allow usage of scalafix-interfaces without scalafix-properties

This non-deterministic fallback will allow scalafixEnable to work in
sbt-scalafix even though no scalafix-interfaces was explicitly provided.

Author: bjaglin

build.sbt

@@ -380,7 +380,15 @@ lazy val integration = projectMatrix
           .collect { case p @ LocalProject(n) if n.startsWith("cli3") => p }
           .map(_ / publishLocalTransitive)): _*
       )
-      .value
+      .value,
+    // Mimic sbt-scalafix usage of interfaces (without properties per default)
+    // to exercise dynamic loading of latest scalafix-properties artifact
+    Test / internalDependencyClasspath := {
+      val prev = (Test / internalDependencyClasspath).value
+      val propertiesClassDirectory =
+        (properties / Compile / classDirectory).value
+      prev.filter(_.data != propertiesClassDirectory)
+    }
   )
   .defaultAxes(VirtualAxis.jvm)
   .jvmPlatformFull(cliScalaVersions)

scalafix-interfaces/src/main/java/scalafix/interfaces/Scalafix.java

@@ -163,11 +163,26 @@ static Scalafix fetchAndClassloadInstance(String requestedScalaVersion, List<Rep
 
         Properties properties = new Properties();
         String propertiesPath = "scalafix-interfaces.properties";
-        InputStream stream = Scalafix.class.getClassLoader().getResourceAsStream(propertiesPath);
         try {
+            InputStream stream = Scalafix.class.getClassLoader().getResourceAsStream(propertiesPath);
             properties.load(stream);
-        } catch (IOException | NullPointerException e) {
-            throw new ScalafixException("Failed to load '" + propertiesPath + "' to lookup versions", e);
+        } catch (Exception e) {
+            System.err.println(
+                "Failed to load '" + propertiesPath +  "' from local artifact, " +
+                "falling back to fetching the latest scalafix version...");
+
+            try {
+                List<URL> jars = ScalafixCoursier.latestScalafixPropertiesJars(repositories);
+                URLClassLoader classLoader =
+                    new URLClassLoader(jars.stream().toArray(URL[]::new), null);
+                
+                InputStream stream = classLoader.getResourceAsStream(propertiesPath);
+                properties.load(stream);
+            } catch (Exception ee) {
+                throw new ScalafixException(
+                    "Failed to load '" + propertiesPath + "' from local & remote artifacts",
+                    ee);
+            }
         }
 
         String scalafixVersion = properties.getProperty("scalafixVersion");

scalafix-interfaces/src/main/java/scalafix/internal/interfaces/ScalafixCoursier.java

@@ -15,6 +15,21 @@
 
 public class ScalafixCoursier {
 
+    private static VersionListing versions(
+            List<Repository> repositories,
+            coursierapi.Module module
+    ) throws ScalafixException {
+        try {
+            return Versions.create()
+                    .withModule(module)
+                    .withRepositories(repositories.stream().toArray(Repository[]::new))
+                    .versions()
+                    .getMergedListings();
+        } catch (CoursierError e) {
+            throw new ScalafixException("Failed to list versions for " + module + " from " + repositories, e);
+        }
+    }
+
     private static FetchResult fetch(
             List<Repository> repositories,
             List<Dependency> dependencies,
@@ -44,6 +59,16 @@ private static List<URL> toURLs(FetchResult result) throws ScalafixException {
         return urls;
     }
 
+    public static List<URL> latestScalafixPropertiesJars(
+            List<Repository> repositories
+    ) throws ScalafixException {
+        Module module = Module.of("ch.epfl.scala", "scalafix-properties");
+        // TODO: switch to release once we have a stable cut
+        String version = versions(repositories, module).getLatest();
+        Dependency scalafixProperties = Dependency.of(module, version);
+        return toURLs(fetch(repositories, Collections.singletonList(scalafixProperties), ResolutionParams.create()));
+    }
+
     public static List<URL> scalafixCliJars(
             List<Repository> repositories,
             String scalafixVersion,