[2.x] fix: Parallelize dependency resolution when no progress bar is rendered (#9270)

update held a process-global lock around coursier resolution and artifact
fetching whenever a logger was set OR coursier was not in fallback mode. That
lock exists only to serialize coursier's interactive progress bar, which is
rendered solely when no custom logger is supplied and coursier is not in
fallback mode. The loggerOpt.nonEmpty clause therefore over-serialized the
common non-interactive case (IntelliJ re-imports, CI, any non-TTY run, where
sbt supplies a quiet debug-only logger), making update scale with the number
of modules rather than the number of distinct artifacts.

Fixes #5508.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -41,7 +41,10 @@ object ArtifactsRun {
     }
 
     Lock.maybeSynchronized(needsLock =
-      params.loggerOpt.nonEmpty || !RefreshLogger.defaultFallbackMode
+      Lock.progressBarActive(
+        hasCustomLogger = params.loggerOpt.nonEmpty,
+        fallbackMode = RefreshLogger.defaultFallbackMode
+      )
     ) {
       result(params, coursierLogger)
     }

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

@@ -3,6 +3,14 @@ package lmcoursier.internal
 private[lmcoursier] object Lock {
   private val lock = new Object
 
+  /* The lock guards coursier's interactive progress bar (ProgressBarRefreshDisplay), the only thing
+   * here that cannot be driven from more than one module at a time. coursier renders it only when no
+   * custom cache logger is supplied and it is not in fallback mode; otherwise (a custom logger, or
+   * the line-based fallback display) modules can be resolved in parallel. A CacheLogger is already
+   * invoked concurrently within a single resolution, so it must be thread-safe regardless. */
+  def progressBarActive(hasCustomLogger: Boolean, fallbackMode: Boolean): Boolean =
+    !hasCustomLogger && !fallbackMode
+
   /* Progress bars require us to only work on one module at the time. Without those we can go faster */
   def maybeSynchronized[T](needsLock: Boolean)(f: => T): T =
     if (needsLock) lock.synchronized(f)

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

@@ -189,7 +189,10 @@ object ResolutionRun {
     SbtCoursierCache.default.resolutionOpt(params.resolutionKey).map(Right(_)).getOrElse {
       val resOrError =
         Lock.maybeSynchronized(needsLock =
-          params.loggerOpt.nonEmpty || !RefreshLogger.defaultFallbackMode
+          Lock.progressBarActive(
+            hasCustomLogger = params.loggerOpt.nonEmpty,
+            fallbackMode = RefreshLogger.defaultFallbackMode
+          )
         ) {
           val map = new mutable.HashMap[Configuration, Resolution]
           val either = params.orderedConfigs.foldLeft[Either[coursier.error.ResolutionError, Unit]](

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

@@ -0,0 +1,23 @@
+package lmcoursier.internal
+
+import verify.*
+
+// progressBarActive decides whether resolution must be serialized. It is true only when coursier
+// renders its interactive progress bar (no custom logger and not in fallback mode) -- the one thing
+// that cannot be driven from more than one module at a time.
+object LockSpec extends BasicTestSuite:
+
+  test("a custom cache logger does not require the lock") {
+    assert(!Lock.progressBarActive(hasCustomLogger = true, fallbackMode = false))
+    assert(!Lock.progressBarActive(hasCustomLogger = true, fallbackMode = true))
+  }
+
+  test("the fallback (line-based) display does not require the lock") {
+    assert(!Lock.progressBarActive(hasCustomLogger = false, fallbackMode = true))
+  }
+
+  test("only the interactive progress bar requires the lock") {
+    assert(Lock.progressBarActive(hasCustomLogger = false, fallbackMode = false))
+  }
+
+end LockSpec

notes/2.0.0/parallel-dependency-resolution.md

@@ -0,0 +1,21 @@
+### `update` resolves modules in parallel
+
+Dependency resolution (`update`) no longer holds a process-global lock while
+coursier is *not* rendering its interactive progress bar -- that is, whenever a
+custom `csrLogger` is set, or coursier is in fallback display mode (the default
+under IntelliJ, CI, and any non-TTY environment, or with `COURSIER_PROGRESS=false`).
+
+Previously the lock was taken even for sbt's quiet default logger, so `update`
+processed one module at a time and its duration grew with the number of modules
+rather than the number of distinct artifacts. Large multi-module builds and
+IDE/CI re-imports could spend minutes serializing resolution.
+
+The lock is still held while coursier draws its live per-module progress bars in
+an interactive terminal, because that renderer is not safe to drive concurrently;
+to parallelize there as well, run with `COURSIER_PROGRESS=false`. Fully parallel
+resolution *with* live progress bars is tracked by [#5627][i5627].
+
+This addresses [#5508][i5508].
+
+[i5508]: https://github.com/sbt/sbt/issues/5508
+[i5627]: https://github.com/sbt/sbt/issues/5627