[2.x] Custom user agent (#9786)

**Problem**
We want to track the network request created by sbt.

**Solution**
This sets both sbt version and Coursier version into the user agent.

build.sbt

@@ -1284,6 +1284,15 @@ lazy val lmCoursierDependencies = Def.settings(
   excludeDependencies ++= Seq(
     ExclusionRule("org.scala-lang.modules", "scala-xml_2.13"),
   ),
+  // lmCoursierShaded merges every dependency into one assembly with a single surviving
+  // MANIFEST.MF, so coursier's own Implementation-Version doesn't survive there to be read back
+  // at runtime. Stash coursierVersion in a resource under our own package instead, which assembly
+  // merges in unchanged.
+  Compile / resourceGenerators += Def.task {
+    val file = (Compile / resourceManaged).value / "lmcoursier" / "coursier.properties"
+    IO.write(file, s"version=$coursierVersion\n")
+    Seq(file)
+  }.taskValue,
 )
 
 lazy val lmCoursier = project

lm-coursier/definitions/src/main/scala/lmcoursier/CoursierConfiguration.scala

@@ -73,4 +73,6 @@ import scala.concurrent.duration.{ Duration, FiniteDuration }
     localArtifactsShouldBeCached: Boolean = false,
     @since
     lockFile: Option[File] = None,
+    @since
+    userAgent: Option[String] = None,
 )

lm-coursier/src/main/scala/lmcoursier/CoursierDependencyResolution.scala

@@ -256,7 +256,8 @@ class CoursierDependencyResolution(
         checksums = checksums,
         credentials = conf.credentials.map(ToCoursier.credentials),
         followHttpToHttpsRedirections = conf.followHttpToHttpsRedirections.getOrElse(true),
-        localArtifactsShouldBeCached = conf.localArtifactsShouldBeCached
+        localArtifactsShouldBeCached = conf.localArtifactsShouldBeCached,
+        userAgent = conf.userAgent
       )
 
     val excludeDependencies = conf.excludeDependencies.map { (strOrg, strName) =>
@@ -547,6 +548,30 @@ class CoursierDependencyResolution(
 end CoursierDependencyResolution
 
 object CoursierDependencyResolution:
+  // Built at runtime, not as a single literal: lmCoursierShaded's relocation rewrites any string
+  // constant shaped like a "coursier."-prefixed path, which would otherwise silently turn this
+  // into "lmcoursier.internal.shaded.coursier.http.agent" and break the override.
+  private val userAgentPropertyKey: String = Seq("coursier", "http", "agent").mkString(".")
+
+  lazy val coursierUserAgent: String =
+    sys.props.get(userAgentPropertyKey).getOrElse(coursierUserAgent0)
+  // Reads the version from a resource rather than the jar manifest, which lmCoursierShaded's assembly merge clobbers.
+  private def coursierUserAgent0: String =
+    val version =
+      Option(getClass.getResourceAsStream("/lmcoursier/coursier.properties"))
+        .flatMap { in =>
+          scala.util
+            .Using(in) { in0 =>
+              val props = new java.util.Properties
+              props.load(in0)
+              Option(props.getProperty("version"))
+            }
+            .toOption
+            .flatten
+        }
+        .getOrElse("2.1")
+    s"Coursier/$version (+https://github.com/coursier)"
+
   def apply(configuration: CoursierConfiguration): DependencyResolution =
     DependencyResolution(new CoursierDependencyResolution(configuration))
 

lm-coursier/src/main/scala/lmcoursier/syntax/package.scala

@@ -79,6 +79,7 @@ package object syntax:
         sameVersions = Nil,
         localArtifactsShouldBeCached = false,
         lockFile = None,
+        userAgent = None,
       )
   end extension
 
@@ -120,6 +121,8 @@ package object syntax:
 
     def withRetry(retry: (FiniteDuration, Int)): CoursierConfiguration =
       value.withRetry(Some((retry._1, retry._2)))
+    def withUserAgent(userAgent: String): CoursierConfiguration =
+      value.withUserAgent(Option(userAgent))
   end extension
 
   extension (value: Publication)

main/src/main/scala/sbt/coursierint/LMCoursier.scala

@@ -35,6 +35,12 @@ object LMCoursier:
   private val credentialRegistry: ConcurrentHashMap[(String, String), IvyCredentials] =
     new ConcurrentHashMap
 
+  def defaultUserAgent(sbtVer: String): String =
+    sys.props.get("sbt.http.agent").getOrElse {
+      val cs = CoursierDependencyResolution.coursierUserAgent
+      s"$cs sbt/$sbtVer (+https://www.scala-sbt.org/)".trim
+    }
+
   def defaultCacheLocation: File =
     def absoluteFile(path: String): File = new File(path).getAbsoluteFile()
     def windowsCacheDirectory: File =
@@ -119,6 +125,7 @@ object LMCoursier:
     val missingOk = updateConfig match
       case Some(uc) => uc.missingOk
       case _        => false
+    val sbtVer = appConfig.provider.id.version
     CoursierConfiguration()
       .withResolvers(rs.toVector)
       .withInterProjectDependencies(interProjectDependencies.toVector)
@@ -144,6 +151,7 @@ object LMCoursier:
       .withSameVersions(sameVersions)
       .withLocalArtifactsShouldBeCached(localArtifactsShouldBeCached)
       .withLockFile(lockFile)
+      .withUserAgent(defaultUserAgent(sbtVer))
   end coursierConfiguration
 
   def coursierConfigurationTask: Def.Initialize[Task[CoursierConfiguration]] = Def.task {