Snapshot fallback should consider build.snapshot

When determining if a build is a snapshot build, we look for a field in
the JAR manifest. However, when running tests, we are not running with a
compiled core Elasticsearch JAR, we are running with the compiled core
classes on the classpath. We have a fallback for this, we always assume
such a situation is a snapshot build. However, when running builds with
-Dbuild.snapshot=false, this is not the case. As such, we need to
fallback to the value of build.snapshot. However, there are cases where
we are not running with a compiled core Elasticsearch JAR (e.g., when
the transport client is embedded in a web container) so we should only
do this fallback if we are in tests. To verify we are in tests, we check
if randomized runner is on the classpath.

Relates elastic#26554

core/src/main/java/org/elasticsearch/Build.java

@@ -19,6 +19,7 @@
 
 package org.elasticsearch;
 
+import org.elasticsearch.common.Booleans;
 import org.elasticsearch.common.io.FileSystemUtils;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
@@ -59,7 +60,18 @@ public class Build {
             // not running from the official elasticsearch jar file (unit tests, IDE, uber client jar, shadiness)
             shortHash = "Unknown";
             date = "Unknown";
-            isSnapshot = true;
+            final String buildSnapshot = System.getProperty("build.snapshot");
+            if (buildSnapshot != null) {
+                try {
+                    Class.forName("com.carrotsearch.randomizedtesting.RandomizedContext");
+                } catch (final ClassNotFoundException e) {
+                    // we are not in tests but build.snapshot is set, bail hard
+                    throw new IllegalStateException("build.snapshot set to [" + buildSnapshot + "] but not running tests");
+                }
+                isSnapshot = Booleans.parseBoolean(buildSnapshot);
+            } else {
+                isSnapshot = true;
+            }
         }
         if (shortHash == null) {
             throw new IllegalStateException("Error finding the build shortHash. " +

test/framework/src/main/java/org/elasticsearch/test/junit/listeners/ReproduceInfoPrinter.java

@@ -141,7 +141,7 @@ public ReproduceErrorMessageBuilder appendESProperties() {
                 appendProperties(ESIntegTestCase.TESTS_ENABLE_MOCK_MODULES);
             }
             appendProperties("tests.assertion.disabled", "tests.security.manager", "tests.nightly", "tests.jvms",
-                             "tests.client.ratio", "tests.heap.size", "tests.bwc", "tests.bwc.version");
+                             "tests.client.ratio", "tests.heap.size", "tests.bwc", "tests.bwc.version", "build.snapshot");
             if (System.getProperty("tests.jvm.argline") != null && !System.getProperty("tests.jvm.argline").isEmpty()) {
                 appendOpt("tests.jvm.argline", "\"" + System.getProperty("tests.jvm.argline") + "\"");
             }