Fix MustacheEnvironmentCollector to not ignore native fetcher

When the context is a map (as it is in a web View for instance)
you can't assume a non-null fetcher actually contains the property
you are searching for. This change alters the logic so that the
native fetcher is always consulted if it exists, but there is
always a fallback.

Fixes gh-21045

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/mustache/MustacheEnvironmentCollector.java

@@ -17,6 +17,7 @@
 package org.springframework.boot.autoconfigure.mustache;
 
 import com.samskivert.mustache.DefaultCollector;
+import com.samskivert.mustache.Template;
 import com.samskivert.mustache.Mustache.Collector;
 import com.samskivert.mustache.Mustache.VariableFetcher;
 
@@ -35,8 +36,6 @@ public class MustacheEnvironmentCollector extends DefaultCollector implements En
 
 	private ConfigurableEnvironment environment;
 
-	private final VariableFetcher propertyFetcher = new PropertyVariableFetcher();
-
 	@Override
 	public void setEnvironment(Environment environment) {
 		this.environment = (ConfigurableEnvironment) environment;
@@ -46,18 +45,39 @@ public void setEnvironment(Environment environment) {
 	public VariableFetcher createFetcher(Object ctx, String name) {
 		VariableFetcher fetcher = super.createFetcher(ctx, name);
 		if (fetcher != null) {
-			return fetcher;
+			return new PropertyVariableFetcher(fetcher);
 		}
 		if (this.environment.containsProperty(name)) {
-			return this.propertyFetcher;
+			return new PropertyVariableFetcher();
 		}
 		return null;
 	}
 
 	private class PropertyVariableFetcher implements VariableFetcher {
 
+		private final VariableFetcher nativeFetcher;
+
+		public PropertyVariableFetcher() {
+			this.nativeFetcher = null;
+		}
+
+		public PropertyVariableFetcher(VariableFetcher nativeFetcher) {
+			this.nativeFetcher = nativeFetcher;
+		}
+
 		@Override
 		public Object get(Object ctx, String name) {
+			if (this.nativeFetcher != null) {
+				Object result;
+				try {
+					result = this.nativeFetcher.get(ctx, name);
+					if (result != null && result != Template.NO_FETCHER_FOUND) {
+						return result;
+					}
+				} catch (Exception e) {
+					// fall through
+				}
+			}
 			return MustacheEnvironmentCollector.this.environment.getProperty(name);
 		}
 

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/mustache/MustacheStandaloneIntegrationTests.java

@@ -60,11 +60,22 @@ void environmentCollectorCompoundKeyStandard() {
 				.isEqualTo("Hello: There");
 	}
 
+	@Test
+	void environmentCollectorCompoundKeyStandardMap() {
+		assertThat(this.compiler.standardsMode(true).compile("Hello: {{env.foo}}").execute(Collections.singletonMap("world", "World")))
+				.isEqualTo("Hello: There");
+	}
+
 	@Test
 	void environmentCollectorSimpleKey() {
 		assertThat(this.compiler.compile("Hello: {{foo}}").execute(new Object())).isEqualTo("Hello: World");
 	}
 
+	@Test
+	void environmentCollectorSimpleKeyMap() {
+		assertThat(this.compiler.compile("Hello: {{foo}}").execute(Collections.singletonMap("world", "Foo"))).isEqualTo("Hello: World");
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	@Import({ MustacheAutoConfiguration.class, PropertyPlaceholderAutoConfiguration.class })
 	static class Application {