fix: address review feedback on fetch-events.ts (#315)

- Replace hardcoded Infura API key with INFURA_API_KEY env var; fail fast
  with a helpful message if unset and no --rpc-url is given
- Add onchain/script/backtesting/.gitignore (cache/) instead of relying on
  the opaque root pattern; remove force-tracked cache/.gitkeep (mkdirSync
  creates the directory at runtime)
- Document resume constraint: reliable only when both --start-block and
  --end-block are explicit, or --output is set
- Fix batch-number display: derive batchNum inside the loop from the actual
  `from` block so it stays correct when resumeFromBlock isn't BATCH_SIZE-aligned
- Guard log.logIndex === null consistently with blockNumber/transactionHash
- console.warn on decode errors instead of silently discarding them

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

onchain/script/backtesting/fetch-events.ts

@@ -7,6 +7,13 @@
  *   npx tsx fetch-events.ts --pool 0x0d59... --days 7
  *   npx tsx fetch-events.ts --pool 0x0d59... --start-block 12345678 --end-block 12989078
  *   npx tsx fetch-events.ts --pool 0x0d59... --days 7 --output /tmp/events.jsonl
+ *
+ * Requires the INFURA_API_KEY environment variable unless --rpc-url is given.
+ *
+ * NOTE: Resume (re-run with the same flags) only works reliably when both
+ * --start-block and --end-block are provided, or when --output is explicit.
+ * Without --end-block the output filename embeds the runtime latestBlock,
+ * which changes on subsequent runs and creates a new file instead of resuming.
  */
 
 import { createPublicClient, http, parseAbi, decodeEventLog } from "viem";
@@ -20,8 +27,19 @@ import { fileURLToPath } from "url";
 // Constants
 // ---------------------------------------------------------------------------
 
-const DEFAULT_INFURA_URL =
-  "https://base-mainnet.infura.io/v3/409c42ecaa4e405bb5735faac0f7aec2";
+/** Build the default RPC URL from the INFURA_API_KEY environment variable. */
+function buildDefaultRpcUrl(): string {
+  const key = process.env.INFURA_API_KEY;
+  if (!key) {
+    console.error(
+      "Error: INFURA_API_KEY environment variable is not set.\n" +
+        "  Set it with: export INFURA_API_KEY=<your-infura-project-id>\n" +
+        "  or pass --rpc-url <url> to use a different RPC endpoint."
+    );
+    process.exit(1);
+  }
+  return `https://base-mainnet.infura.io/v3/${key}`;
+}
 
 /** Base mainnet produces ~1 block every 2 seconds → ~43 200 blocks/day. */
 const BASE_BLOCKS_PER_DAY = 43_200;
@@ -88,7 +106,7 @@ function parseArgs(): Args {
     startBlock: startBlockRaw !== undefined ? parseInt(startBlockRaw, 10) : null,
     endBlock: endBlockRaw !== undefined ? parseInt(endBlockRaw, 10) : null,
     output: getFlag("--output") ?? null,
-    rpcUrl: getFlag("--rpc-url") ?? DEFAULT_INFURA_URL,
+    rpcUrl: getFlag("--rpc-url") ?? buildDefaultRpcUrl(),
   };
 }
 
@@ -208,15 +226,14 @@ async function main(): Promise<void> {
   // Fetch in batches
   // ------------------------------------------------------------------
   const totalBatches = Math.ceil((endBlock - startBlock + 1) / BATCH_SIZE);
-  // Batch index that corresponds to resumeFromBlock (1-based for display)
-  const startBatchNum =
-    Math.floor((resumeFromBlock - startBlock) / BATCH_SIZE) + 1;
 
   let totalEvents = 0;
-  let batchNum = startBatchNum;
 
   for (let from = resumeFromBlock; from <= endBlock; from += BATCH_SIZE) {
     const to = Math.min(from + BATCH_SIZE - 1, endBlock);
+    // Derive batch number from actual `from` position so it stays correct
+    // even when resumeFromBlock is not aligned to a BATCH_SIZE boundary.
+    const batchNum = Math.floor((from - startBlock) / BATCH_SIZE) + 1;
 
     process.stdout.write(`Fetching blocks ${from}-${to}... `);
 
@@ -256,7 +273,7 @@ async function main(): Promise<void> {
     let batchEvents = 0;
 
     for (const log of logs) {
-      if (log.blockNumber === null || log.transactionHash === null) continue;
+      if (log.blockNumber === null || log.transactionHash === null || log.logIndex === null) continue;
 
       try {
         const decoded = decodeEventLog({
@@ -276,13 +293,17 @@ async function main(): Promise<void> {
         appendFileSync(outputPath, JSON.stringify(entry) + "\n");
         batchEvents++;
         totalEvents++;
-      } catch {
-        // Skip any log we cannot decode (shouldn't happen for our known pool)
+      } catch (err) {
+        // Decode failure is unexpected for a filtered-by-address+topic0 log;
+        // warn so any ABI drift is visible rather than silently discarded.
+        console.warn(
+          `Warning: could not decode log tx=${log.transactionHash} logIndex=${log.logIndex}:`,
+          err instanceof Error ? err.message : err
+        );
       }
     }
 
     console.log(`${batchEvents} events (${batchNum}/${totalBatches} batches)`);
-    batchNum++;
 
     // Polite inter-batch delay (skip after last batch)
     if (from + BATCH_SIZE <= endBlock) {