Provide better error message if Docker is not running

Previously, if the Spring Boot build plugins got a connection error
when attempting to communicate with a Docker daemon (for example,
when the daemon isn't running), the error message made it appear that
the daemon returned an HTTP error code. This commit makes a connection
error distinct from an HTTP error response code to make it easier for
the user to diagnose the root cause of the problem.

Fixes gh-21554

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/main/java/org/springframework/boot/buildpack/platform/docker/transport/DockerConnectionException.java

@@ -0,0 +1,55 @@
+/*
+ * 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.springframework.boot.buildpack.platform.docker.transport;
+
+import org.springframework.util.Assert;
+
+/**
+ * Exception thrown when connection to the Docker daemon fails.
+ *
+ * @author Scott Frederick
+ * @since 2.3.0
+ */
+public class DockerConnectionException extends RuntimeException {
+
+	private static final String JNA_EXCEPTION_CLASS_NAME = "com.sun.jna.LastErrorException";
+
+	public DockerConnectionException(String host, Exception cause) {
+		super(buildMessage(host, cause), cause);
+	}
+
+	private static String buildMessage(String host, Exception cause) {
+		Assert.notNull(host, "host must not be null");
+		Assert.notNull(cause, "cause must not be null");
+		StringBuilder message = new StringBuilder("Connection to the Docker daemon at '" + host + "' failed");
+		String causeMessage = getCauseMessage(cause);
+		if (causeMessage != null && !causeMessage.isEmpty()) {
+			message.append(" with error \"").append(causeMessage).append("\"");
+		}
+		message.append("; ensure the Docker daemon is running and accessible");
+		return message.toString();
+	}
+
+	private static String getCauseMessage(Exception cause) {
+		if (cause.getCause() != null && cause.getCause().getClass().getName().equals(JNA_EXCEPTION_CLASS_NAME)) {
+			return cause.getCause().getMessage();
+		}
+
+		return cause.getMessage();
+	}
+
+}

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/main/java/org/springframework/boot/buildpack/platform/docker/transport/DockerEngineException.java

@@ -21,7 +21,7 @@
 import org.springframework.util.Assert;
 
 /**
- * Exception throw when the Docker API fails.
+ * Exception thrown when a call to the Docker API fails.
  *
  * @author Phillip Webb
  * @author Scott Frederick

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/main/java/org/springframework/boot/buildpack/platform/docker/transport/HttpClientTransport.java

@@ -137,7 +137,7 @@ private Response execute(HttpUriRequest request) {
 			return new HttpClientResponse(response);
 		}
 		catch (IOException ex) {
-			throw new DockerEngineException(this.host.toHostString(), request.getURI(), 500, ex.getMessage(), null);
+			throw new DockerConnectionException(this.host.toHostString(), ex);
 		}
 	}
 

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/test/java/org/springframework/boot/buildpack/platform/docker/transport/DockerConnectionExceptionTests.java

@@ -0,0 +1,62 @@
+/*
+ * 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.springframework.boot.buildpack.platform.docker.transport;
+
+import java.io.IOException;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+
+/**
+ * Tests for {@link DockerEngineException}.
+ *
+ * @author Scott Frederick
+ */
+class DockerConnectionExceptionTests {
+
+	private static final String HOST = "docker://localhost/";
+
+	@Test
+	void createWhenHostIsNullThrowsException() {
+		assertThatIllegalArgumentException().isThrownBy(() -> new DockerConnectionException(null, null))
+				.withMessage("host must not be null");
+	}
+
+	@Test
+	void createWhenCauseIsNullThrowsException() {
+		assertThatIllegalArgumentException().isThrownBy(() -> new DockerConnectionException(HOST, null))
+				.withMessage("cause must not be null");
+	}
+
+	@Test
+	void createWithIOException() {
+		DockerConnectionException exception = new DockerConnectionException(HOST, new IOException("error"));
+		assertThat(exception.getMessage())
+				.contains("Connection to the Docker daemon at 'docker://localhost/' failed with error \"error\"");
+	}
+
+	@Test
+	void createWithLastErrorException() {
+		DockerConnectionException exception = new DockerConnectionException(HOST,
+				new IOException(new com.sun.jna.LastErrorException("root cause")));
+		assertThat(exception.getMessage())
+				.contains("Connection to the Docker daemon at 'docker://localhost/' failed with error \"root cause\"");
+	}
+
+}

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/test/java/org/springframework/boot/buildpack/platform/docker/transport/HttpClientTransportTests.java

@@ -195,10 +195,8 @@ void executeWhenResponseIsIn500RangeShouldThrowDockerException() {
 	void executeWhenClientThrowsIOExceptionRethrowsAsDockerException() throws IOException {
 		given(this.client.execute(any(HttpHost.class), any(HttpRequest.class)))
 				.willThrow(new IOException("test IO exception"));
-		assertThatExceptionOfType(DockerEngineException.class).isThrownBy(() -> this.http.get(this.uri))
-				.satisfies((ex) -> assertThat(ex.getErrors()).isNull()).satisfies(DockerEngineException::getStatusCode)
-				.withMessageContaining("500")
-				.satisfies((ex) -> assertThat(ex.getReasonPhrase()).contains("test IO exception"));
+		assertThatExceptionOfType(DockerConnectionException.class).isThrownBy(() -> this.http.get(this.uri))
+				.satisfies((ex) -> assertThat(ex.getMessage()).contains("test IO exception"));
 	}
 
 	private String writeToString(HttpEntity entity) throws IOException {