Fix file hashes being deleted

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/BazelLockFileModule.java

@@ -16,7 +16,6 @@
 
 import static com.google.common.collect.ImmutableMap.toImmutableMap;
 import static java.nio.charset.StandardCharsets.UTF_8;
-import static java.util.Comparator.comparing;
 
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSortedMap;
@@ -83,6 +82,27 @@ public void doAfterCommand() {
       return;
     }
 
+    BazelLockFileValue oldLockfile = moduleResolutionEvent.getOnDiskLockfileValue();
+
+    ImmutableMap<String, Optional<Checksum>> fileHashes;
+    if (downloadEvents.isEmpty()) {
+      // BazelDepGraphFunction took the dep graph from the lockfile and didn't cause evaluation of
+      // BazelModuleResolutionFunction. The file hashes in the lockfile are still up-to-date.
+      fileHashes = oldLockfile.getFileHashes();
+    } else {
+      // BazelModuleResolutionFunction ran and emitted events with the up-to-date list of accessed
+      // registry files.
+      fileHashes =
+          downloadEvents.stream()
+              .collect(
+                  toImmutableMap(
+                      RegistryFileDownloadEvent::getUri,
+                      RegistryFileDownloadEvent::getChecksum,
+                      // This should never be called since we take care to download the same file
+                      // only once per build.
+                      BazelLockFileModule::failOnDifferentChecksums));
+    }
+
     // All nodes corresponding to module extensions that have been evaluated in the current build
     // are done at this point. Look up entries by unvalidated keys to record results even if
     // validation later fails due to invalid imports.
@@ -94,26 +114,15 @@ public void doAfterCommand() {
                     entry -> ((SingleExtensionEvalValue.UnvalidatedKey) entry.getKey()).argument(),
                     entry ->
                         ((SingleExtensionEvalValue) entry.getValue()).getLockFileInfo().get()));
+    var combinedExtensionInfos =
+        combineModuleExtensions(oldLockfile.getModuleExtensions(), newExtensionInfos);
 
-    BazelLockFileValue oldLockfile = moduleResolutionEvent.getOnDiskLockfileValue();
     // Create an updated version of the lockfile, keeping only the extension results from the old
     // lockfile that are still up-to-date and adding the newly resolved extension results.
     BazelLockFileValue newLockfile =
         moduleResolutionEvent.getResolutionOnlyLockfileValue().toBuilder()
-            .setModuleExtensions(
-                combineModuleExtensions(oldLockfile.getModuleExtensions(), newExtensionInfos))
-            .setFileHashes(
-                // We ensure that the same file is not downloaded multiple times and can thus
-                // collect without worrying about duplicates.
-                downloadEvents.stream()
-                    .sorted(comparing(RegistryFileDownloadEvent::getUri))
-                    .collect(
-                        toImmutableMap(
-                            RegistryFileDownloadEvent::getUri,
-                            RegistryFileDownloadEvent::getChecksum,
-                            // This should never be called since we take care to download the same
-                            // file only once per build.
-                            BazelLockFileModule::failOnDifferentChecksums)))
+            .setModuleExtensions(combinedExtensionInfos)
+            .setFileHashes(fileHashes)
             .build();
 
     // Write the new value to the file, but only if needed. This is not just a performance