Make os.pwd modifiable via the os.pwd0 dynamic variable (#298)

Basically the same thing we did in
#295 and
#283, except rather than being
`os.SubProcess`-specific this applies to all usage of `os.pwd`. And with
similar limitations: this works for code paths that go through OS-Lib,
but not for code that uses `java.io` or `java.nio` directly.

AFAIK this is the last of the "global" things we need to redirect in
Mill tasks, after env variables and default subprocess streams. I'd like
it to go into Mill 0.12.0 together with the other sandboxing-related
changes

Added unit tests and updated the documentation

Pull Request: #298

Author: lihaoyi

Readme.adoc

@@ -1828,8 +1828,8 @@ wd / os.up
 wd / os.up / os.up
 ----
 
-Note that there are no in-built operations to change the `os.pwd`. In general,
-you should not need to: simply defining a new path, e.g.
+`os.pwd` can be modified in certain scopes via the `os.dynamicPwd` dynamic variable, but
+best practice is not to change it. Instead simply define a new path, e.g.
 
 [source,scala]
 ----

os/src-jvm/package.scala

@@ -2,6 +2,7 @@ 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 {
   type Generator[+T] = geny.Generator[T]
@@ -38,7 +39,9 @@ package object os {
   /**
    * The current working directory for this process.
    */
-  val pwd: Path = os.Path(java.nio.file.Paths.get(".").toAbsolutePath)
+  def pwd: Path = dynamicPwd.value
+  val dynamicPwd: DynamicVariable[Path] =
+    new DynamicVariable(os.Path(java.nio.file.Paths.get(".").toAbsolutePath))
 
   val up: RelPath = RelPath.up
 

os/src-native/package.scala

@@ -1,5 +1,6 @@
 import java.nio.file.FileSystem
 import java.nio.file.FileSystems
+import scala.util.DynamicVariable
 package object os {
   type Generator[+T] = geny.Generator[T]
   val Generator = geny.Generator
@@ -31,7 +32,9 @@ package object os {
   /**
    * The current working directory for this process.
    */
-  val pwd: Path = os.Path(java.nio.file.Paths.get(".").toAbsolutePath)
+  def pwd: Path = dynamicPwd.value
+  val dynamicPwd: DynamicVariable[Path] =
+    new DynamicVariable(os.Path(java.nio.file.Paths.get(".").toAbsolutePath))
 
   val up: RelPath = RelPath.up
 

os/test/src/PathTests.scala

@@ -434,6 +434,14 @@ object PathTests extends TestSuite {
       System.err.printf("p[%s]\n", posix(p))
       assert(posix(p) contains "/omg")
     }
+    test("dynamicPwd") {
+      val x = os.pwd
+      val y = os.dynamicPwd.withValue(os.pwd / "hello") {
+        os.pwd
+      }
+
+      assert(x / "hello" == y)
+    }
   }
   // compare absolute paths
   def sameFile(a: java.nio.file.Path, b: java.nio.file.Path): Boolean = {

os/test/src/SubprocessTests.scala

@@ -225,5 +225,24 @@ object SubprocessTests extends TestSuite {
         assert(output.out.lines() == Seq("HELLO /usr"))
       }
     }
+    test("dynamicPwd") {
+      // Windows doesnt have bash installed so a bit inconvenient
+      // to run these subprocesses for testing
+      if (!scala.util.Properties.isWin) {
+        val outsidePwd = os.pwd
+        val tmp0 = os.temp.dir()
+        val tmp = os.followLink(tmp0).getOrElse(tmp0)
+        val x = proc("bash", "-c", "pwd").call()
+        val y = os.dynamicPwd.withValue(tmp) {
+          proc("bash", "-c", "pwd").call()
+        }
+
+        val z = proc("bash", "-c", "pwd").call()
+        assert(outsidePwd.toString != tmp.toString)
+        assert(x.out.trim() == outsidePwd.toString)
+        assert(y.out.trim() == tmp.toString)
+        assert(z.out.trim() == outsidePwd.toString)
+      }
+    }
   }
 }