Allow the user that runs the app to be specified via an env var

See gh-16973

spring-boot-project/spring-boot-docs/src/main/asciidoc/deployment.adoc

@@ -708,6 +708,11 @@ The following environment properties are supported with the default script:
   The default depends on the way the jar was built but is usually `auto` (meaning it tries to guess if it is an init script by checking if it is a symlink in a directory called `init.d`).
   You can explicitly set it to `service` so that the `stop\|start\|status\|restart` commands work or to `run` if you want to run the script in the foreground.
 
+| `RUN_AS_USER`
+| If set, the application will be executed as the informed user.
+  For security reasons, you should never run an user space application as `root`, therefore it's recommended to set this property.
+  Defaults to the user who owns the jar file.
+
 | `USE_START_STOP_DAEMON`
 | Whether the `start-stop-daemon` command, when it's available, should be used to control the process.
   Defaults to `true`.

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/main/resources/org/springframework/boot/loader/tools/launch.script

@@ -128,6 +128,26 @@ log_file="$LOG_FOLDER/$LOG_FILENAME"
 # shellcheck disable=SC2012
 [[ $(id -u) == "0" ]] && run_user=$(ls -ld "$jarfile" | awk '{print $3}')
 
+# Force run as informed user (from environment variable)
+if [[ -n "$RUN_AS_USER" ]]; then
+    # checks performed for all actions except 'status' and 'run'
+    if ! [[ "$action" =~ ^(status|run)$ ]]; then
+        # Issue a error if informed user is not valid
+        id -u "$RUN_AS_USER" || {
+            echoRed "Cannot run as '$RUN_AS_USER': no such user"
+            exit 5
+        }
+
+        # Issue a error if we are not root
+        [[ $(id -u) == 0 ]] || {
+            echoRed "root required to run as '$RUN_AS_USER'"
+            exit 6
+        }
+    fi
+
+    run_user="$RUN_AS_USER"
+fi
+
 # Issue a warning if the application will run as root
 [[ $(id -u ${run_user}) == "0" ]] && { echoYellow "Application is running as root (UID 0). This is considered insecure."; }
 

spring-boot-tests/spring-boot-integration-tests/spring-boot-launch-script-tests/src/test/java/org/springframework/boot/launchscript/SysVinitLaunchScriptIT.java

@@ -267,6 +267,36 @@ public class SysVinitLaunchScriptIT {
 		assertThat(output).contains("Log written");
 	}
 
+	@ParameterizedTest(name = "{0} {1}")
+	@MethodSource("parameters")
+	public void launchWithRunAs(String os, String version) throws Exception {
+		String output = doTest(os, version, "launch-with-run-as.sh");
+		assertThat(output).contains("wagner root");
+	}
+
+	@ParameterizedTest(name = "{0} {1}")
+	@MethodSource("parameters")
+	public void launchWithRunAsInvalidUser(String os, String version) throws Exception {
+		String output = doTest(os, version, "launch-with-run-as-invalid-user.sh");
+		assertThat(output).contains("Status: 5");
+		assertThat(output).has(coloredString(AnsiColor.RED, "Cannot run as 'johndoe': no such user"));
+	}
+
+	@ParameterizedTest(name = "{0} {1}")
+	@MethodSource("parameters")
+	public void launchWithRunAsPreferUserInformed(String os, String version) throws Exception {
+		String output = doTest(os, version, "launch-with-run-as-prefer-user-informed.sh");
+		assertThat(output).contains("wagner root");
+	}
+
+	@ParameterizedTest(name = "{0} {1}")
+	@MethodSource("parameters")
+	public void launchWithRunAsRootRequired(String os, String version) throws Exception {
+		String output = doTest(os, version, "launch-with-run-as-root-required.sh");
+		assertThat(output).contains("Status: 6");
+		assertThat(output).has(coloredString(AnsiColor.RED, "root required to run as 'wagner'"));
+	}
+
 	static List<Object[]> parameters() {
 		List<Object[]> parameters = new ArrayList<>();
 		for (File os : new File("src/test/resources/conf").listFiles()) {

spring-boot-tests/spring-boot-integration-tests/spring-boot-launch-script-tests/src/test/resources/scripts/launch-with-run-as-invalid-user.sh

+source ./test-functions.sh
+install_service
+
+echo 'RUN_AS_USER=johndoe' > /test-service/spring-boot-app.conf
+
+start_service
+echo "Status: $?"

spring-boot-tests/spring-boot-integration-tests/spring-boot-launch-script-tests/src/test/resources/scripts/launch-with-run-as-prefer-user-informed.sh

+source ./test-functions.sh
+install_service
+
+useradd wagner
+echo 'RUN_AS_USER=wagner' > /test-service/spring-boot-app.conf
+
+useradd phil
+chown phil /test-service/spring-boot-app.jar
+
+start_service
+await_app
+
+ls -la /var/log/spring-boot-app.log

spring-boot-tests/spring-boot-integration-tests/spring-boot-launch-script-tests/src/test/resources/scripts/launch-with-run-as-root-required.sh

+source ./test-functions.sh
+install_service
+
+useradd wagner
+echo 'RUN_AS_USER=wagner' > /test-service/spring-boot-app.conf
+echo "JAVA_HOME='$JAVA_HOME'" >> /test-service/spring-boot-app.conf
+
+su - wagner -c "$(which service) spring-boot-app start"
+echo "Status: $?"

spring-boot-tests/spring-boot-integration-tests/spring-boot-launch-script-tests/src/test/resources/scripts/launch-with-run-as.sh

+source ./test-functions.sh
+install_service
+
+useradd wagner
+echo 'RUN_AS_USER=wagner' > /test-service/spring-boot-app.conf
+
+start_service
+await_app
+
+ls -la /var/log/spring-boot-app.log