fix: Handle race conditions when getting the compiler bridge

It was identified in #738 that a race condition could occur when building a project using multiple threads.

To mitigate the issue, I wrapped the installation of the compiler with a synchronized block on the Static instance.

As a result, if multiple threads try to install the compiler at the same time, only one thread will be able to do so. When they will get unlocked, it will check if the file was created meanwhile and if so, to will early return.

src/main/java/sbt_inc/CompilerBridgeFactory.java

@@ -130,88 +130,101 @@ private static File getScala2CompilerBridgeJar(
     }
 
     if (!cachedCompiledBridgeJar.exists()) {
-      mavenLogger.info("Compiler bridge file is not installed yet");
-      // compile and install
-      RawCompiler rawCompiler =
-          new RawCompiler(
-              scalaInstance, ClasspathOptionsUtil.auto(), new MavenLoggerSbtAdapter(mavenLogger));
-
-      File bridgeSources =
-          resolver.getJar(SBT_GROUP_ID, bridgeArtifactId, zincVersion, "sources").getFile();
-
-      Set<Path> bridgeSourcesDependencies =
-          resolver
-              .getJarAndDependencies(SBT_GROUP_ID, bridgeArtifactId, zincVersion, "sources")
-              .stream()
-              .filter(
-                  artifact ->
-                      artifact.getScope() != null && !artifact.getScope().equals("provided"))
-              .map(Artifact::getFile)
-              .map(File::toPath)
-              .collect(Collectors.toSet());
-
-      bridgeSourcesDependencies.addAll(
-          Arrays.stream(scalaInstance.allJars())
-              .sequential()
-              .map(File::toPath)
-              .collect(Collectors.toList()));
-
-      Path sourcesDir = Files.createTempDirectory("scala-maven-plugin-compiler-bridge-sources");
-      Path classesDir = Files.createTempDirectory("scala-maven-plugin-compiler-bridge-classes");
-
-      IO.unzip(bridgeSources, sourcesDir.toFile(), AllPassFilter$.MODULE$, true);
-
-      List<Path> bridgeSourcesScalaFiles =
-          FileUtils.listDirectoryContent(
-              sourcesDir,
-              file ->
-                  Files.isRegularFile(file) && file.getFileName().toString().endsWith(".scala"));
-      List<Path> bridgeSourcesNonScalaFiles =
-          FileUtils.listDirectoryContent(
-              sourcesDir,
-              file ->
-                  Files.isRegularFile(file)
-                      && !file.getFileName().toString().endsWith(".scala")
-                      && !file.getFileName().toString().equals("MANIFEST.MF"));
-
-      try {
-        rawCompiler.apply(
-            IterableHasAsScala(bridgeSourcesScalaFiles).asScala().toSeq(), // sources:Seq[File]
-            IterableHasAsScala(bridgeSourcesDependencies).asScala().toSeq(), // classpath:Seq[File],
-            classesDir, // outputDirectory:Path,
-            IterableHasAsScala(Collections.<String>emptyList())
-                .asScala()
-                .toSeq() // options:Seq[String]
-            );
-
-        Manifest manifest = new Manifest();
-        Path sourcesManifestFile = sourcesDir.resolve("META-INF").resolve("MANIFEST.MF");
-        try (InputStream is = Files.newInputStream(sourcesManifestFile)) {
-          manifest.read(is);
+      // Install the compiler in a synchronized block to prevent multithreaded compiles to corrupt
+      // the installer.
+      synchronized (CompilerBridgeFactory.class) {
+        boolean compiledWasInstalledWhileThisThreadWasWaiting = cachedCompiledBridgeJar.exists();
+
+        if(compiledWasInstalledWhileThisThreadWasWaiting){
+          return cachedCompiledBridgeJar;
         }
 
-        List<Tuple2<File, String>> scalaCompiledClasses =
-            computeZipEntries(FileUtils.listDirectoryContent(classesDir, file -> true), classesDir);
-        List<Tuple2<File, String>> resources =
-            computeZipEntries(bridgeSourcesNonScalaFiles, sourcesDir);
-        List<Tuple2<File, String>> allZipEntries = new ArrayList<>();
-        allZipEntries.addAll(scalaCompiledClasses);
-        allZipEntries.addAll(resources);
-
-        IO.jar(
-            IterableHasAsScala(
-                    allZipEntries.stream()
-                        .map(x -> scala.Tuple2.apply(x._1, x._2))
-                        .collect(Collectors.toList()))
-                .asScala(),
-            cachedCompiledBridgeJar,
-            manifest);
-
-        mavenLogger.info("Compiler bridge installed");
-
-      } finally {
-        FileUtils.deleteDirectory(sourcesDir);
-        FileUtils.deleteDirectory(classesDir);
+        mavenLogger.info("Compiler bridge file is not installed yet");
+        // compile and install
+        RawCompiler rawCompiler =
+            new RawCompiler(
+                scalaInstance, ClasspathOptionsUtil.auto(), new MavenLoggerSbtAdapter(mavenLogger));
+
+        File bridgeSources =
+            resolver.getJar(SBT_GROUP_ID, bridgeArtifactId, zincVersion, "sources").getFile();
+
+        Set<Path> bridgeSourcesDependencies =
+            resolver
+                .getJarAndDependencies(SBT_GROUP_ID, bridgeArtifactId, zincVersion, "sources")
+                .stream()
+                .filter(
+                    artifact ->
+                        artifact.getScope() != null && !artifact.getScope().equals("provided"))
+                .map(Artifact::getFile)
+                .map(File::toPath)
+                .collect(Collectors.toSet());
+
+        bridgeSourcesDependencies.addAll(
+            Arrays.stream(scalaInstance.allJars())
+                .sequential()
+                .map(File::toPath)
+                .collect(Collectors.toList()));
+
+        Path sourcesDir = Files.createTempDirectory("scala-maven-plugin-compiler-bridge-sources");
+        Path classesDir = Files.createTempDirectory("scala-maven-plugin-compiler-bridge-classes");
+
+        IO.unzip(bridgeSources, sourcesDir.toFile(), AllPassFilter$.MODULE$, true);
+
+        List<Path> bridgeSourcesScalaFiles =
+            FileUtils.listDirectoryContent(
+                sourcesDir,
+                file ->
+                    Files.isRegularFile(file) && file.getFileName().toString().endsWith(".scala"));
+        List<Path> bridgeSourcesNonScalaFiles =
+            FileUtils.listDirectoryContent(
+                sourcesDir,
+                file ->
+                    Files.isRegularFile(file)
+                        && !file.getFileName().toString().endsWith(".scala")
+                        && !file.getFileName().toString().equals("MANIFEST.MF"));
+
+        try {
+          rawCompiler.apply(
+              IterableHasAsScala(bridgeSourcesScalaFiles).asScala().toSeq(), // sources:Seq[File]
+              IterableHasAsScala(bridgeSourcesDependencies)
+                  .asScala()
+                  .toSeq(), // classpath:Seq[File],
+              classesDir, // outputDirectory:Path,
+              IterableHasAsScala(Collections.<String>emptyList())
+                  .asScala()
+                  .toSeq() // options:Seq[String]
+              );
+
+          Manifest manifest = new Manifest();
+          Path sourcesManifestFile = sourcesDir.resolve("META-INF").resolve("MANIFEST.MF");
+          try (InputStream is = Files.newInputStream(sourcesManifestFile)) {
+            manifest.read(is);
+          }
+
+          List<Tuple2<File, String>> scalaCompiledClasses =
+              computeZipEntries(
+                  FileUtils.listDirectoryContent(classesDir, file -> true), classesDir);
+          List<Tuple2<File, String>> resources =
+              computeZipEntries(bridgeSourcesNonScalaFiles, sourcesDir);
+          List<Tuple2<File, String>> allZipEntries = new ArrayList<>();
+          allZipEntries.addAll(scalaCompiledClasses);
+          allZipEntries.addAll(resources);
+
+          IO.jar(
+              IterableHasAsScala(
+                      allZipEntries.stream()
+                          .map(x -> scala.Tuple2.apply(x._1, x._2))
+                          .collect(Collectors.toList()))
+                  .asScala(),
+              cachedCompiledBridgeJar,
+              manifest);
+
+          mavenLogger.info("Compiler bridge installed");
+
+        } finally {
+          FileUtils.deleteDirectory(sourcesDir);
+          FileUtils.deleteDirectory(classesDir);
+        }
       }
     }