Persist PR diff ranges early during init

We don't use them yet and will re-save them during analysis.

src/init-action.ts

@@ -45,7 +45,7 @@ import {
   runDatabaseInitCluster,
 } from "./init";
 import { KnownLanguage } from "./languages";
-import { getActionsLogger, Logger } from "./logging";
+import { getActionsLogger, Logger, withGroupAsync } from "./logging";
 import {
   downloadOverlayBaseDatabaseFromCache,
   OverlayBaseDatabaseDownloadStats,
@@ -66,6 +66,11 @@ import { ZstdAvailability } from "./tar";
 import { ToolsDownloadStatusReport } from "./tools-download";
 import { ToolsFeature } from "./tools-features";
 import { getCombinedTracerConfig } from "./tracer-config";
+import {
+  getPullRequestEditedDiffRanges,
+  writeDiffRangesJsonFile,
+  getDiffInformedAnalysisBranches,
+} from "./diff-informed-analysis-utils";
 import {
   checkDiskUsage,
   checkForTimeout,
@@ -336,6 +341,8 @@ async function run() {
     });
 
     await checkInstallPython311(config.languages, codeql);
+
+  await computeAndPersistDiffRangesEarly(codeql, features, logger);
   } catch (unwrappedError) {
     const error = wrapError(unwrappedError);
     core.setFailed(error.message);
@@ -748,6 +755,43 @@ async function run() {
   );
 }
 
+/**
+ * Compute and persist diff ranges early during init when diff-informed analysis
+ * is enabled (feature flag + PR context). This writes the standard pr-diff-range.json
+ * file for later reuse in the analyze step. Failures are logged but non-fatal.
+ */
+async function computeAndPersistDiffRangesEarly(
+  codeql: CodeQL,
+  features: Features,
+  logger: Logger,
+): Promise<void> {
+  try {
+    await withGroupAsync("Compute PR diff ranges", async () => {
+      const branches = await getDiffInformedAnalysisBranches(
+        codeql,
+        features,
+        logger,
+      );
+      if (!branches) {
+        return;
+      }
+      const ranges = await getPullRequestEditedDiffRanges(branches, logger);
+      if (ranges === undefined) {
+        return;
+      }
+      writeDiffRangesJsonFile(logger, ranges);
+      const distinctFiles = new Set(ranges.map((r) => r.path)).size;
+      logger.info(
+        `Persisted ${ranges.length} diff range(s) across ${distinctFiles} file(s) for reuse during analyze step.`,
+      );
+    });
+  } catch (e) {
+    logger.warning(
+      `Failed to compute and persist PR diff ranges early: ${getErrorMessage(e)}`,
+    );
+  }
+}
+
 function getTrapCachingEnabled(): boolean {
   // If the workflow specified something always respect that
   const trapCaching = getOptionalInput("trap-caching");