Merge pull request #6976 from onflow/alex/result-approvals-godoc

documentation of storage abstraction interface `storage.ResultApprovals`

Author: AlexHentschel

storage/approvals.go

@@ -4,17 +4,41 @@ import (
 	"github.com/onflow/flow-go/model/flow"
 )
 
+// ResultApprovals implements persistent storage for result approvals.
+// Implementations of this interface must be concurrency safe.
+//
+// CAUTION suitable only for _Verification Nodes_ for persisting their _own_ approvals!
+//   - In general, the Flow protocol requires multiple approvals for the same chunk from different
+//     verification nodes. In other words, there are multiple different approvals for the same chunk.
+//   - Internally, ResultApprovals populates an index from Executed Chunk ➜ ResultApproval. This is
+//     *only safe* for Verification Nodes when tracking their own approvals (for the same ExecutionResult,
+//     a Verifier will always produce the same approval)
 type ResultApprovals interface {
 
-	// Store stores a ResultApproval
+	// Store stores a ResultApproval by its ID.
+	// No errors are expected during normal operations.
 	Store(result *flow.ResultApproval) error
 
-	// Index indexes a ResultApproval by result ID and chunk index
+	// Index indexes a ResultApproval by result ID and chunk index.
+	//
+	// CAUTION: the Flow protocol requires multiple approvals for the same chunk from different verification
+	// nodes. In other words, there are multiple different approvals for the same chunk. Therefore, the index
+	// Executed Chunk ➜ ResultApproval ID (populated here) is *only safe* to be used by Verification Nodes
+	// for tracking their own approvals.
+	//
+	// For the same ExecutionResult, a Verifier will always produce the same approval. Therefore, this operation
+	// is idempotent, i.e. repeated calls with the *same inputs* are equivalent to just calling the method once;
+	// still the method succeeds on each call. However, when attempting to index *different* ResultApproval IDs
+	// for the same key (resultID, chunkIndex) this method returns an exception, as this should never happen for
+	// a correct Verification Node indexing its own approvals.
+	// No errors are expected during normal operations.
 	Index(resultID flow.Identifier, chunkIndex uint64, approvalID flow.Identifier) error
 
-	// ByID retrieves a ResultApproval by its ID
+	// ByID retrieves a ResultApproval by its ID.
+	// Returns [storage.ErrNotFound] if no Approval with the given ID has been stored.
 	ByID(approvalID flow.Identifier) (*flow.ResultApproval, error)
 
-	// ByChunk retrieves a ResultApproval by result ID and chunk index
+	// ByChunk retrieves a ResultApproval by result ID and chunk index.
+	// Returns [storage.ErrNotFound] if no Approval for  the given key (resultID, chunkIndex) has been stored.
 	ByChunk(resultID flow.Identifier, chunkIndex uint64) (*flow.ResultApproval, error)
 }