Restore support for non-EnumerablePropertySource in PropertySourcesPlaceholderConfigurer

Commit 3295289e17 fixed a number issues with placeholder resolution in
PropertySourcesPlaceholderConfigurer. However, in doing so, it replaced
a raw PropertySource with a CompositePropertySource which implements
EnumerablePropertySource.

Consequently, all property sources registered in the Environment must
now implement EnumerablePropertySource (which is not an actual
requirement). Otherwise, invocations of getPropertyNames() on the
CompositePropertySource result in an IllegalStateException, and that is
a breaking change which resulted in numerous build failures within the
Spring portfolio.

To address that regression, this commit introduces a private
ConfigurableEnvironmentPropertySource in
PropertySourcesPlaceholderConfigurer which is a "raw" PropertySource
that delegates directly to the PropertySources in a
ConfigurableEnvironment.

This commit also extracts the raw PropertySource for direct Environment
delegation into a new FallbackEnvironmentPropertySource.

See gh-17385
Closes gh-34861

spring-context/src/test/java/org/springframework/context/support/PropertySourcesPlaceholderConfigurerTests.java

@@ -16,6 +16,9 @@
 
 package org.springframework.context.support;
 
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
 import java.util.Optional;
 import java.util.Properties;
 
@@ -34,6 +37,7 @@ import org.springframework.context.annotation.AnnotationConfigApplicationContext
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.convert.support.DefaultConversionService;
+import org.springframework.core.env.EnumerablePropertySource;
 import org.springframework.core.env.MutablePropertySources;
 import org.springframework.core.env.PropertySource;
 import org.springframework.core.env.StandardEnvironment;
@@ -300,6 +304,60 @@ class PropertySourcesPlaceholderConfigurerTests {
 		assertThat(bf.getBean(TestBean.class).getName()).isEqualTo("bar");
 	}
 
+	@Test  // gh-34861
+	void withEnumerableAndNonEnumerablePropertySourcesInTheEnvironmentAndLocalProperties() {
+		DefaultListableBeanFactory bf = new DefaultListableBeanFactory();
+		bf.registerBeanDefinition("testBean",
+				genericBeanDefinition(TestBean.class)
+					.addPropertyValue("name", "${foo:bogus}")
+					.addPropertyValue("jedi", "${local:false}")
+					.getBeanDefinition());
+
+		// 1) MockPropertySource is an EnumerablePropertySource.
+		MockPropertySource mockPropertySource = new MockPropertySource("mockPropertySource")
+				.withProperty("foo", "${bar}");
+
+		// 2) PropertySource is not an EnumerablePropertySource.
+		PropertySource<?> rawPropertySource = new PropertySource<>("rawPropertySource", new Object()) {
+			@Override
+			public Object getProperty(String key) {
+				return ("bar".equals(key) ? "quux" : null);
+			}
+		};
+
+		MockEnvironment env = new MockEnvironment();
+		env.getPropertySources().addFirst(mockPropertySource);
+		env.getPropertySources().addLast(rawPropertySource);
+
+		PropertySourcesPlaceholderConfigurer ppc = new PropertySourcesPlaceholderConfigurer();
+		ppc.setEnvironment(env);
+		// 3) Local properties are stored in a PropertiesPropertySource which is an EnumerablePropertySource.
+		ppc.setProperties(new Properties() {{
+			setProperty("local", "true");
+		}});
+		ppc.postProcessBeanFactory(bf);
+
+		// Verify all properties can be resolved via the Environment.
+		assertThat(env.getProperty("foo")).isEqualTo("quux");
+		assertThat(env.getProperty("bar")).isEqualTo("quux");
+
+		// Verify that placeholder resolution works.
+		TestBean testBean = bf.getBean(TestBean.class);
+		assertThat(testBean.getName()).isEqualTo("quux");
+		assertThat(testBean.isJedi()).isTrue();
+
+		// Verify that the presence of a non-EnumerablePropertySource does not prevent
+		// accessing EnumerablePropertySources via getAppliedPropertySources().
+		List<String> propertyNames = new ArrayList<>();
+		for (PropertySource<?> propertySource : ppc.getAppliedPropertySources()) {
+			if (propertySource instanceof EnumerablePropertySource<?> enumerablePropertySource) {
+				Collections.addAll(propertyNames, enumerablePropertySource.getPropertyNames());
+			}
+		}
+		// Should not contain "foo" or "bar" from the Environment.
+		assertThat(propertyNames).containsOnly("local");
+	}
+
 	@Test
 	void customPlaceholderPrefixAndSuffix() {
 		PropertySourcesPlaceholderConfigurer ppc = new PropertySourcesPlaceholderConfigurer();