Add hooks into serializing/constructing os.Paths and launching subprocesses (#365)

Necessary for com-lihaoyi/mill#3660 in Mill.
Essentially we need a way to (a) serialize absolute paths as relative
paths and (b) set up the necessary symlinks in any subprocess folder
such that the relative paths point to the correct absolute location

Author: lihaoyi

os/src-jvm/ResourceApi.scala

@@ -0,0 +1,7 @@
+package os
+trait ResourceApi {
+  def resource(implicit resRoot: ResourceRoot = Thread.currentThread().getContextClassLoader) = {
+    os.ResourcePath.resource(resRoot)
+  }
+
+}

os/src-jvm/package.scala

@@ -0,0 +1,2 @@
+package os 
+trait ResourceApi

os/src-native/ResourceApi.scala

@@ -7,7 +7,7 @@ import scala.language.implicitConversions
 import acyclic.skipped
 import os.PathError.{InvalidSegment, NonCanonicalLiteral}
 
-import scala.util.Try //needed for cross-version defined macros
+import scala.util.{DynamicVariable, Try} //needed for cross-version defined macros
 
 trait PathChunk {
   def segments: Seq[String]
@@ -438,6 +438,31 @@ object SubPath extends SubPathMacros {
 }
 
 object Path extends PathMacros {
+  @experimental trait Serializer {
+    def serializeString(p: os.Path): String
+    def serializeFile(p: os.Path): java.io.File
+    def serializePath(p: os.Path): java.nio.file.Path
+    def deserialize(s: String): java.nio.file.Path
+    def deserialize(s: java.io.File): java.nio.file.Path
+    def deserialize(s: java.nio.file.Path): java.nio.file.Path
+    def deserialize(s: java.net.URI): java.nio.file.Path
+  }
+  @experimental val pathSerializer = new DynamicVariable[Serializer](defaultPathSerializer)
+  @experimental object defaultPathSerializer extends Serializer {
+    def serializeString(p: os.Path): String = p.wrapped.toString
+    def serializeFile(p: os.Path): java.io.File = p.wrapped.toFile
+    def serializePath(p: os.Path): java.nio.file.Path = p.wrapped
+    def deserialize(s: String) = Paths.get(s)
+    def deserialize(s: java.io.File) = Paths.get(s.getPath)
+    def deserialize(s: java.nio.file.Path) = s
+    def deserialize(s: java.net.URI) = s.getScheme() match {
+      case "file" => Paths.get(s)
+      case uriType =>
+        throw new IllegalArgumentException(
+          s"""os.Path can only be created from a "file" URI scheme, but found "${uriType}""""
+        )
+    }
+  }
   def apply(p: FilePath, base: Path) = p match {
     case p: RelPath => base / p
     case p: SubPath => base / p
@@ -562,7 +587,7 @@ class Path private[os] (val wrapped: java.nio.file.Path)
     val resolved = wrapped.resolve(chunk.toString).normalize()
     new Path(resolved)
   }
-  override def toString = wrapped.toString
+  override def toString = Path.pathSerializer.value.serializeString(this)
 
   override def equals(o: Any): Boolean = o match {
     case p: Path => wrapped.equals(p.wrapped)
@@ -593,8 +618,8 @@ class Path private[os] (val wrapped: java.nio.file.Path)
     new RelPath(segments.drop(nonUpIndex), nonUpIndex)
   }
 
-  def toIO: java.io.File = wrapped.toFile
-  def toNIO: java.nio.file.Path = wrapped
+  def toIO: java.io.File = Path.pathSerializer.value.serializeFile(this)
+  def toNIO: java.nio.file.Path = Path.pathSerializer.value.serializePath(this)
 
   def resolveFrom(base: os.Path) = this
 
@@ -608,23 +633,18 @@ sealed trait PathConvertible[T] {
 
 object PathConvertible {
   implicit object StringConvertible extends PathConvertible[String] {
-    def apply(t: String) = Paths.get(t)
+    def apply(t: String) = Path.pathSerializer.value.deserialize(t)
   }
   implicit object JavaIoFileConvertible extends PathConvertible[java.io.File] {
-    def apply(t: java.io.File) = Paths.get(t.getPath)
+    def apply(t: java.io.File) = Path.pathSerializer.value.deserialize(t)
   }
   implicit object NioPathConvertible extends PathConvertible[java.nio.file.Path] {
-    def apply(t: java.nio.file.Path) = t
+    def apply(t: java.nio.file.Path) = Path.pathSerializer.value.deserialize(t)
     override def isCustomFs(t: java.nio.file.Path): Boolean =
       t.getFileSystem() != java.nio.file.FileSystems.getDefault()
   }
   implicit object UriPathConvertible extends PathConvertible[URI] {
-    def apply(uri: URI) = uri.getScheme() match {
-      case "file" => Paths.get(uri)
-      case uriType =>
-        throw new IllegalArgumentException(
-          s"""os.Path can only be created from a "file" URI scheme, but found "${uriType}""""
-        )
-    }
+    def apply(uri: URI) = Path.pathSerializer.value.deserialize(uri)
+
   }
 }

os/src/Path.scala

@@ -607,7 +607,9 @@ case class ProcGroup private[os] (commands: Seq[proc]) {
   def pipeTo(next: proc) = ProcGroup(commands :+ next)
 }
 
-private[os] object ProcessOps {
+@experimental
+object ProcessOps {
+  val spawnHook = new scala.util.DynamicVariable[os.Path => Unit]({ p => () })
   def buildProcess(
       command: Seq[String],
       cwd: Path = null,
@@ -644,7 +646,9 @@ private[os] object ProcessOps {
 
     addToProcessEnv(env)
 
-    builder.directory(Option(cwd).getOrElse(os.pwd).toIO)
+    val dir = Option(cwd).getOrElse(os.pwd)
+    builder.directory(dir.toIO)
+    spawnHook.value.apply(dir)
 
     builder
       .command(command: _*)

os/src/ProcessOps.scala

@@ -1,7 +1,10 @@
+import scala.language.implicitConversions
 import java.nio.file.FileSystem
 import java.nio.file.FileSystems
+import java.nio.file.Paths
 import scala.util.DynamicVariable
-package object os {
+
+package object os extends ResourceApi {
   type Generator[+T] = geny.Generator[T]
   val Generator = geny.Generator
   implicit def GlobSyntax(s: StringContext): GlobInterpolator = new GlobInterpolator(s)