From 08f4ea7f09037be492d3426531f27f4a024705e8 Mon Sep 17 00:00:00 2001
From: Sam Robson <samrobson321@gmail.com>
Date: Thu, 26 Mar 2026 17:23:59 +0000
Subject: [PATCH] fix: fall back to non-overlay analysis when diff-informed
 analysis is unavailable

---
 lib/init-action.js                  | 16 ++++++++++++++++
 src/diff-informed-analysis-utils.ts |  4 ++++
 src/init-action.ts                  | 26 ++++++++++++++++++++++++++
 3 files changed, 46 insertions(+)

diff --git a/lib/init-action.js b/lib/init-action.js
index dcf3d2bea9..ba52abd01b 100644
--- a/lib/init-action.js
+++ b/lib/init-action.js
@@ -106260,6 +106260,9 @@ function writeDiffRangesJsonFile(logger, ranges) {
 ${jsonContents}`
   );
 }
+function hasDiffRangesJsonFile() {
+  return fs5.existsSync(getDiffRangesJsonFilePath());
+}
 async function getPullRequestEditedDiffRanges(branches, logger) {
   const fileDiffs = await getFileDiffsWithBasehead(branches, logger);
   if (fileDiffs === void 0) {
@@ -110333,6 +110336,19 @@ async function run(startedAt) {
     }
     return;
   }
+  try {
+    if (config.overlayDatabaseMode === "overlay" /* Overlay */ && await shouldPerformDiffInformedAnalysis(codeql, features, logger) && !hasDiffRangesJsonFile()) {
+      logger.warning(
+        `Diff-informed analysis is not available for this pull request. Reverting overlay database mode to ${"none" /* None */}.`
+      );
+      config.overlayDatabaseMode = "none" /* None */;
+    }
+  } catch (e) {
+    logger.warning(
+      `Failed to determine diff-informed analysis availability, reverting overlay database mode to ${"none" /* None */}: ${getErrorMessage(e)}`
+    );
+    config.overlayDatabaseMode = "none" /* None */;
+  }
   let overlayBaseDatabaseStats;
   let dependencyCachingStatus;
   try {
diff --git a/src/diff-informed-analysis-utils.ts b/src/diff-informed-analysis-utils.ts
index 1c98d4caca..3be9c2cb2d 100644
--- a/src/diff-informed-analysis-utils.ts
+++ b/src/diff-informed-analysis-utils.ts
@@ -88,6 +88,10 @@ export function writeDiffRangesJsonFile(
   );
 }
 
+export function hasDiffRangesJsonFile(): boolean {
+  return fs.existsSync(actionsUtil.getDiffRangesJsonFilePath());
+}
+
 export function readDiffRangesJsonFile(
   logger: Logger,
 ): DiffThunkRange[] | undefined {
diff --git a/src/init-action.ts b/src/init-action.ts
index b06903d5c5..6fda0c448c 100644
--- a/src/init-action.ts
+++ b/src/init-action.ts
@@ -40,6 +40,8 @@ import {
 import {
   getDiffInformedAnalysisBranches,
   getPullRequestEditedDiffRanges,
+  hasDiffRangesJsonFile,
+  shouldPerformDiffInformedAnalysis,
   writeDiffRangesJsonFile,
 } from "./diff-informed-analysis-utils";
 import { EnvVar } from "./environment";
@@ -438,6 +440,30 @@ async function run(startedAt: Date) {
     return;
   }
 
+  // If overlay is enabled and diff-informed analysis should have run but
+  // failed to produce output, revert to non-overlay analysis. Overlay
+  // without diff-informed is an untested combination that can produce
+  // inaccurate results.
+  try {
+    if (
+      config.overlayDatabaseMode === OverlayDatabaseMode.Overlay &&
+      (await shouldPerformDiffInformedAnalysis(codeql, features, logger)) &&
+      !hasDiffRangesJsonFile()
+    ) {
+      logger.warning(
+        "Diff-informed analysis is not available for this pull request. " +
+          `Reverting overlay database mode to ${OverlayDatabaseMode.None}.`,
+      );
+      config.overlayDatabaseMode = OverlayDatabaseMode.None;
+    }
+  } catch (e) {
+    logger.warning(
+      `Failed to determine diff-informed analysis availability, ` +
+        `reverting overlay database mode to ${OverlayDatabaseMode.None}: ${getErrorMessage(e)}`,
+    );
+    config.overlayDatabaseMode = OverlayDatabaseMode.None;
+  }
+
   let overlayBaseDatabaseStats: OverlayBaseDatabaseDownloadStats | undefined;
   let dependencyCachingStatus: DependencyCacheRestoreStatusReport | undefined;
   try {