Core: add only_ancient_segments to upgrade API, so only segments with an old Lucene version are upgraded

This option defaults to false, because it is also important to upgrade
the "merely old" segments since many Lucene improvements happen within
minor releases.

But you can pass true to do the minimal work necessary to upgrade to
the next major Elasticsearch release.

The HTTP GET upgrade request now also breaks out how many bytes of
ancient segments need upgrading.

Closes #10213

Closes #10540

Conflicts:
	dev-tools/create_bwc_index.py
	rest-api-spec/api/indices.upgrade.json
	src/main/java/org/elasticsearch/action/admin/indices/optimize/OptimizeRequest.java
	src/main/java/org/elasticsearch/action/admin/indices/optimize/ShardOptimizeRequest.java
	src/main/java/org/elasticsearch/action/admin/indices/optimize/TransportOptimizeAction.java
	src/main/java/org/elasticsearch/index/engine/InternalEngine.java
	src/test/java/org/elasticsearch/bwcompat/StaticIndexBackwardCompatibilityTest.java
	src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java
	src/test/java/org/elasticsearch/rest/action/admin/indices/upgrade/UpgradeReallyOldIndexTest.java

dev-tools/create_bwc_index_with_some_ancient_segments.py

@@ -0,0 +1,113 @@
+import create_bwc_index
+import logging
+import os
+import random
+import shutil
+import subprocess
+import sys
+import tempfile
+
+def fetch_version(version):
+  logging.info('fetching ES version %s' % version)
+  if subprocess.call([sys.executable, os.path.join(os.path.split(sys.argv[0])[0], 'get-bwc-version.py'), version]) != 0:
+    raise RuntimeError('failed to download ES version %s' % version)
+
+def main():
+  '''
+  Creates a static back compat index (.zip) with mixed 0.20 (Lucene 3.x) and 0.90 (Lucene 4.x) segments. 
+  '''
+  
+  logging.basicConfig(format='[%(levelname)s] [%(asctime)s] %(message)s', level=logging.INFO,
+                      datefmt='%Y-%m-%d %I:%M:%S %p')
+  logging.getLogger('elasticsearch').setLevel(logging.ERROR)
+  logging.getLogger('urllib3').setLevel(logging.WARN)
+
+  tmp_dir = tempfile.mkdtemp()
+  try:
+    data_dir = os.path.join(tmp_dir, 'data')
+    logging.info('Temp data dir: %s' % data_dir)
+
+    first_version = '0.20.6'
+    second_version = '0.90.6'
+    index_name = 'index-%s-and-%s' % (first_version, second_version)
+
+    # Download old ES releases if necessary:
+    release_dir = os.path.join('backwards', 'elasticsearch-%s' % first_version)
+    if not os.path.exists(release_dir):
+      fetch_version(first_version)
+
+    node = create_bwc_index.start_node(first_version, release_dir, data_dir, cluster_name=index_name)
+    client = create_bwc_index.create_client()
+
+    # Creates the index & indexes docs w/ first_version:
+    create_bwc_index.generate_index(client, first_version, index_name)
+
+    # Make sure we write segments:
+    flush_result = client.indices.flush(index=index_name)
+    if not flush_result['ok']:
+      raise RuntimeError('flush failed: %s' % str(flush_result))
+
+    segs = client.indices.segments(index=index_name)
+    shards = segs['indices'][index_name]['shards']
+    if len(shards) != 1:
+      raise RuntimeError('index should have 1 shard but got %s' % len(shards))
+
+    first_version_segs = shards['0'][0]['segments'].keys()
+
+    create_bwc_index.shutdown_node(node)
+    print('%s server output:\n%s' % (first_version, node.stdout.read().decode('utf-8')))
+    node = None
+
+    release_dir = os.path.join('backwards', 'elasticsearch-%s' % second_version)
+    if not os.path.exists(release_dir):
+      fetch_version(second_version)
+
+    # Now also index docs with second_version:
+    node = create_bwc_index.start_node(second_version, release_dir, data_dir, cluster_name=index_name)
+    client = create_bwc_index.create_client()
+
+    # If we index too many docs, the random refresh/flush causes the ancient segments to be merged away:
+    num_docs = 10
+    create_bwc_index.index_documents(client, index_name, 'doc', num_docs)
+
+    # Make sure we get a segment:
+    flush_result = client.indices.flush(index=index_name)
+    if not flush_result['ok']:
+      raise RuntimeError('flush failed: %s' % str(flush_result))
+
+    # Make sure we see mixed segments (it's possible Lucene could have "accidentally" merged away the first_version segments):
+    segs = client.indices.segments(index=index_name)
+    shards = segs['indices'][index_name]['shards']
+    if len(shards) != 1:
+      raise RuntimeError('index should have 1 shard but got %s' % len(shards))
+
+    second_version_segs = shards['0'][0]['segments'].keys()
+    #print("first: %s" % first_version_segs)
+    #print("second: %s" % second_version_segs)
+
+    for segment_name in first_version_segs:
+      if segment_name in second_version_segs:
+        # Good: an ancient version seg "survived":
+        break
+    else:
+      raise RuntimeError('index has no first_version segs left')
+
+    for segment_name in second_version_segs:
+      if segment_name not in first_version_segs:
+        # Good: a second_version segment was written
+        break
+    else:
+      raise RuntimeError('index has no second_version segs left')
+
+    create_bwc_index.shutdown_node(node)
+    print('%s server output:\n%s' % (second_version, node.stdout.read().decode('utf-8')))
+    node = None
+    create_bwc_index.compress_index('%s-and-%s' % (first_version, second_version), tmp_dir, 'src/test/resources/org/elasticsearch/rest/action/admin/indices/upgrade')
+  finally:
+    if node is not None:
+      create_bwc_index.shutdown_node(node)
+    shutil.rmtree(tmp_dir)
+    
+if __name__ == '__main__':
+  main()
+