Make Parsing SnapshotInfo more Efficient (#74005) (#74047)

Flatting the logic for parsing `SnapshotInfo` to go field by field like we do for `RepositoryData`
which is both easier to read and also faster (mostly when moving to batch multiple of these blobs into one
and doing on-the-fly filtering in an upcoming PR where the approach allows for more tricks).
Also, simplified/deduplicated parsing out (mostly/often) empty lists in the deserialization code
and used the new utility in a few more spots as well to save empty lists.

Author: original-brownbear

server/src/main/java/org/elasticsearch/action/admin/cluster/snapshots/create/CreateSnapshotResponse.java

@@ -54,7 +54,7 @@ public CreateSnapshotResponse(@Nullable SnapshotInfo snapshotInfo) {
 
     public CreateSnapshotResponse(StreamInput in) throws IOException {
         super(in);
-        snapshotInfo = in.readOptionalWriteable(SnapshotInfo::new);
+        snapshotInfo = in.readOptionalWriteable(SnapshotInfo::readFrom);
     }
 
     private void setSnapshotInfoFromBuilder(SnapshotInfoBuilder snapshotInfoBuilder) {

server/src/main/java/org/elasticsearch/action/admin/cluster/snapshots/get/GetSnapshotsResponse.java

@@ -53,7 +53,7 @@ public GetSnapshotsResponse(List<SnapshotInfo> snapshots) {
 
     GetSnapshotsResponse(StreamInput in) throws IOException {
         super(in);
-        snapshots = Collections.unmodifiableList(in.readList(SnapshotInfo::new));
+        snapshots = Collections.unmodifiableList(in.readList(SnapshotInfo::readFrom));
     }
 
     /**

server/src/main/java/org/elasticsearch/common/settings/Setting.java

@@ -27,10 +27,10 @@
 import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.common.xcontent.XContentFactory;
 import org.elasticsearch.common.xcontent.XContentParser;
+import org.elasticsearch.common.xcontent.XContentParserUtils;
 import org.elasticsearch.common.xcontent.XContentType;
 
 import java.io.IOException;
-import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.EnumSet;
@@ -1535,18 +1535,11 @@ private static List<String> parseableStringToList(String parsableString) {
         // fromXContent doesn't use named xcontent or deprecation.
         try (XContentParser xContentParser = XContentType.JSON.xContent()
                 .createParser(NamedXContentRegistry.EMPTY, DeprecationHandler.THROW_UNSUPPORTED_OPERATION, parsableString)) {
-            XContentParser.Token token = xContentParser.nextToken();
-            if (token != XContentParser.Token.START_ARRAY) {
-                throw new IllegalArgumentException("expected START_ARRAY but got " + token);
-            }
-            ArrayList<String> list = new ArrayList<>();
-            while ((token = xContentParser.nextToken()) != XContentParser.Token.END_ARRAY) {
-                if (token != XContentParser.Token.VALUE_STRING) {
-                    throw new IllegalArgumentException("expected VALUE_STRING but got " + token);
-                }
-                list.add(xContentParser.text());
-            }
-            return list;
+            xContentParser.nextToken();
+            return XContentParserUtils.parseList(xContentParser, p -> {
+                XContentParserUtils.ensureExpectedToken(XContentParser.Token.VALUE_STRING, p.currentToken(), p);
+                return p.text();
+            });
         } catch (IOException e) {
             throw new IllegalArgumentException("failed to parse array", e);
         }

server/src/main/java/org/elasticsearch/common/xcontent/XContentParserUtils.java

@@ -12,8 +12,11 @@
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.xcontent.XContentParser.Token;
+import org.elasticsearch.core.CheckedFunction;
 
 import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
 import java.util.Locale;
 import java.util.function.Consumer;
 
@@ -151,4 +154,25 @@ public static <T> void parseTypedKeysObject(XContentParser parser, String delimi
             throw new ParsingException(parser.getTokenLocation(), "Failed to parse object: empty key");
         }
     }
+
+    /**
+     * Parses a list of a given type from the given {@code parser}. Assumes that the parser is currently positioned on a
+     * {@link Token#START_ARRAY} token and will fail if it is not. The returned list may or may not be mutable.
+     *
+     * @param parser      x-content parser
+     * @param valueParser parser for expected list value type
+     * @return list parsed from parser
+     */
+    public static <T> List<T> parseList(XContentParser parser,
+                                        CheckedFunction<XContentParser, T, IOException> valueParser) throws IOException {
+        XContentParserUtils.ensureExpectedToken(Token.START_ARRAY, parser.currentToken(), parser);
+        if (parser.nextToken() == Token.END_ARRAY) {
+            return org.elasticsearch.core.List.of();
+        }
+        final ArrayList<T> list = new ArrayList<>();
+        do {
+            list.add(valueParser.apply(parser));
+        } while (parser.nextToken() != Token.END_ARRAY);
+        return list;
+    }
 }

server/src/main/java/org/elasticsearch/index/snapshots/blobstore/BlobStoreIndexShardSnapshot.java

@@ -23,8 +23,6 @@
 import org.elasticsearch.index.store.StoreFileMetadata;
 
 import java.io.IOException;
-import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 import java.util.stream.IntStream;
 
@@ -379,7 +377,7 @@ public BlobStoreIndexShardSnapshot(
         assert indexVersion >= 0;
         this.snapshot = snapshot;
         this.indexVersion = indexVersion;
-        this.indexFiles = Collections.unmodifiableList(new ArrayList<>(indexFiles));
+        this.indexFiles = org.elasticsearch.core.List.copyOf(indexFiles);
         this.startTime = startTime;
         this.time = time;
         this.incrementalFileCount = incrementalFileCount;
@@ -516,7 +514,7 @@ public static BlobStoreIndexShardSnapshot fromXContent(XContentParser parser) th
         int incrementalFileCount = 0;
         long incrementalSize = 0;
 
-        List<FileInfo> indexFiles = new ArrayList<>();
+        List<FileInfo> indexFiles = null;
         if (parser.currentToken() == null) { // fresh parser? move to the first token
             parser.nextToken();
         }
@@ -545,9 +543,7 @@ public static BlobStoreIndexShardSnapshot fromXContent(XContentParser parser) th
                 }
             } else if (token == XContentParser.Token.START_ARRAY) {
                 if (PARSE_FILES.match(currentFieldName, parser.getDeprecationHandler())) {
-                    while ((parser.nextToken()) != XContentParser.Token.END_ARRAY) {
-                        indexFiles.add(FileInfo.fromXContent(parser));
-                    }
+                    indexFiles = XContentParserUtils.parseList(parser, FileInfo::fromXContent);
                 } else {
                     XContentParserUtils.throwUnknownField(currentFieldName, parser.getTokenLocation());
                 }
@@ -559,7 +555,7 @@ public static BlobStoreIndexShardSnapshot fromXContent(XContentParser parser) th
         return new BlobStoreIndexShardSnapshot(
             snapshot,
             indexVersion,
-            Collections.unmodifiableList(indexFiles),
+            indexFiles == null ? org.elasticsearch.core.List.of() : indexFiles,
             startTime,
             time,
             incrementalFileCount,

server/src/main/java/org/elasticsearch/index/snapshots/blobstore/BlobStoreIndexShardSnapshots.java

@@ -273,11 +273,7 @@ public static BlobStoreIndexShardSnapshots fromXContent(XContentParser parser) t
                             currentFieldName = parser.currentName();
                             if (ParseFields.FILES.match(currentFieldName, parser.getDeprecationHandler())
                                 && parser.nextToken() == XContentParser.Token.START_ARRAY) {
-                                List<String> fileNames = new ArrayList<>();
-                                while (parser.nextToken() != XContentParser.Token.END_ARRAY) {
-                                    fileNames.add(parser.text());
-                                }
-                                snapshotsMap.put(snapshot, fileNames);
+                                snapshotsMap.put(snapshot, XContentParserUtils.parseList(parser, XContentParser::text));
                             } else if (ParseFields.SHARD_STATE_ID.match(currentFieldName, parser.getDeprecationHandler())) {
                                 parser.nextToken();
                                 historyUUIDs.put(snapshot, parser.text());