imap: speed up HEADER.FIELDS[.NOT] range fetches

While we can't memoize the regexp forever like we do with other
Eml users, we can still benefit from caching regexp compilation
on a per-request basis.

A FETCH request from mutt on a 4K message inbox is around 8%
faster after this.  Since regexp compilation via qr// isn't
unbearably slow, a shared cache probably isn't worth the
trouble of implementing.  A per-request cache seems enough.

1 files changed, 8 insertions, 9 deletions

diff --git a/lib/PublicInbox/IMAP.pm b/lib/PublicInbox/IMAP.pm
index 0852ffab..39667199 100644
--- a/lib/PublicInbox/IMAP.pm
+++ b/lib/PublicInbox/IMAP.pm
@@ -544,25 +544,23 @@ sub hdrs_regexp ($) {
 
 # BODY[($SECTION_IDX.)?HEADER.FIELDS.NOT ($HDRS)]<$offset.$bytes>
 sub partial_hdr_not {
-        my ($eml, $section_idx, $hdrs) = @_;
+        my ($eml, $section_idx, $hdrs_re) = @_;
         if (defined $section_idx) {
                 $eml = eml_body_idx($eml, $section_idx) or return;
         }
         my $str = $eml->header_obj->as_string;
-        my $re = hdrs_regexp($hdrs);
-        $str =~ s/$re//g;
+        $str =~ s/$hdrs_re//g;
         $str .= "\r\n";
 }
 
 # BODY[($SECTION_IDX.)?HEADER.FIELDS ($HDRS)]<$offset.$bytes>
 sub partial_hdr_get {
-        my ($eml, $section_idx, $hdrs) = @_;
+        my ($eml, $section_idx, $hdrs_re) = @_;
         if (defined $section_idx) {
                 $eml = eml_body_idx($eml, $section_idx) or return;
         }
         my $str = $eml->header_obj->as_string;
-        my $re = hdrs_regexp($hdrs);
-        join('', ($str =~ m/($re)/g), "\r\n");
+        join('', ($str =~ m/($hdrs_re)/g), "\r\n");
 }
 
 sub partial_prepare ($$$) {
@@ -583,9 +581,10 @@ sub partial_prepare ($$$) {
                                 (?:HEADER\.FIELDS(\.NOT)?)\x20 # 2
                                 \(([A-Z0-9\-\x20]+)\) # 3 - hdrs
                         \](?:<([0-9]+)(?:\.([0-9]+))?>)?\z/sx) { # 4 5
-                $partial->{$att} = [ $2 ? \&partial_hdr_not
-                                        : \&partial_hdr_get,
-                                        $1, $3, $4, $5 ];
+                my $tmp = $partial->{$att} = [ $2 ? \&partial_hdr_not
+                                                : \&partial_hdr_get,
+                                                $1, undef, $4, $5 ];
+                $tmp->[2] = hdrs_regexp($3);
         } else {
                 undef;
         }