Fix 500 error when diff context line has broken encoding

Rugged sometimes chops a context line in between bytes, resulting in the context
line having an invalid encoding: https://github.com/libgit2/rugged/issues/716

When that happens, we will try to detect the encoding for the diff, and
sometimes we'll get it wrong. If that difference in encoding results in a
difference in string lengths between the diff and the underlying blobs, we'd
fail to highlight any inline diffs, and return a 500 status to the user.

As we're using the underlying blobs, the encoding is 'correct' anyway, so if the
string range is invalid, we can just discard the inline diff highlighting. We
still report to Sentry to ensure that we can investigate further in future.

changelogs/unreleased/42332-actionview-template-error-366-524-out-of-range.yml

+---
+title: Fix 500 error being shown when diff has context marker with invalid encoding
+merge_request:
+author:
+type: fixed

lib/gitlab/diff/highlight.rb

@@ -27,7 +27,17 @@ module Gitlab
           rich_line = highlight_line(diff_line) || diff_line.text
 
           if line_inline_diffs = inline_diffs[i]
-            rich_line = InlineDiffMarker.new(diff_line.text, rich_line).mark(line_inline_diffs)
+            begin
+              rich_line = InlineDiffMarker.new(diff_line.text, rich_line).mark(line_inline_diffs)
+            # This should only happen when the encoding of the diff doesn't
+            # match the blob, which is a bug. But we shouldn't fail to render
+            # completely in that case, even though we want to report the error.
+            rescue RangeError => e
+              if Gitlab::Sentry.enabled?
+                Gitlab::Sentry.context
+                Raven.capture_exception(e)
+              end
+            end
           end
 
           diff_line.text = rich_line

spec/lib/gitlab/diff/highlight_spec.rb

@@ -72,6 +72,28 @@ describe Gitlab::Diff::Highlight do
         expect(subject[5].text).to eq(code)
         expect(subject[5].text).to be_html_safe
       end
+
+      context 'when the inline diff marker has an invalid range' do
+        before do
+          allow_any_instance_of(Gitlab::Diff::InlineDiffMarker).to receive(:mark).and_raise(RangeError)
+        end
+
+        it 'keeps the original rich line' do
+          code = %q{+      raise RuntimeError, "System commands must be given as an array of strings"}
+
+          expect(subject[5].text).to eq(code)
+          expect(subject[5].text).not_to be_html_safe
+        end
+
+        it 'reports to Sentry if configured' do
+          allow(Gitlab::Sentry).to receive(:enabled?).and_return(true)
+
+          expect(Gitlab::Sentry).to receive(:context)
+          expect(Raven).to receive(:capture_exception)
+
+          subject
+        end
+      end
     end
   end
 end