core, graph: Ensure subgraphs have at least one data source

Author: Jannis

core/src/subgraph/validation.rs

@@ -5,6 +5,11 @@ pub fn validate_manifest(
 ) -> Result<SubgraphManifest, SubgraphRegistrarError> {
     let mut errors: Vec<SubgraphManifestValidationError> = Vec::new();
 
+    // Validate that the manifest has at least one data source
+    if manifest.data_sources.is_empty() {
+        errors.push(SubgraphManifestValidationError::NoDataSources);
+    }
+
     // Validate that the manifest has a `source` address in each data source
     // which has call or block handlers
     let has_invalid_data_source = manifest.data_sources.iter().any(|data_source| {

graph/src/data/subgraph/mod.rs

@@ -321,6 +321,8 @@ pub enum SubgraphAssignmentProviderEvent {
 
 #[derive(Fail, Debug)]
 pub enum SubgraphManifestValidationError {
+    #[fail(display = "subgraph has no data sources")]
+    NoDataSources,
     #[fail(display = "subgraph source address is required")]
     SourceAddressRequired,
     #[fail(display = "subgraph cannot index data from different Ethereum networks")]