[2.x] Retry on HTTP 5xx during dependency resolution (#8903)

lm-coursier/src/main/scala/lmcoursier/internal/ResolutionRun.scala

@@ -144,12 +144,7 @@ object ResolutionRun {
         resolveTask.io.attempt
           .flatMap {
             case Left(e: ResolutionError) =>
-              val hasConnectionTimeouts = e.errors.exists {
-                case err: CantDownloadModule =>
-                  err.perRepositoryErrors.exists(_.contains("Connection timed out"))
-                case _ => false
-              }
-              if (hasConnectionTimeouts)
+              if (isTransientResolutionError(e))
                 if (attempt + 1 >= maxAttempts) {
                   log.error(s"Failed, maximum iterations ($maxAttempts) reached")
                   Task.point(Left(e))
@@ -280,4 +275,16 @@ object ResolutionRun {
   }
 
   private lazy val retryScheduler = ThreadUtil.fixedScheduledThreadPool(1)
+
+  private[internal] def isTransientResolutionError(e: ResolutionError): Boolean =
+    e.errors.exists {
+      case err: CantDownloadModule => isTimeout(err) || isServerError(err)
+      case _                       => false
+    }
+
+  private def isTimeout(err: CantDownloadModule): Boolean =
+    err.perRepositoryErrors.exists(_.contains("Connection timed out"))
+
+  private def isServerError(err: CantDownloadModule): Boolean =
+    err.perRepositoryErrors.exists(_.contains("Server returned HTTP response code: 5"))
 }

lm-coursier/src/test/scala/lmcoursier/internal/ResolutionRunSpec.scala

@@ -0,0 +1,38 @@
+package lmcoursier.internal
+
+import coursier.core.{ Module, ModuleName, Organization, Resolution }
+import coursier.error.ResolutionError.CantDownloadModule
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class ResolutionRunSpec extends AnyFunSuite with Matchers:
+
+  private def cantDownload(errors: String*): CantDownloadModule =
+    new CantDownloadModule(
+      Resolution(),
+      Module(Organization("org"), ModuleName("mod"), Map.empty),
+      "1.0",
+      errors.toSeq
+    )
+
+  test("503 is a transient resolution error"):
+    val err = cantDownload(
+      "Server returned HTTP response code: 503 for URL: https://repo.example.com/org/mod/1.0/mod-1.0.pom"
+    )
+    ResolutionRun.isTransientResolutionError(err) shouldBe true
+
+  test("500 is a transient resolution error"):
+    val err = cantDownload(
+      "Server returned HTTP response code: 500 for URL: https://repo.example.com/org/mod/1.0/mod-1.0.pom"
+    )
+    ResolutionRun.isTransientResolutionError(err) shouldBe true
+
+  test("connection timeout is a transient resolution error"):
+    val err = cantDownload("Connection timed out")
+    ResolutionRun.isTransientResolutionError(err) shouldBe true
+
+  test("404 is not a transient resolution error"):
+    val err = cantDownload(
+      "Server returned HTTP response code: 404 for URL: https://repo.example.com/org/mod/1.0/mod-1.0.pom"
+    )
+    ResolutionRun.isTransientResolutionError(err) shouldBe false