[maven-plugin] Prevent always re-generating code

Description

The codegen plugin currently always re-generates code, even though neither the plugin configuration nor the input specification file changed. This results in later Maven build steps to be also re-executed for no reason, which finally make build times much longer than necessary for rebuilds.

Swagger-codegen version

3.0.18

Swagger declaration file content or url

not relevant

Command line used for generation

Plugin configuration:

<plugin>
    <groupId>io.swagger.codegen.v3</groupId>
    <artifactId>swagger-codegen-maven-plugin</artifactId>
    <version>3.0.18</version>
    <executions>
        <execution>
            <id>generate-api-interfaces</id>
            <goals>
                <goal>generate</goal>
            </goals>
            <configuration>
                <inputSpec>${basedir}/api/openapi.yaml</inputSpec>
                <artifactId>my-service</artifactId>
                <output>${basedir}/target/generated-sources</output>
                <language>spring</language>
                <apiPackage>com.mycorp.svc.api</apiPackage>
                <modelPackage>com.mycorp.svc.model</modelPackage>
                <generateApis>true</generateApis>
                <generateSupportingFiles>false</generateSupportingFiles>
                <configOptions>
                    <sourceFolder>swagger</sourceFolder>
                    <interfaceOnly>true</interfaceOnly>
                    <library>spring-boot</library>
                    <configPackage>com.mycorp.svc</configPackage>
                    <dateLibrary>legacy</dateLibrary>
                </configOptions>
            </configuration>
        </execution>
    </executions>
</plugin>

Steps to reproduce

mvn clean package
mvn package

Related issues/PRs

none

Suggest a fix/enhancement

The plugin could write out a "stamp" file at a well-known location, containing a hash of the plugin configuration. The timestamp of the file itself serves as a marker for the last generation run and can be compared to the file specified as inputSpec parameter.

The plugin should then only re-generate code if either of the following statements is true:

• the inputSpec file is newer than our marker file
• the plugin configuration hash does not match

Start at the Maven plugin's generate goal and reproduce the behavior with the provided mvn clean package and mvn package sequence. Review how the inputSpec and plugin configuration are handled, then verify that unchanged inputs skip regeneration while a newer specification or changed configuration triggers it.