Fix EOF detection with CI artifacts metadata

There are some corner cases where a perfectly correct GZIP stream may
not hit the EOF until another read is attempted. We now skip the entry
if we don't see any valid data, which allows the EOF check to work
properly.

Closes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22479

changelogs/unreleased/sh-pages-eof-error.yml

+---
+title: Fix EOF detection with CI artifacts metadata
+merge_request: 22479
+author:
+type: fixed

lib/gitlab/ci/build/artifacts/metadata.rb

@@ -59,9 +59,12 @@ module Gitlab
 
             until gz.eof?
               begin
-                path = read_string(gz).force_encoding('UTF-8')
-                meta = read_string(gz).force_encoding('UTF-8')
+                path = read_string(gz)&.force_encoding('UTF-8')
+                meta = read_string(gz)&.force_encoding('UTF-8')
 
+                # We might hit an EOF while reading either value, so we should
+                # abort if we don't get any data.
+                next unless path && meta
                 next unless path.valid_encoding? && meta.valid_encoding?
                 next unless path =~ match_pattern
                 next if path =~ INVALID_PATH_PATTERN