perf: parallelize user directory scanning and XML unmarshaling at sta…

core/src/main/java/hudson/model/User.java

@@ -1124,40 +1124,53 @@ public static void scanAll() throws IOException {
             }
             var byName = instance.byName;
             var idStrategy = idStrategy();
-            for (var dir : subdirectories) {
-                var dirName = dir.getName();
-                if (!HASHED_DIRNAMES.matcher(dirName).matches()) {
-                    LOGGER.fine(() -> "ignoring unrecognized dir " + dir);
-                    continue;
-                }
-                var xml = new XmlFile(XSTREAM, new File(dir, CONFIG_XML));
-                if (!xml.exists()) {
-                    LOGGER.fine(() -> "ignoring dir " + dir + " with no " + CONFIG_XML);
-                    continue;
-                }
-                var user = new User();
-                try {
-                    xml.unmarshal(user);
-                } catch (Exception x) {
-                    LOGGER.log(Level.WARNING, "failed to load " + xml, x);
-                    continue;
-                }
-                if (user.id == null) {
-                    LOGGER.warning(() -> "ignoring " + xml + " with no <id>");
-                    continue;
-                }
-                var expectedFolderName = getUserFolderNameFor(user.id);
-                if (!dirName.equals(expectedFolderName)) {
-                    LOGGER.warning(() -> "ignoring " + xml + " with <id> " + user.id + " expected to be in " + expectedFolderName);
-                    continue;
-                }
-                user.fixUpAfterLoad();
-                var old = byName.put(idStrategy.keyFor(user.id), user);
-                if (old != null) {
-                    LOGGER.warning(() -> "entry for " + user.id + " in " + dir + " duplicates one seen earlier for " + old.id);
-                } else {
-                    LOGGER.fine(() -> "successfully loaded " + user.id + " from " + xml);
-                }
+            int concurrency = Integer.getInteger("hudson.model.User.scanConcurrency", Math.min(4, Runtime.getRuntime().availableProcessors()));
+            var pool = new java.util.concurrent.ForkJoinPool(concurrency);
+            try {
+                pool.submit(() -> {
+                    Arrays.stream(subdirectories).parallel().forEach(dir -> {
+                        var dirName = dir.getName();
+                        if (!HASHED_DIRNAMES.matcher(dirName).matches()) {
+                            LOGGER.fine(() -> "ignoring unrecognized dir " + dir);
+                            return;
+                        }
+                        var xml = new XmlFile(XSTREAM, new File(dir, CONFIG_XML));
+                        if (!xml.exists()) {
+                            LOGGER.fine(() -> "ignoring dir " + dir + " with no " + CONFIG_XML);
+                            return;
+                        }
+                        var user = new User();
+                        try {
+                            xml.unmarshal(user);
+                        } catch (Exception x) {
+                            LOGGER.log(Level.WARNING, "failed to load " + xml, x);
+                            return;
+                        }
+                        if (user.id == null) {
+                            LOGGER.warning(() -> "ignoring " + xml + " with no <id>");
+                            return;
+                        }
+                        var expectedFolderName = getUserFolderNameFor(user.id);
+                        if (!dirName.equals(expectedFolderName)) {
+                            LOGGER.warning(() -> "ignoring " + xml + " with <id> " + user.id + " expected to be in " + expectedFolderName);
+                            return;
+                        }
+                        user.fixUpAfterLoad();
+                        var old = byName.put(idStrategy.keyFor(user.id), user);
+                        if (old != null) {
+                            LOGGER.warning(() -> "entry for " + user.id + " in " + dir + " duplicates one seen earlier for " + old.id);
+                        } else {
+                            LOGGER.fine(() -> "successfully loaded " + user.id + " from " + xml);
+                        }
+                    });
+                }).get();
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
+                LOGGER.log(Level.WARNING, "User directory scanning was interrupted", e);
+            } catch (ExecutionException e) {
+                LOGGER.log(Level.WARNING, "User directory scanning failed", e);
+            } finally {
+                pool.shutdown();
             }
             LOGGER.fine(() -> "loaded " + byName.size() + " entries");
         }

test/src/test/java/hudson/model/UserTest.java

@@ -818,4 +818,68 @@ public UserDetails loadUserByUsername2(String username) throws UsernameNotFoundE
             throw new UserMayOrMayNotExistException2(username + " not found");
         }
     }
+
+    @Test
+    void parallelScanAllWithMalformedConfig() throws Exception {
+        List<String> validIds = List.of("valid1", "valid2", "valid3", "valid4", "valid5", "valid6", "valid7", "valid8", "valid9", "valid10");
+        for (String id : validIds) {
+            User.getById(id, true).save();
+        }
+
+        File malformedDir1 = User.getUserFolderFor("malformed1");
+        File malformedDir2 = User.getUserFolderFor("malformed2");
+
+        assertTrue(malformedDir1.mkdirs());
+        assertTrue(malformedDir2.mkdirs());
+
+        // Write a malformed config.xml (syntactically invalid XML) for malformed1
+        java.nio.file.Files.writeString(
+                new File(malformedDir1, "config.xml").toPath(),
+                "<hudson.model.User>\n  <id>malformed1</id>\n  <invalid>...\n"
+        );
+
+        // Write a malformed config.xml (syntactically valid but unmarshal-failing XML) for malformed2
+        java.nio.file.Files.writeString(
+                new File(malformedDir2, "config.xml").toPath(),
+                "<hudson.model.Descriptor>\n</hudson.model.Descriptor>\n"
+        );
+
+        try {
+            // Clear the memory cache of users before scanning to make sure scanAll actually loads them
+            User.clear();
+
+            // Verify they are not in the registry
+            for (String id : validIds) {
+                assertNull(User.getById(id, false));
+            }
+            assertNull(User.getById("malformed1", false));
+            assertNull(User.getById("malformed2", false));
+
+            // Execute the parallel unmarshaling / scan
+            User.AllUsers.scanAll();
+
+            // Verify that all valid users are successfully unmarshaled and populated in the registry
+            for (String id : validIds) {
+                User u = User.getById(id, false);
+                assertNotNull(u, "User " + id + " should have been loaded");
+                assertEquals(id, u.getId());
+            }
+
+            // Verify that malformed users are not populated in the registry
+            assertNull(User.getById("malformed1", false), "Malformed user 1 should not be loaded");
+            assertNull(User.getById("malformed2", false), "Malformed user 2 should not be loaded");
+
+        } finally {
+            // Clean up files on disk
+            for (String id : validIds) {
+                hudson.Util.deleteRecursive(User.getUserFolderFor(id));
+            }
+            hudson.Util.deleteRecursive(malformedDir1);
+            hudson.Util.deleteRecursive(malformedDir2);
+
+            // Reset the memory cache again
+            User.clear();
+        }
+    }
 }
+