fetch: ignore non-writable epoch dirs

This will eventually be useful for maintaing partial mirrors.

Keeping inline with the original public-inbox-fetch philosophy,
there are no additional config files to manage:
the user merely needs to remove write permissions to an $N.git
directory to prevent it from being updated.

Re-enabling updates just requires restoring write permission.

3 files changed, 57 insertions, 6 deletions

diff --git a/Documentation/public-inbox-fetch.pod b/Documentation/public-inbox-fetch.pod
index 28d5638d..833df862 100644
--- a/Documentation/public-inbox-fetch.pod
+++ b/Documentation/public-inbox-fetch.pod
@@ -19,7 +19,19 @@ It does not run L<public-inbox-index(1)>, making it suitable
 for maintaining git-only backups.
 
 For v2 inboxes, it will maintain C<$INBOX_DIR/manifest.js.gz>
-file to speed up future invocations.
+file to speed up future invocations.  It always safe to remove
+manifest.js.gz, it is merely an optimization and will be
+restored on the next invocation.
+
+To prevent fetches on any v2 epoch, use L<chmod(1)> to remove
+write permissions to the top-level of the epoch.  For example,
+to disable fetches on epoch 4:
+
+        chmod a-w $INBOX_DIR/git/4.git
+
+If you wish to re-enable fetches to the epoch:
+
+        chmod u+w $INBOX_DIR/git/4.git
 
 =head1 OPTIONS
 
diff --git a/lib/PublicInbox/Fetch.pm b/lib/PublicInbox/Fetch.pm
index 993e5b19..0bd6502c 100644
--- a/lib/PublicInbox/Fetch.pm
+++ b/lib/PublicInbox/Fetch.pm
@@ -96,7 +96,7 @@ sub do_fetch { # main entry point
         my $ibx_ver;
         $lei->{curl} //= PublicInbox::LeiCurl->new($lei) or return;
         my $dir = PublicInbox::Admin::resolve_inboxdir($cd, \$ibx_ver);
-        my ($ibx_uri, @git_dir, @epochs, $mg, @new_epoch);
+        my ($ibx_uri, @git_dir, @epochs, $mg, @new_epoch, $skip);
         if ($ibx_ver == 1) {
                 my $url = remote_url($lei, $dir) //
                         die "E: $dir missing remote.origin.url\n";
@@ -108,6 +108,10 @@ sub do_fetch { # main entry point
                 my ($git_url, $epoch);
                 for my $nr (@epochs) { # try newest epoch, first
                         my $edir = "$dir/git/$nr.git";
+                        unless (-d $edir && -w _) { # must be writable dir
+                                $skip->{$nr} = 1;
+                                next;
+                        }
                         if (defined(my $url = remote_url($lei, $edir))) {
                                 $git_url = $url;
                                 $epoch = $nr;
@@ -116,6 +120,8 @@ sub do_fetch { # main entry point
                                 warn "W: $edir missing remote.origin.url\n";
                         }
                 }
+                @epochs = grep { !$skip->{$_} } @epochs if $skip;
+                $skip //= {}; # makes code below easier
                 $git_url or die "Unable to determine git URL\n";
                 my $inbox_url = $git_url;
                 $inbox_url =~ s!/git/$epoch(?:\.git)?/?\z!! or
@@ -132,7 +138,10 @@ EOM
                 # any pre-manifest.js.gz instances running? Just fetch all
                 # existing ones and unconditionally try cloning the next
                 $v2_epochs = [ map { "$dir/git/$_.git" } @epochs ];
-                push @$v2_epochs, "$dir/git/".($epochs[-1] + 1) if @epochs;
+                if (@epochs) {
+                        my $n = $epochs[-1] + 1;
+                        push @$v2_epochs, "$dir/git/$n.git" if !$skip->{$n};
+                }
         } else {
                 $code == 200 or die "BUG unexpected code $code\n";
         }
@@ -140,8 +149,10 @@ EOM
                 defined($v1_path) and warn <<EOM;
 E: got v1 `$v1_path' when expecting v2 epoch(s) in <$muri>, WTF?
 EOM
-                @git_dir = map { "$dir/git/$_.git" } sort { $a <=> $b }
-                        map { my ($nr) = (m!/([0-9]+)\.git\z!g) } @$v2_epochs;
+                @git_dir = map { "$dir/git/$_.git" } sort { $a <=> $b } map {
+                                my ($nr) = (m!/([0-9]+)\.git\z!g);
+                                $skip->{$nr} ? () : $nr;
+                        } @$v2_epochs;
         } else {
                 $git_dir[0] = $dir;
         }
diff --git a/t/v2mirror.t b/t/v2mirror.t
index 3df5d053..665a4d59 100644
--- a/t/v2mirror.t
+++ b/t/v2mirror.t
@@ -3,7 +3,7 @@
 use strict;
 use v5.10.1;
 use PublicInbox::TestCommon;
-use File::Path qw(remove_tree);
+use File::Path qw(remove_tree make_path);
 use Cwd qw(abs_path);
 require_git(2.6);
 require_cmd('curl');
@@ -235,6 +235,34 @@ for my $d (@new_epochs) {
                 'include.path set');
 }
 
+if ('test read-only epoch dirs') {
+        my @git = ('git', "--git-dir=$new_epochs[0]");
+        my $get_objs = [@git,
+                qw(cat-file --buffer --batch-check --batch-all-objects)];
+        my $before = [sort xqx($get_objs)];
+
+        remove_tree(map { "$new_epochs[0]/$_" } qw(objects refs/heads));
+        chmod(0555, $new_epochs[0]) or xbail "chmod: $!";
+
+        # force a refetch
+        unlink("$tmpdir/m/manifest.js.gz") or xbail "unlink: $!";
+
+        run_script([qw(-fetch -q)], undef, {-C => "$tmpdir/m"}) or
+                xbail '-fetch failed';
+
+        ok(!-d "$new_epochs[0]/objects", 'no objects after fetch to R/O dir');
+
+        chmod(0755, $new_epochs[0]) or xbail "chmod: $!";
+        mkdir("$new_epochs[0]/objects") or xbail "mkdir: $!";
+        mkdir("$new_epochs[0]/refs/heads") or xbail "mkdir: $!";
+
+        my $err = '';
+        run_script([qw(-fetch -q)], undef, {-C => "$tmpdir/m", 2 => \$err}) or
+                xbail '-fetch failed '.$err;
+        is_deeply([ sort xqx($get_objs) ], $before,
+                'fetch restored objects once GIT_DIR became writable');
+}
+
 ok($td->kill, 'killed httpd');
 $td->join;