Allow empty env entries when building an image

Prior to this commit, an entry in the environment map provided to the
build plugin image building goal or task that had a null value would
result in a failure with a message that was difficult to diagnose.

This commit treats env map entries with a null value as an empty
entry to prevent the failure and also make it easier to provide an
explicit empty entry in the Maven XML.

Fixes gh-22703

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/main/java/org/springframework/boot/buildpack/platform/build/EphemeralBuilder.java

@@ -74,7 +74,7 @@ private Layer getEnvLayer(Map<String, String> env) throws IOException {
 		return Layer.of((layout) -> {
 			for (Map.Entry<String, String> entry : env.entrySet()) {
 				String name = "/platform/env/" + entry.getKey();
-				Content content = Content.of(entry.getValue());
+				Content content = Content.of((entry.getValue() != null) ? entry.getValue() : "");
 				layout.file(name, Owner.ROOT, content);
 			}
 		});

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/test/java/org/springframework/boot/buildpack/platform/build/EphemeralBuilderTests.java

@@ -25,7 +25,7 @@
 import java.time.Instant;
 import java.time.OffsetDateTime;
 import java.time.ZoneId;
-import java.util.Collections;
+import java.util.HashMap;
 import java.util.Map;
 
 import org.apache.commons.compress.archivers.ArchiveEntry;
@@ -68,7 +68,9 @@ class EphemeralBuilderTests extends AbstractJsonTests {
 	void setup() throws Exception {
 		this.image = Image.of(getContent("image.json"));
 		this.metadata = BuilderMetadata.fromImage(this.image);
-		this.env = Collections.singletonMap("spring", "boot");
+		this.env = new HashMap<>();
+		this.env.put("spring", "boot");
+		this.env.put("empty", null);
 	}
 
 	@Test
@@ -113,6 +115,7 @@ void getArchiveContainsEnvLayer() throws Exception {
 		EphemeralBuilder builder = new EphemeralBuilder(this.owner, this.image, this.metadata, this.creator, this.env);
 		File directory = unpack(getLayer(builder.getArchive(), 0), "env");
 		assertThat(new File(directory, "platform/env/spring")).usingCharset(StandardCharsets.UTF_8).hasContent("boot");
+		assertThat(new File(directory, "platform/env/empty")).usingCharset(StandardCharsets.UTF_8).hasContent("");
 	}
 
 	private TarArchiveInputStream getLayer(ImageArchive archive, int index) throws Exception {

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/intTest/java/org/springframework/boot/maven/BuildImageTests.java

@@ -130,6 +130,24 @@ void whenBuildImageIsInvokedWithCustomBuilderImageAndRunImage(MavenBuild mavenBu
 		});
 	}
 
+	@TestTemplate
+	void whenBuildImageIsInvokedWithEmptyEnvEntry(MavenBuild mavenBuild) {
+		mavenBuild.project("build-image-empty-env-entry").goals("package").prepare(this::writeLongNameResource)
+				.execute((project) -> {
+					assertThat(buildLog(project)).contains("Building image").contains("paketo-buildpacks/builder")
+							.contains("docker.io/library/build-image-empty-env-entry:0.0.1.BUILD-SNAPSHOT")
+							.contains("Successfully built image");
+					ImageReference imageReference = ImageReference.of(ImageName.of("build-image-empty-env-entry"),
+							"0.0.1.BUILD-SNAPSHOT");
+					try (GenericContainer<?> container = new GenericContainer<>(imageReference.toString())) {
+						container.waitingFor(Wait.forLogMessage("Launched\\n", 1)).start();
+					}
+					finally {
+						removeImage(imageReference);
+					}
+				});
+	}
+
 	@TestTemplate
 	void failsWhenBuilderFails(MavenBuild mavenBuild) {
 		mavenBuild.project("build-image-builder-error").goals("package")

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/intTest/projects/build-image-empty-env-entry/pom.xml

@@ -0,0 +1,36 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+	<groupId>org.springframework.boot.maven.it</groupId>
+	<artifactId>build-image-empty-env-entry</artifactId>
+	<version>0.0.1.BUILD-SNAPSHOT</version>
+	<properties>
+		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+		<maven.compiler.source>@java.version@</maven.compiler.source>
+		<maven.compiler.target>@java.version@</maven.compiler.target>
+	</properties>
+	<build>
+		<plugins>
+			<plugin>
+				<groupId>@project.groupId@</groupId>
+				<artifactId>@project.artifactId@</artifactId>
+				<version>@project.version@</version>
+				<executions>
+					<execution>
+						<goals>
+							<goal>build-image</goal>
+						</goals>
+						<configuration>
+							<image>
+								<env>
+									<EMPTY_KEY></EMPTY_KEY>
+								</env>
+							</image>
+						</configuration>
+					</execution>
+				</executions>
+			</plugin>
+		</plugins>
+	</build>
+</project>

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/intTest/projects/build-image-empty-env-entry/src/main/java/org/test/SampleApplication.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.test;
+
+public class SampleApplication {
+
+	public static void main(String[] args) throws Exception {
+		System.out.println("Launched");
+		synchronized(args) {
+			args.wait(); // Prevent exit"
+		}
+	}
+
+}