Fix #433: Make classpath hashing more lightweight

zinc/src/main/scala/sbt/internal/inc/MixedAnalyzingCompiler.scala

@@ -11,6 +11,7 @@ package inc
 
 import java.io.File
 import java.lang.ref.{ Reference, SoftReference }
+import java.nio.file.Files
 import java.util.Optional
 
 import inc.javac.AnalyzingJavaCompiler
@@ -20,6 +21,7 @@ import xsbti.compile._
 import sbt.io.IO
 import sbt.util.{ InterfaceUtil, Logger }
 import sbt.internal.inc.JavaInterfaceUtil.EnrichOption
+import sbt.internal.inc.caching.ClasspathCache
 import xsbti.compile.ClassFileManager
 
 /** An instance of an analyzing compiler that can run both javac + scalac. */
@@ -181,13 +183,11 @@ object MixedAnalyzingCompiler {
       incrementalCompilerOptions: IncOptions,
       extra: List[(String, String)]
   ): CompileConfiguration = {
-    val classpathHash = classpath map { x =>
-      FileHash.of(x, Stamper.forHash(x).hashCode)
-    }
+    val classpathHash = ClasspathCache.hashClasspath(classpath)
     val compileSetup = MiniSetup.of(
       output,
       MiniOptions.of(
-        classpathHash.toArray,
+        classpathHash,
         options.toArray,
         javacOptions.toArray
       ),

zinc/src/main/scala/sbt/internal/inc/caching/ClasspathCache.scala

@@ -0,0 +1,43 @@
+package sbt.internal.inc.caching
+
+import java.io.File
+import java.nio.file.Files
+import java.util.concurrent.ConcurrentHashMap
+import java.nio.file.attribute.{ BasicFileAttributes, FileTime }
+
+import xsbti.compile.FileHash
+import sbt.internal.inc.{ EmptyStamp, Stamper }
+
+object ClasspathCache {
+  // For more safety, store both the time and size
+  private type JarMetadata = (FileTime, Long)
+  private[this] val cacheMetadataJar = new ConcurrentHashMap[File, (JarMetadata, FileHash)]()
+  private[this] final val emptyStampCode = EmptyStamp.hashCode()
+  private def emptyFileHash(file: File) = FileHash.of(file, emptyStampCode)
+  private def genFileHash(file: File, metadata: JarMetadata): FileHash = {
+    val newHash = FileHash.of(file, Stamper.forHash(file).hashCode())
+    cacheMetadataJar.put(file, (metadata, newHash))
+    newHash
+  }
+
+  def hashClasspath(classpath: Seq[File]): Array[FileHash] = {
+    // #433: Cache jars with their metadata to avoid recomputing hashes transitively in other projects
+    def fromCacheOrHash(file: File): FileHash = {
+      if (!file.exists()) emptyFileHash(file)
+      else {
+        // `readAttributes` needs to be guarded by `file.exists()`, otherwise it fails
+        val attrs = Files.readAttributes(file.toPath, classOf[BasicFileAttributes])
+        if (attrs.isDirectory) emptyFileHash(file)
+        else {
+          val currentMetadata = (attrs.lastModifiedTime(), attrs.size())
+          Option(cacheMetadataJar.get(file)) match {
+            case Some((metadata, hashHit)) if metadata == currentMetadata => hashHit
+            case None                                                     => genFileHash(file, currentMetadata)
+          }
+        }
+      }
+    }
+
+    classpath.toParArray.map(fromCacheOrHash).toArray
+  }
+}

zinc/src/test/scala/sbt/inc/cached/CachedHashingSpec.scala

@@ -0,0 +1,58 @@
+package sbt.inc.cached
+
+import java.nio.file.Paths
+
+import sbt.inc.{ BaseCompilerSpec, SourceFiles }
+import sbt.internal.inc.{ Analysis, CompileOutput, MixedAnalyzingCompiler }
+import sbt.io.IO
+
+class CachedHashingSpec extends BaseCompilerSpec {
+  def timeMs[R](block: => R): Long = {
+    val t0 = System.nanoTime()
+    block // call-by-name
+    val t1 = System.nanoTime()
+    (t1 - t0) / 1000000
+  }
+
+  "zinc" should "cache jar generation" in {
+    IO.withTemporaryDirectory { tempDir =>
+      val classes = Seq(SourceFiles.Good)
+      val sources0 = Map(Paths.get("src") -> classes.map(path => Paths.get(path)))
+      val projectSetup = ProjectSetup(tempDir.toPath(), sources0, Nil)
+      val compiler = projectSetup.createCompiler()
+
+      import compiler.in.{ setup, options, compilers, previousResult }
+      import sbt.internal.inc.JavaInterfaceUtil._
+      import sbt.io.syntax.{ file, fileToRichFile, singleFileFinder }
+
+      val javac = compilers.javaTools.javac
+      val scalac = compilers.scalac
+      val giganticClasspath = file(sys.props("user.home"))./(".ivy2").**("*.jar").get.take(500)
+
+      def genConfig = MixedAnalyzingCompiler.makeConfig(
+        scalac,
+        javac,
+        options.sources,
+        giganticClasspath,
+        CompileOutput(options.classesDirectory),
+        setup.cache,
+        setup.progress.toOption,
+        options.scalacOptions,
+        options.javacOptions,
+        Analysis.empty,
+        previousResult.setup.toOption,
+        setup.perClasspathEntryLookup,
+        setup.reporter,
+        options.order,
+        setup.skip,
+        setup.incrementalCompilerOptions,
+        setup.extra.toList.map(_.toScalaTuple)
+      )
+
+      val hashingTime = timeMs(genConfig)
+      val cachedHashingTime = timeMs(genConfig)
+      assert(cachedHashingTime < (hashingTime * 0.20),
+             s"Cache jar didn't work: $cachedHashingTime is >= than 20% of $hashingTime.")
+    }
+  }
+}