fix: ensure HTTP code is returned when image file is missing (404) or unreadable (500)

[eduardomozart]

Description

This PR resolves a critical network stream bug in /appasset (Moonlight streaming protocol) endpoint.

When Sunshine is asked to serve an image file & fallback image file (box.png) that is missing or locked/unreadable due to insufficient permissions, std::ifstream silently fails to open the file. Prior to this fix, the code did not verify if the stream successfully opened (in.is_open()), and instead blindly proceeded to calculate the content length using tellg(). On a failed stream, tellg() returns -1, which was incorrectly cast to a 64-bit unsigned integer (yielding a massive impossible Content-Length like 18446744073709551615). This invalid header abruptly aborted the connection via libcurl.

This issue specifically impacted Moonlight clients (like Moonlight Tizen). When a custom app image was inaccessible and the fallback box.png was also unreadable, the connection abortion threw a cryptic Weird server reply (libcurl: 8) error but HTTP 200, breaking the fetch process completely.

This commit adds if (!in.is_open()) safeguards to appasset() in nvhttp.cpp. It also adds a std::filesystem::exists() check to differentiate the HTTP response correctly:

• If the file is missing, it returns 404 Not Found.
• If the file exists but is locked/unreadable, it returns 500 Internal Server Error.

After the fix:

Sunshine gracefully catches the file read failure, logs the warning, and safely returns a 500 or 404 response without breaking the network stream. The stream stays alive to handle subsequent background /serverinfo pings:
text [2026-08-18 18:41:56.480]: Debug: METHOD :: GET [2026-08-18 18:41:56.480]: Debug: DESTINATION :: /appasset ... [2026-08-18 18:41:56.481]: Warning: Invalid PNG file at path ["../Resources/assets/desktop-alt.png"] [2026-08-18 18:41:56.920]: Debug: /CN=NVIDIA GameStream Client -- verified [2026-08-18 18:41:56.920]: Debug: TUNNEL :: HTTPS [2026-08-18 18:41:56.920]: Debug: METHOD :: GET [2026-08-18 18:41:56.920]: Debug: DESTINATION :: /serverinfo
Moonlight Tizen safely catches the standard HTTP error, gracefully abandons the download, and uses its local fallback placeholder:
CURL: https://192.168.1.73:47984/appasset?uniqueid=... -> HTTP response code said error

Before the fix:

Sunshine attempts to serve the file and immediately crashes the libcurl connection due to the -1 Content-Length. The Sunshine logs abruptly end after the /appasset request because the stream dies:
text [2026-08-18 18:41:56.480]: Debug: METHOD :: GET [2026-08-18 18:41:56.480]: Debug: DESTINATION :: /appasset [2026-08-18 18:41:56.480]: Debug: appid -- 427450027
Moonlight Tizen fails to fetch the image and receives a crashed connection because of the massive unsigned Content-Length, logging:
CURL: https://192.168.1.73:47984/appasset?uniqueid=... -> Weird server reply

Reproducible Steps (using Web UI proxy)

Because hitting the mutual-TLS /appasset endpoint is complex from a standard CLI, you can easily reproduce the exact same C++ tellg() bug via the Web UI .ico endpoint:

1. Make the icon unreadable: chmod 000 assets/web/images/sunshine.ico
2. Run Sunshine.
3. Attempt to fetch it: curl -v -k https://localhost:47990/images/sunshine.ico
4. Before: Curl crashes with curl: (8) Weird server reply and an impossible Content-Length.
5. After: Curl gracefully prints < HTTP/1.1 500 Internal Server Error (or 404 Not Found if you delete the file).

Screenshot

Issues Fixed or Closed

Roadmap Issues

Type of Change

• feat: New feature (non-breaking change which adds functionality)
• fix: Bug fix (non-breaking change which fixes an issue)
• docs: Documentation only changes
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semicolons, etc.)
• refactor: Code change that neither fixes a bug nor adds a feature
• perf: Code change that improves performance
• test: Adding missing tests or correcting existing tests
• build: Changes that affect the build system or external dependencies
• ci: Changes to CI configuration files and scripts
• chore: Other changes that don't modify src or test files
• revert: Reverts a previous commit
• BREAKING CHANGE: Introduces a breaking change (can be combined with any type above)

Checklist

• Code follows the style guidelines of this project
• Code has been self-reviewed
• Code has been commented, particularly in hard-to-understand areas
• Code docstring/documentation-blocks for new or existing methods/components have been added or updated
• Unit tests have been added or updated for any new or modified functionality

AI Usage

See our AI usage policy.

• None: No AI tools were used in creating this PR
• Light: AI provided minor assistance (formatting, simple suggestions)
• Moderate: AI helped with code generation or debugging specific parts
• Heavy: AI generated most or all of the code changes

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud