Viewing full files in Merge requests results in "Error loading full diff. Please try again."

Description

Users attempting to view some merge request diffs encounter the error message: Error loading full diff. Please try again.
Looking at the browser's developer tools console shows the error message: net::ERR_INCOMPLETE_CHUNKED_ENCODING 200 (OK)
The browser's developer tools Network tab shows an empty response for /diff?full=true&from_merge_request=true

Environment

Self-managed GitLab instances behind an Apache reverse proxy

Impacted offerings:

GitLab Self-Managed

Solution

Ensure that the SubstituteMaxLineLength setting for mod_substitute in httpd.conf is set to a high enough value. 5M is a good place to start:

SubstituteMaxLineLength 5M

After raising the value, restart Apache.

Cause

This issue occurs when using Apache as a reverse proxy in front of GitLab with a too-low SubstituteMaxLineLength value. This causes mod_substitute to truncate reponses from GitLab, which breaks certain requests.

Additional Information

Further details on this issue can be found in Apache bug 56176.
According to the mod_substitute documentation, raising SubstituteMaxLineLength can slightly increase memory usage.