Write sbt.version to project/build.properties

Have sbt.version set in project/build.properties is a best practice
because it makes the build more deterministic and reproducible.

With this change sbt, after ensuring that the base directory is probably
an sbt project, writes out sbt.version in project/build.properties if it
is missing.

Fixes #754

main/src/main/scala/sbt/Main.scala

@@ -13,12 +13,12 @@ import scala.annotation.tailrec
 import Path._
 import StandardMain._
 
-import java.io.File
+import java.io.{ File, IOException }
 import java.net.URI
 import java.util.Locale
 import scala.util.control.NonFatal
 
-/** This class is the entry point for sbt.*/
+/** This class is the entry point for sbt. */
 final class xMain extends xsbti.AppMain {
   def run(configuration: xsbti.AppConfiguration): xsbti.MainResult =
     {
@@ -26,12 +26,42 @@ final class xMain extends xsbti.AppMain {
       import BasicCommandStrings.runEarly
       import BuiltinCommands.{ initialize, defaults }
       import CommandStrings.{ BootCommand, DefaultsCommand, InitCommand }
+      setSbtVersion(configuration.baseDirectory(), configuration.provider().id().version())
       runManaged(initialState(configuration,
         Seq(defaults, early),
         runEarly(DefaultsCommand) :: runEarly(InitCommand) :: BootCommand :: Nil)
       )
     }
+
+  private val sbtVersionRegex = """sbt\.version\s*=.*""".r
+  private def isSbtVersionLine(s: String) = sbtVersionRegex.pattern matcher s matches ()
+
+  private def isSbtProject(baseDir: File, projectDir: File) =
+    projectDir.exists() || (baseDir * "*.sbt").get.nonEmpty
+
+  private def setSbtVersion(baseDir: File, sbtVersion: String) = {
+    val projectDir = baseDir / "project"
+    val buildProps = projectDir / "build.properties"
+
+    val buildPropsLines = if (buildProps.canRead) IO.readLines(buildProps) else Nil
+
+    val sbtVersionAbsent = buildPropsLines forall (!isSbtVersionLine(_))
+
+    if (sbtVersionAbsent) {
+      val errorMessage = s"WARN: No sbt.version set in project/build.properties, base directory: $baseDir"
+      try {
+        if (isSbtProject(baseDir, projectDir)) {
+          val newBuildPropsLines = s"sbt.version=$sbtVersion" :: buildPropsLines
+          IO.writeLines(buildProps, newBuildPropsLines)
+        } else
+          println(errorMessage)
+      } catch {
+        case _: IOException => println(errorMessage)
+      }
+    }
+  }
 }
+
 final class ScriptMain extends xsbti.AppMain {
   def run(configuration: xsbti.AppConfiguration): xsbti.MainResult =
     {

notes/0.13.14.markdown

@@ -8,6 +8,7 @@
 - Adds "local-preloaded" repository for offline installation. See below.
 - Notifies and enables users to stay in sbt's shell on the warm JVM by hitting `[ENTER]` while sbt is running. [#2987][2987]/[#2996][2996] by [@dwijnand][@dwijnand]
 - Adds an `Append` instance to support `sourceGenerators += Def.task { ... }`, instead of needing `.taskValue`. [#2943][2943] by [@eed3si9n][@eed3si9n]
+- Writes out the sbt.version in project/build.properties if it is missing. [#754][]/[#3025][] by [@dwijnand][]
 - XML generated by JUnitXmlTestsListener now correctly flags ignored, skipped and pending tests. [#2198][2198]/[#2854][2854] by [@ashleymercer][@ashleymercer]
 - When sbt detects that the project is compiled with dotty, it now automatically
   sets `scalaCompilerBridgeSource` correctly, this reduces the boilerplate needed
@@ -63,6 +64,7 @@ Special thanks to the contributors for making this release a success. According
 
   [143]: https://github.com/sbt/sbt-launcher-package/pull/143
   [145]: https://github.com/sbt/sbt-launcher-package/pull/145
+  [754]: https://github.com/sbt/sbt/issues/754
   [2766]: https://github.com/sbt/sbt/issues/2766
   [1855]: https://github.com/sbt/sbt/issues/1855
   [1466]: https://github.com/sbt/sbt/issues/1466
@@ -86,6 +88,7 @@ Special thanks to the contributors for making this release a success. According
   [2993]: https://github.com/sbt/sbt/pull/2993
   [2943]: https://github.com/sbt/sbt/pull/2943
   [3011]: https://github.com/sbt/sbt/issues/3011
+  [3025]: https://github.com/sbt/sbt/issues/3025
   [@eed3si9n]: https://github.com/eed3si9n
   [@dwijnand]: https://github.com/dwijnand
   [@Duhemm]: https://github.com/Duhemm