solver: rewrite to use Qspawn->psgi_qx and pi-httpd.async

The psgi_qx routine in the now-abandoned "repobrowse" branch
allows us to break down blob-solving at each process execution
point.  It reuses the Qspawn facility for git-http-backend(1),
allowing us to limit parallel subprocesses independently of Perl
worker count.

This is actually a 2-3% slower a fully-synchronous execution;
but it is fair to other clients as it won't monopolize the server
for hundreds of milliseconds (or even seconds) at a time.

3 files changed, 298 insertions, 229 deletions

diff --git a/lib/PublicInbox/HTTPD/Async.pm b/lib/PublicInbox/HTTPD/Async.pm
index a1f75518..a647f10d 100644
--- a/lib/PublicInbox/HTTPD/Async.pm
+++ b/lib/PublicInbox/HTTPD/Async.pm
@@ -14,6 +14,15 @@ require PublicInbox::EvCleanup;
 
 sub new {
         my ($class, $io, $cb, $cleanup) = @_;
+
+        # no $io? call $cb at the top of the next event loop to
+        # avoid recursion:
+        unless (defined($io)) {
+                PublicInbox::EvCleanup::asap($cb) if $cb;
+                PublicInbox::EvCleanup::next_tick($cleanup) if $cleanup;
+                return;
+        }
+
         my $self = fields::new($class);
         IO::Handle::blocking($io, 0);
         $self->SUPER::new($io);
diff --git a/lib/PublicInbox/SolverGit.pm b/lib/PublicInbox/SolverGit.pm
index 53a62626..a7a9a0a1 100644
--- a/lib/PublicInbox/SolverGit.pm
+++ b/lib/PublicInbox/SolverGit.pm
@@ -15,23 +15,41 @@ use Fcntl qw(SEEK_SET);
 use PublicInbox::Git qw(git_unquote git_quote);
 use PublicInbox::Spawn qw(spawn popen_rd);
 use PublicInbox::MsgIter qw(msg_iter msg_part_text);
+use PublicInbox::Qspawn;
 use URI::Escape qw(uri_escape_utf8);
 
+# di = diff info / a hashref with information about a diff ($di):
+# {
+#        oid_a => abbreviated pre-image oid,
+#        oid_b => abbreviated post-image oid,
+#        tmp => anonymous file handle with the diff,
+#        hdr_lines => arrayref of various header lines for mode information
+#        mode_a => original mode of oid_a (string, not integer),
+#        ibx => PublicInbox::Inbox object containing the diff
+#        smsg => PublicInbox::SearchMsg object containing diff
+#        path_a => pre-image path
+#        path_b => post-image path
+# }
+
 # don't bother if somebody sends us a patch with these path components,
 # it's junk at best, an attack attempt at worse:
 my %bad_component = map { $_ => 1 } ('', '.', '..');
 
-sub new {
-        my ($class, $gits, $inboxes) = @_;
-        bless {
-                gits => $gits,
-                inboxes => $inboxes,
-        }, $class;
+sub dbg ($$) {
+        print { $_[0]->{out} } $_[1], "\n" or ERR($_[0], "print(dbg): $!");
+}
+
+sub ERR ($$) {
+        my ($self, $err) = @_;
+        print { $self->{out} } $err, "\n";
+        my $ucb = delete($self->{user_cb});
+        eval { $ucb->($err) } if $ucb;
+        die $err;
 }
 
 # look for existing blobs already in git repos
-sub solve_existing ($$$) {
-        my ($self, $out, $want) = @_;
+sub solve_existing ($$) {
+        my ($self, $want) = @_;
         my $oid_b = $want->{oid_b};
         my @ambiguous; # Array of [ git, $oids]
         foreach my $git (@{$self->{gits}}) {
@@ -50,25 +68,13 @@ sub solve_existing ($$$) {
                 # TODO: do something with the ambiguous array?
                 # push @ambiguous, [ $git, @oids ];
 
-                print $out "`$oid_b' ambiguous in ",
-                                join("\n", $git->pub_urls), "\n",
-                                join('', map { "$_ blob\n" } @oids), "\n";
+                dbg($self, "`$oid_b' ambiguous in " .
+                                join("\n\t", $git->pub_urls) . "\n" .
+                                join('', map { "$_ blob\n" } @oids));
         }
         scalar(@ambiguous) ? \@ambiguous : undef;
 }
 
-# returns a hashref with information about a diff ($di):
-# {
-#        oid_a => abbreviated pre-image oid,
-#        oid_b => abbreviated post-image oid,
-#        tmp => anonymous file handle with the diff,
-#        hdr_lines => arrayref of various header lines for mode information
-#        mode_a => original mode of oid_a (string, not integer),
-#        ibx => PublicInbox::Inbox object containing the diff
-#        smsg => PublicInbox::SearchMsg object containing diff
-#        path_a => pre-image path
-#        path_b => post-image path
-# }
 sub extract_diff ($$$$) {
         my ($p, $re, $ibx, $smsg) = @_;
         my ($part) = @$p; # ignore $depth and @idx;
@@ -182,11 +188,51 @@ sub find_extract_diff ($$$) {
         }
 }
 
+sub prepare_index ($) {
+        my ($self) = @_;
+        my $patches = $self->{patches};
+        $self->{nr} = 0;
+        $self->{tot} = scalar @$patches;
+
+        my $di = $patches->[0] or die 'no patches';
+        my $oid_a = $di->{oid_a} or die '{oid_a} unset';
+        my $existing = $self->{found}->{$oid_a};
+
+        # no index creation for added files
+        $oid_a =~ /\A0+\z/ and return next_step($self);
+
+        die "BUG: $oid_a not not found" unless $existing;
+
+        my $oid_full = $existing->[1];
+        my $path_a = $di->{path_a} or die "BUG: path_a missing for $oid_full";
+        my $mode_a = $di->{mode_a} || extract_old_mode($di);
+
+        open my $in, '+>', undef or die "open: $!";
+        print $in "$mode_a $oid_full\t$path_a\0" or die "print: $!";
+        $in->flush or die "flush: $!";
+        sysseek($in, 0, 0) or die "seek: $!";
+
+        dbg($self, 'preparing index');
+        my $rdr = { 0 => fileno($in) };
+        my $cmd = [ qw(git -C), $self->{wt_dir},
+                        qw(update-index -z --index-info) ];
+        my $qsp = PublicInbox::Qspawn->new($cmd, undef, $rdr);
+        $qsp->psgi_qx($self->{psgi_env}, undef, sub {
+                my ($bref) = @_;
+                if (my $err = $qsp->{err}) {
+                        ERR($self, "git update-index error: $err");
+                }
+                dbg($self, "index prepared:\n" .
+                        "$mode_a $oid_full\t" . git_quote($path_a));
+                next_step($self); # onto do_git_apply
+        });
+}
+
 # pure Perl "git init"
 sub do_git_init_wt ($) {
         my ($self) = @_;
         my $wt = File::Temp->newdir('solver.wt-XXXXXXXX', TMPDIR => 1);
-        my $dir = $wt->dirname;
+        my $dir = $self->{wt_dir} = $wt->dirname;
 
         foreach ('', qw(objects refs objects/info refs/heads)) {
                 mkdir("$dir/.git/$_") or die "mkdir $_: $!";
@@ -211,7 +257,9 @@ EOF
         print($fh (map { "$_->{git_dir}/objects\n" } @{$self->{gits}})) or
                 die "print $f: $!";
         close $fh or die "close: $f: $!";
-        $wt;
+        my $wt_git = $self->{wt_git} = PublicInbox::Git->new("$dir/.git");
+        $wt_git->{-wt} = $wt;
+        prepare_index($self);
 }
 
 sub extract_old_mode ($) {
@@ -222,232 +270,227 @@ sub extract_old_mode ($) {
         '100644';
 }
 
-sub reap ($$) {
-        my ($pid, $msg) = @_;
-        waitpid($pid, 0) == $pid or die "waitpid($msg): $!";
-        $? == 0 or die "$msg failed: $?";
+sub do_step ($) {
+        my ($self) = @_;
+        eval {
+                # step 1: resolve blobs to patches in the todo queue
+                if (my $want = pop @{$self->{todo}}) {
+                        # this populates {patches} and {todo}
+                        resolve_patch($self, $want);
+
+                # step 2: then we instantiate a working tree once
+                # the todo queue is finally empty:
+                } elsif (!defined($self->{wt_git})) {
+                        do_git_init_wt($self);
+
+                # step 3: apply each patch in the stack
+                } elsif (scalar @{$self->{patches}}) {
+                        do_git_apply($self);
+
+                # step 4: execute the user-supplied callback with
+                # our result: (which may be undef)
+                # Other steps may call user_cb to terminate prematurely
+                # on error
+                } elsif (my $ucb = delete($self->{user_cb})) {
+                        $ucb->($self->{found}->{$self->{oid_want}});
+                } else {
+                        die 'about to call user_cb twice'; # Oops :x
+                }
+        }; # eval
+        my $err = $@;
+        if ($err) {
+                $err =~ s/^\s*Exception:\s*//; # bad word to show users :P
+                dbg($self, "E: $err");
+                my $ucb = delete($self->{user_cb});
+                eval { $ucb->($err) } if $ucb;
+        }
 }
 
-sub prepare_index ($$$$) {
-        my ($out, $wt_dir, $existing, $di) = @_;
-        my $oid_full = $existing->[1];
-        my ($r, $w);
-        my $path_a = $di->{path_a} or die "BUG: path_a missing for $oid_full";
-        my $mode_a = $di->{mode_a} || extract_old_mode($di);
-
-        # unlike git-apply(1), this only gets called once in a patch
-        # series and happens too quickly to be worth making async:
-        pipe($r, $w) or die "pipe: $!";
-        my $rdr = { 0 => fileno($r) };
-        my $pid = spawn([qw(git -C), $wt_dir,
-                         qw(update-index -z --index-info)], undef, $rdr);
-        close $r or die "close pipe(r): $!";
-        print $w "$mode_a $oid_full\t$path_a\0" or die "print update-index: $!";
-
-        close $w or die "close update-index: $!";
-        reap($pid, 'update-index -z --index-info');
-
-        print $out "index prepared:\n",
-                "$mode_a $oid_full\t", git_quote($path_a), "\n";
+sub step_cb ($) {
+        my ($self) = @_;
+        sub { do_step($self) };
 }
 
-sub do_apply_begin ($$$) {
-        my ($out, $wt_dir, $di) = @_;
-
-        my $tmp = delete $di->{tmp} or die "BUG: no tmp ", di_url($di);
-        $tmp->flush or die "tmp->flush failed: $!";
-        $out->flush or die "err->flush failed: $!";
-        sysseek($tmp, 0, SEEK_SET) or die "sysseek(tmp) failed: $!";
-
-        defined(my $err_fd = fileno($out)) or die "fileno(out): $!";
-        my $rdr = { 0 => fileno($tmp), 1 => $err_fd, 2 => $err_fd };
-
-        # we need --ignore-whitespace because some patches are CRLF
-        my $cmd = [ qw(git -C), $wt_dir,
-                    qw(apply --cached --ignore-whitespace
-                       --whitespace=warn --verbose) ];
-        spawn($cmd, undef, $rdr);
+sub next_step ($) {
+        my ($self) = @_;
+        # if outside of public-inbox-httpd, caller is expected to be
+        # looping step_cb, anyways
+        my $async = $self->{psgi_env}->{'pi-httpd.async'} or return;
+        # PublicInbox::HTTPD::Async->new
+        $async->(undef, step_cb($self));
 }
 
-sub do_apply_continue ($$) {
-        my ($wt_dir, $apply_pid) = @_;
-        reap($apply_pid, 'apply');
-        popen_rd([qw(git -C), $wt_dir, qw(ls-files -s -z)]);
+sub mark_found ($$$) {
+        my ($self, $oid, $found_info) = @_;
+        $self->{found}->{$oid} = $found_info;
 }
 
-sub do_apply_end ($$$$) {
-        my ($out, $wt_git, $rd, $di) = @_;
+sub parse_ls_files ($$$$) {
+        my ($self, $qsp, $bref, $di) = @_;
+        if (my $err = $qsp->{err}) {
+                die "git ls-files error: $err";
+        }
 
-        local $/ = "\0";
-        defined(my $line = <$rd>) or die "failed to read ls-files: $!";
-        chomp $line or die "no trailing \\0 in [$line] from ls-files";
+        my ($line, @extra) = split(/\0/, $$bref);
+        scalar(@extra) and die "BUG: extra files in index: <",
+                                join('> <', @extra), ">";
 
         my ($info, $file) = split(/\t/, $line, 2);
         my ($mode_b, $oid_b_full, $stage) = split(/ /, $info);
+        if ($file ne $di->{path_b}) {
+                die
+"BUG: index mismatch: file=$file != path_b=$di->{path_b}";
+        }
 
-        defined($line = <$rd>) and die "extra files in index: $line";
-        close $rd or die "close ls-files: $?";
-
-        $file eq $di->{path_b} or
-                die "index mismatch: file=$file != path_b=$di->{path_b}";
-
+        my $wt_git = $self->{wt_git} or die 'no git working tree';
         my (undef, undef, $size) = $wt_git->check($oid_b_full);
+        defined($size) or die "check $oid_b_full failed";
 
-        defined($size) or die "failed to read_size from $oid_b_full";
-
-        print $out "$mode_b $oid_b_full\t$file\n";
-        [ $wt_git, $oid_b_full, 'blob', $size, $di ];
+        dbg($self, "index at:\n$mode_b $oid_b_full\t$file");
+        my $created = [ $wt_git, $oid_b_full, 'blob', $size, $di ];
+        mark_found($self, $di->{oid_b}, $created);
+        next_step($self); # onto the next patch
 }
 
-sub di_url ($) {
-        my ($di) = @_;
-        # note: we don't pass the PSGI env here, different inboxes
-        # can have different HTTP_HOST on the same instance.
-        my $url = $di->{ibx}->base_url;
-        my $mid = $di->{smsg}->{mid};
-        defined($url) ? "$url$mid/" : "<$mid>";
+sub start_ls_files ($$) {
+        my ($self, $di) = @_;
+        my $cmd = [qw(git -C), $self->{wt_dir}, qw(ls-files -s -z)];
+        my $qsp = PublicInbox::Qspawn->new($cmd);
+        $qsp->psgi_qx($self->{psgi_env}, undef, sub {
+                my ($bref) = @_;
+                eval { parse_ls_files($self, $qsp, $bref, $di) };
+                ERR($self, $@) if $@;
+        });
 }
 
-# reconstruct the oid_b blob using patches we found:
-sub apply_patches_cb ($$$$$) {
-        my ($self, $out, $found, $patches, $oid_b) = @_;
-
-        my $tot = scalar(@$patches) or return sub {
-                print $out "no patch(es) for $oid_b\n";
-                undef;
-        };
-
-        my $wt = do_git_init_wt($self);
-        my $wt_dir = $wt->dirname;
-        my $wt_git = PublicInbox::Git->new("$wt_dir/.git");
-        $wt_git->{-wt} = $wt;
-
-        my $cur = 0;
-        my ($apply_pid, $rd, $di);
-
-        # returns an empty string if in progress, undef if not found,
-        # or the final [ ::Git, oid_full, type, size, $di ] arrayref
-        # if found
-        sub {
-                if ($rd) {
-                        $found->{$di->{oid_b}} =
-                                        do_apply_end($out, $wt_git, $rd, $di);
-                        $rd = undef;
-                        # continue to shift @$patches
-                } elsif ($apply_pid) {
-                        $rd = do_apply_continue($wt_dir, $apply_pid);
-                        $apply_pid = undef;
-                        return ''; # $rd => do_apply_ned
-                }
+sub do_git_apply ($) {
+        my ($self) = @_;
 
-                # may return undef here
-                $di = shift @$patches or return $found->{$oid_b};
+        my $di = shift @{$self->{patches}} or die 'empty {patches}';
+        my $tmp = delete $di->{tmp} or die 'no tmp ', di_url($self, $di);
+        $tmp->flush or die "tmp->flush failed: $!";
+        sysseek($tmp, 0, SEEK_SET) or die "sysseek(tmp) failed: $!";
 
-                my $i = ++$cur;
-                my $oid_a = $di->{oid_a};
-                my $existing = $found->{$oid_a};
-                my $empty_oid = $oid_a =~ /\A0+\z/;
+        my $i = ++$self->{nr};
+        dbg($self, "\napplying [$i/$self->{tot}] " . di_url($self, $di) .
+                "\n" . join('', @{$di->{hdr_lines}}));
 
-                if ($empty_oid && $i != 1) {
-                        die "empty oid at [$i/$tot] ", di_url($di);
-                }
-                if (!$existing && !$empty_oid) {
-                        die "missing $oid_a at [$i/$tot] ", di_url($di);
+        # we need --ignore-whitespace because some patches are CRLF
+        my $cmd = [ qw(git -C), $self->{wt_dir},
+                    qw(apply --cached --ignore-whitespace
+                       --whitespace=warn --verbose) ];
+        my $rdr = { 0 => fileno($tmp), 2 => 1 };
+        my $qsp = PublicInbox::Qspawn->new($cmd, undef, $rdr);
+        $qsp->psgi_qx($self->{psgi_env}, undef, sub {
+                my ($bref) = @_;
+                close $tmp;
+                dbg($self, $$bref);
+                if (my $err = $qsp->{err}) {
+                        ERR($self, "git apply error: $err");
                 }
+                eval { start_ls_files($self, $di) };
+                ERR($self, $@) if $@;
+        });
+}
 
-                # prepare the worktree for patch application:
-                if ($i == 1 && $existing) {
-                        prepare_index($out, $wt_dir, $existing, $di);
-                }
+sub di_url ($$) {
+        my ($self, $di) = @_;
+        # note: we don't pass the PSGI env unconditionally, here,
+        # different inboxes can have different HTTP_HOST on the same instance.
+        my $ibx = $di->{ibx};
+        my $env = $self->{psgi_env} if $ibx eq $self->{inboxes}->[0];
+        my $url = $ibx->base_url($env);
+        my $mid = $di->{smsg}->{mid};
+        defined($url) ? "$url$mid/" : "<$mid>";
+}
 
-                print $out "\napplying [$i/$tot] ", di_url($di), "\n",
-                           join('', @{$di->{hdr_lines}}), "\n"
-                        or die "print \$out failed: $!";
+sub resolve_patch ($$) {
+        my ($self, $want) = @_;
 
-                # begin the patch application patch!
-                $apply_pid = do_apply_begin($out, $wt_dir, $di);
-                # next call to this callback will call do_apply_continue
-                '';
+        if (scalar(@{$self->{patches}}) > $self->{max_patch}) {
+                die "Aborting, too many steps to $self->{oid_want}";
         }
-}
 
-# recreate $oid_b
-# Returns an array ref: [ ::Git object, oid_full, type, size, di ]
-# or undef if nothing was found.
-#
-# TODO: complete the migration of this and ViewVCS into an evented
-# model for fairness
-sub solve ($$$$) {
-        my ($self, $out, $oid_b, $hints) = @_;
+        # see if we can find the blob in an existing git repo:
+        my $cur_want = $want->{oid_b};
+        if (my $existing = solve_existing($self, $want)) {
+                dbg($self, "found $cur_want in " .
+                        join("\n", $existing->[0]->pub_urls));
 
-        # should we even get here? Probably not, but somebody
-        # could be manually typing URLs:
-        return if $oid_b =~ /\A0+\z/;
-
-        my $req = { %$hints, oid_b => $oid_b };
-        my @todo = ($req);
-        my $found = {}; # { abbrev => [ ::Git, oid_full, type, size, $di ] }
-        my $patches = []; # [ array of $di hashes ]
-        my $max = $self->{max_patches} || 200;
-        my $apply_cb;
-        my $cb = sub {
-                my $want = pop @todo;
-                unless ($want) {
-                        $apply_cb ||= apply_patches_cb($self, $out, $found,
-                                                       $patches, $oid_b);
-                        return $apply_cb->();
-                }
-
-                if (scalar(@$patches) > $max) {
-                        print $out "Aborting, too many steps to $oid_b\n";
+                if ($cur_want eq $self->{oid_want}) { # all done!
+                        eval { delete($self->{user_cb})->($existing) };
+                        die "E: $@" if $@;
                         return;
                 }
-                # see if we can find the blob in an existing git repo:
-                my $want_oid = $want->{oid_b};
-                if (my $existing = solve_existing($self, $out, $want)) {
-                        print $out "found $want_oid in ",
-                                join("\n", $existing->[0]->pub_urls), "\n";
-
-                        return $existing if $want_oid eq $oid_b; # DONE!
-                        $found->{$want_oid} = $existing;
-                        return ''; # ok, one blob resolved, more to go?
-                }
-
-                # scan through inboxes to look for emails which results in
-                # the oid we want:
-                my $di;
-                foreach my $ibx (@{$self->{inboxes}}) {
-                        $di = find_extract_diff($self, $ibx, $want) or next;
+                mark_found($self, $cur_want, $existing);
+                return next_step($self); # onto patch application
+        }
 
-                        unshift @$patches, $di;
-                        print $out "found $want_oid in ",di_url($di),"\n";
+        # scan through inboxes to look for emails which results in
+        # the oid we want:
+        my $di;
+        foreach my $ibx (@{$self->{inboxes}}) {
+                $di = find_extract_diff($self, $ibx, $want) or next;
 
-                        # good, we can find a path to the oid we $want, now
-                        # lets see if we need to apply more patches:
-                        my $src = $di->{oid_a};
+                unshift @{$self->{patches}}, $di;
+                dbg($self, "found $cur_want in ".di_url($self, $di));
 
-                        last if $src =~ /\A0+\z/;
+                # good, we can find a path to the oid we $want, now
+                # lets see if we need to apply more patches:
+                my $src = $di->{oid_a};
 
+                unless ($src =~ /\A0+\z/) {
                         # we have to solve it using another oid, fine:
                         my $job = { oid_b => $src, path_b => $di->{path_a} };
-                        push @todo, $job;
-                        last; # onto the next @todo item
+                        push @{$self->{todo}}, $job;
                 }
-                unless ($di) {
-                        print $out "$want_oid could not be found\n";
-                        return;
-                }
-                ''; # continue onto next @todo item;
-        };
+                return next_step($self); # onto the next todo item
+        }
+        dbg($self, "could not find $cur_want");
+        eval { delete($self->{user_cb})->(undef) }; # not found! :<
+        die "E: $@" if $@;
+}
 
-        while (1) {
-                my $ret = eval { $cb->() };
-                unless (defined($ret)) {
-                        print $out "E: $@\n" if $@;
-                        return;
-                }
-                return $ret if ref($ret);
-                # $ret == ''; so continue looping here
+# this API is designed to avoid creating self-referential structures;
+# so user_cb never references the SolverGit object
+sub new {
+        my ($class, $ibx, $user_cb) = @_;
+
+        bless {
+                gits => $ibx->{-repo_objs},
+                user_cb => $user_cb,
+                max_patch => 100,
+
+                # TODO: config option for searching related inboxes
+                inboxes => [ $ibx ],
+        }, $class;
+}
+
+# recreate $oid_want using $hints
+# Calls {user_cb} with: [ ::Git object, oid_full, type, size, di (diff_info) ]
+# with found object, or undef if nothing was found
+# Calls {user_cb} with a string error on fatal errors
+sub solve ($$$$$) {
+        my ($self, $env, $out, $oid_want, $hints) = @_;
+
+        # should we even get here? Probably not, but somebody
+        # could be manually typing URLs:
+        return (delete $self->{user_cb})->(undef) if $oid_want =~ /\A0+\z/;
+
+        $self->{oid_want} = $oid_want;
+        $self->{out} = $out;
+        $self->{psgi_env} = $env;
+        $self->{todo} = [ { %$hints, oid_b => $oid_want } ];
+        $self->{patches} = []; # [ $di, $di, ... ]
+        $self->{found} = {}; # { abbr => [ ::Git, oid, type, size, $di ] }
+
+        dbg($self, "solving $oid_want ...");
+        my $step_cb = step_cb($self);
+        if (my $async = $env->{'pi-httpd.async'}) {
+                # PublicInbox::HTTPD::Async->new
+                $async->(undef, $step_cb);
+        } else {
+                $step_cb->() while $self->{user_cb};
         }
 }
 
diff --git a/lib/PublicInbox/ViewVCS.pm b/lib/PublicInbox/ViewVCS.pm
index 4a3896d4..fa76086a 100644
--- a/lib/PublicInbox/ViewVCS.pm
+++ b/lib/PublicInbox/ViewVCS.pm
@@ -27,37 +27,33 @@ my $enc_utf8 = find_encoding('UTF-8');
 
 sub html_page ($$$) {
         my ($ctx, $code, $strref) = @_;
+        my $wcb = delete $ctx->{-wcb};
         $ctx->{-upfx} = '../../'; # from "/$INBOX/$OID/s/"
-        PublicInbox::WwwStream->response($ctx, $code, sub {
+        my $res = PublicInbox::WwwStream->response($ctx, $code, sub {
                 my ($nr, undef) =  @_;
                 $nr == 1 ? $$strref : undef;
         });
+        $wcb->($res);
 }
 
-sub show ($$;$) {
-        my ($ctx, $oid_b, $fn) = @_;
-        my $ibx = $ctx->{-inbox};
-        my $inboxes = [ $ibx ];
-        my $solver = PublicInbox::SolverGit->new($ibx->{-repo_objs}, $inboxes);
-        my $qp = $ctx->{qp};
-        my $hints = {};
-        while (my ($from, $to) = each %QP_MAP) {
-                defined(my $v = $qp->{$from}) or next;
-                $hints->{$to} = $v;
-        }
-
-        open my $log, '+>', undef or die "open: $!";
-        my $res = $solver->solve($log, $oid_b, $hints);
+sub solve_result {
+        my ($ctx, $res, $log, $hints, $fn) = @_;
 
-        seek($log, 0, 0) or die "seek: $!";
+        unless (seek($log, 0, 0)) {
+                $ctx->{env}->{'psgi.errors'}->print("seek(log): $!\n");
+                return html_page($ctx, 500, \'seek error');
+        }
         $log = do { local $/; <$log> };
 
+        my $ref = ref($res);
+        $log .= $res unless $ref;
         my $l = PublicInbox::Linkify->new;
         $l->linkify_1($log);
         $log = '<pre>debug log:</pre><hr /><pre>' .
                 $l->linkify_2(ascii_html($log)) . '</pre>';
 
         $res or return html_page($ctx, 404, \$log);
+        $ref eq 'ARRAY' or return html_page($ctx, 500, \$log);
 
         my ($git, $oid, $type, $size, $di) = @$res;
         if ($size > $max_size) {
@@ -78,7 +74,7 @@ sub show ($$;$) {
         if ($fn) {
                 my $h = [ 'Content-Length', $size, 'Content-Type' ];
                 push(@$h, ($binary ? 'application/octet-stream' : 'text/plain'));
-                return [ 200, $h, [ $$blob ]];
+                return delete($ctx->{-wcb})->([200, $h, [ $$blob ]]);
         }
 
         my $path = to_filename($di->{path_b} || $hints->{path_b} || 'blob');
@@ -107,4 +103,25 @@ sub show ($$;$) {
         html_page($ctx, 200, \$log);
 }
 
+sub show ($$;$) {
+        my ($ctx, $oid_b, $fn) = @_;
+        my $qp = $ctx->{qp};
+        my $hints = {};
+        while (my ($from, $to) = each %QP_MAP) {
+                defined(my $v = $qp->{$from}) or next;
+                $hints->{$to} = $v;
+        }
+
+        open my $log, '+>', undef or die "open: $!";
+        my $solver = PublicInbox::SolverGit->new($ctx->{-inbox}, sub {
+                solve_result($ctx, $_[0], $log, $hints, $fn);
+        });
+
+        # PSGI server will call this and give us a callback
+        sub {
+                $ctx->{-wcb} = $_[0]; # HTTP write callback
+                $solver->solve($ctx->{env}, $log, $oid_b, $hints);
+        };
+}
+
 1;