Fix property handling and add tests and docs (#3791)

Fixes #3789. We just
accidentally flipped the conditional and didn't have any tests that
verify this behavior, so I added some example tests that also serve to
explain the feature in the docs

example/tasks/4-inputs/build.sc

@@ -7,7 +7,7 @@ def myInput = T.input {
     .trim()
 }
 
-// A generalization of <<_sources>>, ``T.input``s are tasks that re-evaluate
+// A generalization of <<_sources>>, ``T.Input``s are tasks that re-evaluate
 // _every time_ (unlike <<_anonymous_tasks>>), containing an
 // arbitrary block of code.
 //
@@ -17,34 +17,34 @@ def myInput = T.input {
 // that target does not have any `Task` inputs and so will never re-compute
 // even if the external `git` status changes:
 
-def gitStatusTarget = T {
-  "v-" +
-  os.proc("git", "log", "-1", "--pretty=format:%h-%B ")
-    .call(cwd = T.workspace)
-    .out
-    .text()
-    .trim()
+def gitStatusTask = T {
+  "version-" +
+    os.proc("git", "log", "-1", "--pretty=format:%h-%B ")
+      .call(cwd = T.workspace)
+      .out
+      .text()
+      .trim()
 }
 
 /** Usage
 
 > git init .
 > git commit --allow-empty -m "Initial-Commit"
 
-> ./mill show gitStatusTarget
-"v-...-Initial-Commit"
+> ./mill show gitStatusTask
+"version-...-Initial-Commit"
 
 > git commit --allow-empty -m "Second-Commit"
 
-> ./mill show gitStatusTarget # Mill didn't pick up the git change!
-"v-...-Initial-Commit"
+> ./mill show gitStatusTask # Mill didn't pick up the git change!
+"version-...-Initial-Commit"
 
 */
 
-// `gitStatusTarget` will not know that `git rev-parse` can change, and will
+// `gitStatusTask` will not know that `git rev-parse` can change, and will
 // not know to re-evaluate when your `git log` _does_ change. This means
-// `gitStatusTarget` will continue to use any previously cached value, and
-// ``gitStatusTarget``'s output will  be out of date!
+// `gitStatusTask` will continue to use any previously cached value, and
+// ``gitStatusTask``'s output will  be out of date!
 
 // To fix this, you can wrap your `git log` in a `T.input`:
 
@@ -55,26 +55,87 @@ def gitStatusInput = T.input {
     .text()
     .trim()
 }
-def gitStatusTarget2 = T { "v-" + gitStatusInput() }
+def gitStatusTask2 = T { "version-" + gitStatusInput() }
 
 // This makes `gitStatusInput` to always re-evaluate every build, and only if
-// the output of `gitStatusInput` changes will `gitStatusTarget2` re-compute
+// the output of `gitStatusInput` changes will `gitStatusTask2` re-compute
 
 /** Usage
 
 > git commit --allow-empty -m "Initial-Commit"
 
-> ./mill show gitStatusTarget2
-"v-...-Initial-Commit"
+> ./mill show gitStatusTask2
+"version-...-Initial-Commit"
 
 > git commit --allow-empty -m "Second-Commit"
 
-> ./mill show gitStatusTarget2 # Mill picked up git change
-"v-...-Second-Commit"
+> ./mill show gitStatusTask2 # Mill picked up git change
+"version-...-Second-Commit"
 
 */
 
 // Note that because ``T.input``s re-evaluate every time, you should ensure that the
 // code you put in `T.input` runs quickly. Ideally it should just be a simple check
 // "did anything change?" and any heavy-lifting should be delegated to downstream
-// targets where it can be cached if possible.
+// tasks where it can be cached if possible.
+//
+// === System Properties Inputs
+//
+// One major use case of `Input` tasks is to make your build configurable via
+// JVM system properties of environment variables. If you directly access
+// `sys.props` or `sys.env` inside a xref:#_cached_tasks[cached Task{}], the
+// cached value will be used even if the property or environment variable changes
+// in subsequent runs, when you really want it to be re-evaluated. Thus, accessing
+// system properties should be done in a `T.input`, and usage of the property
+// should be done downstream in a xref:#_cached_tasks[cached task]:
+
+def myPropertyInput = T.input {
+  sys.props("my-property")
+}
+def myPropertyTask = T {
+  "Hello Prop " + myPropertyInput()
+}
+
+/** Usage
+
+> ./mill show myPropertyTask
+"Hello Prop null"
+
+> ./mill -Dmy-property=world show myPropertyTask # Task is correctly invalidated when prop is added
+"Hello Prop world"
+
+> ./mill show myPropertyTask # Task is correctly invalidated when prop is removed
+"Hello Prop null"
+*/
+
+// Again, `T.input` runs every time, and thus you should only do the bare minimum
+// in your `T.input` that is necessary to detect changes. Any further processing
+// should be done in downstreak xref:#_cached_tasks[cached tasks] to allow for proper
+// caching and re-use
+//
+// === Environment Variable Inputs
+//
+// Like system properties, environment variables should be referenced in `T.input`s. Unlike
+// system properties, you need to use the special API `T.env` to access the environment,
+// due to JVM limitations:
+
+def myEnvInput = T.input {
+  T.env.getOrElse("MY_ENV", null)
+}
+
+def myEnvTask = T {
+  "Hello Env " + myEnvInput()
+}
+
+
+/** Usage
+
+> ./mill show myEnvTask
+"Hello Env null"
+
+> MY_ENV=world ./mill show myEnvTask # Task is correctly invalidated when env is added
+"Hello Env world"
+
+> ./mill show myEnvTask # Task is correctly invalidated when env is removed
+"Hello Env null"
+*/

runner/src/mill/runner/MillMain.scala

@@ -336,7 +336,7 @@ object MillMain {
   ): Unit = {
     val currentProps = sys.props
     val desiredProps = initialSystemProperties ++ userSpecifiedProperties
-    val systemPropertiesToUnset = desiredProps.keySet -- currentProps.keySet
+    val systemPropertiesToUnset = currentProps.keySet -- desiredProps.keySet
 
     for (k <- systemPropertiesToUnset) System.clearProperty(k)
     for ((k, v) <- desiredProps) System.setProperty(k, v)