louis-e/arnis · error

provider chain is never empty

Error message

provider chain is never empty

What it means

`fetch_raw_with_fallback` iterates a chain of elevation data providers and, if the loop ends without returning, hits `unreachable!("provider chain is never empty")`. The invariant assumes the provider chain is constructed with at least one provider, so the only way to reach this line is an empty chain.

Source

Thrown at src/elevation/mod.rs:325

                    name,
                    e,
                    chain[i + 1].name()
                );
                #[cfg(feature = "gui")]
                crate::telemetry::send_log(
                    crate::telemetry::LogLevel::Warning,
                    &format!(
                        "Elevation provider '{}' failed, falling back to '{}'.",
                        name,
                        chain[i + 1].name()
                    ),
                );
                emit_gui_progress_update(10.0, "Downloading data...");
            }
            Err(e) => return Err(e),
        }
    }
    unreachable!("provider chain is never empty")
}

/// Compute the fraction of NaN/non-finite values in a height grid (0.0 to 1.0).
fn compute_nan_ratio(heights: &[Vec<f64>]) -> f64 {
    let mut total = 0usize;
    let mut nan_count = 0usize;
    for row in heights {
        for &h in row {
            total += 1;
            if !h.is_finite() {
                nan_count += 1;
            }
        }
    }
    if total == 0 {
        return 1.0;
    }
    nan_count as f64 / total as f64

View on GitHub (pinned to 34048924d9)

Solutions

1. Ensure at least one elevation provider is configured/enabled before calling `fetch_elevation_data`
2. Validate the provider chain is non-empty early and return a proper `Err(String)` like "no elevation providers configured" instead of relying on unreachable!
3. Enable the default provider feature or add a fallback provider to the chain construction

Example fix

// before
unreachable!("provider chain is never empty")
// after
Err(anyhow!("no elevation providers configured"))

if providers.is_empty() { return Err(anyhow!("no elevation providers configured; enable at least one source")); }

fn has_provider(chain: &[Box<dyn ElevationProvider>]) -> bool { !chain.is_empty() }

When it happens

Trigger: Building the provider list with configuration that selects zero providers (e.g. all elevation sources disabled, an empty/invalid config, or a feature-gated build where every provider is compiled out) and then calling `fetch_elevation_data`.

Common situations: Config file with all elevation providers turned off; CLI flags excluding every source; a build without the default provider features enabled.

Related errors

• select_level_for_cell_size called with empty levels
• Earth has no PDS raster
• internal error: entered unreachable code
• internal error: entered unreachable code
• Elevation data not available

AI-assisted analysis of louis-e/arnis@34048924d9 (2026-09-03). Data as JSON: /api/errors/91f709a54f2004c7. Report an issue: GitHub.