my $ret = <$fh>;
+ close $fh; # caller should check $?
+ $ret;
+ }
}
# check_async and cat_async may trigger the other, so ensure they're
^ permalink raw reply related [flat|nested] 7+ messages in thread
* [PATCH 4/6] import: switch to using ProcessPipe
2021-01-02 9:13 [PATCH 0/6] process pipe improvements Eric Wong
` (2 preceding siblings ...)
2021-01-02 9:13 ` [PATCH 3/6] git: qx: waitpid synchronously via ProcessPipe->CLOSE Eric Wong
@ 2021-01-02 9:13 ` Eric Wong
2021-01-02 9:13 ` [PATCH 5/6] git: manifest_entry: use ProcessPipe via popen_rd Eric Wong
2021-01-02 9:13 ` [PATCH 6/6] qspawn: switch to " Eric Wong
5 siblings, 0 replies; 7+ messages in thread
From: Eric Wong @ 2021-01-02 9:13 UTC (permalink / raw)
To: meta
This saves us a few lines of code, but also prevents misreaping
by sibling processes.
---
lib/PublicInbox/Import.pm | 23 +++++++++--------------
1 file changed, 9 insertions(+), 14 deletions(-)
diff --git a/lib/PublicInbox/Import.pm b/lib/PublicInbox/Import.pm
index 47a529ff..b5780d2b 100644
--- a/lib/PublicInbox/Import.pm
+++ b/lib/PublicInbox/Import.pm
@@ -54,9 +54,9 @@ sub new {
sub gfi_start {
my ($self) = @_;
- return ($self->{in}, $self->{out}) if $self->{pid};
+ return ($self->{in}, $self->{out}) if $self->{in};
- my ($in_r, $pid, $out_r, $out_w);
+ my ($in_r, $out_r, $out_w);
pipe($out_r, $out_w) or die "pipe failed: $!";
$self->lock_acquire;
@@ -72,13 +72,11 @@ sub gfi_start {
chomp @t;
$self->{-tree} = { map { $_ => 1 } @t };
}
- my @cmd = ('git', "--git-dir=$git->{git_dir}",
- qw(fast-import --quiet --done --date-format=raw));
- ($in_r, $pid) = popen_rd(\@cmd, undef, { 0 => $out_r });
+ $in_r = $self->{in} = $git->popen(qw(fast-import
+ --quiet --done --date-format=raw),
+ undef, { 0 => $out_r });
$out_w->autoflush(1);
- $self->{in} = $in_r;
$self->{out} = $out_w;
- $self->{pid} = $pid;
$self->{nchg} = 0;
};
if ($@) {
@@ -162,14 +160,14 @@ sub check_remove_v1 {
sub checkpoint {
my ($self) = @_;
- return unless $self->{pid};
+ return unless $self->{in};
print { $self->{out} } "checkpoint\n" or wfail;
undef;
}
sub progress {
my ($self, $msg) = @_;
- return unless $self->{pid};
+ return unless $self->{in};
print { $self->{out} } "progress $msg\n" or wfail;
readline($self->{in}) eq "progress $msg\n" or die
"progress $msg not received\n";
@@ -218,7 +216,7 @@ sub barrier {
# used for v2
sub get_mark {
my ($self, $mark) = @_;
- die "not active\n" unless $self->{pid};
+ die "not active\n" unless $self->{in};
my ($r, $w) = $self->gfi_start;
print $w "get-mark $mark\n" or wfail;
defined(my $oid = <$r>) or die "get-mark failed, need git 2.6.0+\n";
@@ -481,10 +479,7 @@ sub done {
eval {
my $r = delete $self->{in} or die 'BUG: missing {in} when done';
print $w "done\n" or wfail;
- my $pid = delete $self->{pid} or
- die 'BUG: missing {pid} when done';
- waitpid($pid, 0) == $pid or die 'fast-import did not finish';
- $? == 0 or die "fast-import failed: $?";
+ close $r or die "fast-import failed: $?"; # ProcessPipe::CLOSE
};
my $wait_err = $@;
my $nchg = delete $self->{nchg};
^ permalink raw reply related [flat|nested] 7+ messages in thread
* [PATCH 5/6] git: manifest_entry: use ProcessPipe via popen_rd
2021-01-02 9:13 [PATCH 0/6] process pipe improvements Eric Wong
` (3 preceding siblings ...)
2021-01-02 9:13 ` [PATCH 4/6] import: switch to using ProcessPipe Eric Wong
@ 2021-01-02 9:13 ` Eric Wong
2021-01-02 9:13 ` [PATCH 6/6] qspawn: switch to " Eric Wong
5 siblings, 0 replies; 7+ messages in thread
From: Eric Wong @ 2021-01-02 9:13 UTC (permalink / raw)
To: meta
Only saves us one line of code, but that's better than nothing.
---
lib/PublicInbox/Git.pm | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)
diff --git a/lib/PublicInbox/Git.pm b/lib/PublicInbox/Git.pm
index cdd2b400..3d97300c 100644
--- a/lib/PublicInbox/Git.pm
+++ b/lib/PublicInbox/Git.pm
@@ -507,14 +507,13 @@ sub modified ($) {
# templates/this--description in git.git
sub manifest_entry {
my ($self, $epoch, $default_desc) = @_;
- my ($fh, $pid) = $self->popen('show-ref');
+ my $fh = $self->popen('show-ref');
my $dig = Digest::SHA->new(1);
while (read($fh, my $buf, 65536)) {
$dig->add($buf);
}
- close $fh;
- waitpid($pid, 0);
- return if $?; # empty, uninitialized git repo
+ close $fh or return; # empty, uninitialized git repo
+ undef $fh; # for open, below
my $git_dir = $self->{git_dir};
my $ent = {
fingerprint => $dig->hexdigest,
^ permalink raw reply related [flat|nested] 7+ messages in thread
* [PATCH 6/6] qspawn: switch to ProcessPipe via popen_rd
2021-01-02 9:13 [PATCH 0/6] process pipe improvements Eric Wong
` (4 preceding siblings ...)
2021-01-02 9:13 ` [PATCH 5/6] git: manifest_entry: use ProcessPipe via popen_rd Eric Wong
@ 2021-01-02 9:13 ` Eric Wong
5 siblings, 0 replies; 7+ messages in thread
From: Eric Wong @ 2021-01-02 9:13 UTC (permalink / raw)
To: meta
ProcessPipe has a built-in mechanism to prevent siblings from
reaping children.
---
lib/PublicInbox/Qspawn.pm | 15 ++++++---------
1 file changed, 6 insertions(+), 9 deletions(-)
diff --git a/lib/PublicInbox/Qspawn.pm b/lib/PublicInbox/Qspawn.pm
index 68b71112..7e50a59a 100644
--- a/lib/PublicInbox/Qspawn.pm
+++ b/lib/PublicInbox/Qspawn.pm
@@ -28,7 +28,6 @@ package PublicInbox::Qspawn;
use strict;
use PublicInbox::Spawn qw(popen_rd);
use PublicInbox::GzipFilter;
-use PublicInbox::DS qw(dwaitpid); # doesn't need event loop
# n.b.: we get EAGAIN with public-inbox-httpd, and EINTR on other PSGI servers
use Errno qw(EAGAIN EINTR);
@@ -58,9 +57,9 @@ sub _do_spawn {
$self->{cmd} = $o{quiet} ? undef : $cmd;
eval {
# popen_rd may die on EMFILE, ENFILE
- ($self->{rpipe}, $self->{pid}) = popen_rd($cmd, $cmd_env, \%o);
+ $self->{rpipe} = popen_rd($cmd, $cmd_env, \%o);
- die "E: $!" unless defined($self->{pid});
+ die "E: $!" unless defined($self->{rpipe});
$limiter->{running}++;
$start_cb->($self); # EPOLL_CTL_ADD may ENOSPC/ENOMEM
@@ -117,16 +116,14 @@ sub finalize ($$) {
}
}
-# callback for dwaitpid
+# callback for dwaitpid or ProcessPipe
sub waitpid_err { finalize($_[0], child_err($?)) }
sub finish ($;$) {
my ($self, $err) = @_;
- if (delete $self->{rpipe}) {
- dwaitpid $self->{pid}, \&waitpid_err, $self;
- } else {
- finalize($self, $err);
- }
+ my $tied_pp = delete($self->{rpipe}) or return finalize($self, $err);
+ my PublicInbox::ProcessPipe $pp = tied *$tied_pp;
+ @$pp{qw(cb arg)} = (\&waitpid_err, $self); # for ->DESTROY
}
sub start ($$$) {
^ permalink raw reply related [flat|nested] 7+ messages in thread
end of thread, other threads:[~2021-01-02 9:13 UTC | newest]
Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-01-02 9:13 [PATCH 0/6] process pipe improvements Eric Wong
2021-01-02 9:13 ` [PATCH 1/6] processpipe: allow synchronous close to set $? Eric Wong
2021-01-02 9:13 ` [PATCH 2/6] processpipe: lazy-require PublicInbox::DS for dwaitpid Eric Wong
2021-01-02 9:13 ` [PATCH 3/6] git: qx: waitpid synchronously via ProcessPipe->CLOSE Eric Wong
2021-01-02 9:13 ` [PATCH 4/6] import: switch to using ProcessPipe Eric Wong
2021-01-02 9:13 ` [PATCH 5/6] git: manifest_entry: use ProcessPipe via popen_rd Eric Wong
2021-01-02 9:13 ` [PATCH 6/6] qspawn: switch to " Eric Wong
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).