Only regen in sbt OpenAPI codegen when input YAMLs change

[aelred]

The current behaviour is:

1. Run OpenAPI codegen (unconditionally)
2. Write results to temporary files
3. If a temporary file changed, copy it to the final destination

This saves some work (downstream recompilation etc.) but still means we always run codegen every time.

For larger YAMLs this can take a while, and impacts every build even with no code changes. For me this can add 20-30 seconds to

Changing this behaviour to use FileFunction.cached and / fileInputs to check the input YAML files instead:

1. Check OpenAPI YAMLs have changed (if they haven't changed, stop)
2. Run OpenAPI codegen
3. Write results to final files

Manual test

Verified on my project by compiling the API model twice:

sbt:redzone> apiModel/compile
... first compile ...
sbt:redzone> apiModel/compile
[info] Total time: 80 ms
[info]   apiModel / Compile / compileIncremental                           : 31 ms
[info]   apiModel / Compile / managedSources                               : 12 ms
[info]   apiModel / scalaCompilerBridgeBinaryJar                           : 10 ms
[info]   apiModel / Compile / compile                                      :  9 ms
[info]   apiModel / Compile / generateTapirDefinitions                     :  6 ms
[info]   apiModel / Compile / compileOutputs                               :  4 ms
[info]   apiModel / Compile / scalacOptions                                :  3 ms
[info]   apiModel / update                                                 :  3 ms
[info]   apiModel / sass                                                   :  2 ms
[info]   apiModel / csrRecursiveResolvers                                  :  2 ms
[info]   apiModel / csrFallbackDependencies                                :  1 ms
[info]   apiModel / Compile / unmanagedSources / allInputPathsAndAttributes:  1 ms
[info]   apiModel / csrInterProjectDependencies                            :  1 ms
[success] Total time: 0 s, completed 6 Mar 2026, 15:05:06

Without new plugin:

sbt:redzone> apiModel/compile
... first compile ...
sbt:redzone> apiModel/compile
[info] Zipping file...
[info] Total time: 21062 ms
[info]   apiModel / Compile / managedSources                               : 20976 ms
[info]   apiModel / Compile / compileIncremental                           :    34 ms
[info]   apiModel / csrRecursiveResolvers                                  :    33 ms
[info]   apiModel / csrFallbackDependencies                                :    33 ms
[info]   apiModel / csrInterProjectDependencies                            :    33 ms
[info]   apiModel / scalaCompilerBridgeBinaryJar                           :    31 ms
[info]   apiModel / Compile / scalacOptions                                :    18 ms
[info]   apiModel / Compile / compile                                      :    10 ms
[info]   apiModel / update                                                 :     9 ms
[info]   apiModel / dependencyPositions                                    :     6 ms
[info]   apiModel / Compile / unmanagedSources / allInputPathsAndAttributes:     5 ms
[info]   apiModel / Compile / compileOutputs                               :     4 ms
[info]   apiModel / sass                                                   :     3 ms
[info]   apiModel / Compile / unmanagedSources / inputFileStamps           :     3 ms
[info]   apiModel / projectDescriptors                                     :     3 ms
[info]   apiModel / Compile / unmanagedClasspath                           :     2 ms
[info]   apiModel / Compile / unmanagedJars                                :     2 ms
[info]   apiModel / Compile / managedClasspath                             :     2 ms
[info]   apiModel / scalaInstance                                          :     1 ms
[info]   apiModel / csrProject                                             :     1 ms
[info]   apiModel / ivyModule                                              :     1 ms
[info]   apiModel / csrConfiguration                                       :     1 ms
[info]   apiModel / ivyConfiguration                                       :     1 ms
[info]   streams-manager                                                   :     1 ms
[success] Total time: 21 s, completed 6 Mar 2026, 15:11:56

[hughsimpson]

Thanks @aelred this has annoyed me for ages

[hughsimpson]

Actually I think there are issues with this. If I change the config and rerun, then nothing's regenerated... 🤔

Edit: behaving as desired now, tnx

[hughsimpson]

@adamw a big motivation for wanting this is if you have sourcecode generators to subsequently munge the output of the plugin -- ideally we should make it easier to skip those applications. I think this works as desired since locally it helps me skip a bunch of redundant reprocessing; and at least with my suggested changes, I can't think of any other things other than openapi, config, and plugin version that you'd want to react to.