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Relevant threads:

"AWT-EventQueue-0 3.1#AI-173.4567466 Studio, eap:true, os:Windows 10 10.0, java-version:JetBrains s.r.o 1.8.0_152-release-1024-b02" prio=0 tid=0x0 nid=0x0 blocked
     java.lang.Thread.State: BLOCKED
 on java.lang.Object@1ec03874 owned by "ApplicationImpl pooled thread 819" Id=1288
        at com.android.tools.idea.gradle.project.sync.GradleFiles.checkHashesOfChangedFiles(GradleFiles.java:232)
        at com.android.tools.idea.gradle.project.sync.GradleFiles.areGradleFilesModified(GradleFiles.java:335)
        at com.android.tools.idea.gradle.project.sync.GradleSyncState.isSyncNeeded(GradleSyncState.java:419)
        at com.android.tools.idea.gradle.util.GradleVersions.getGradleVersion(GradleVersions.java:57)
        at com.android.tools.idea.gradle.project.sync.GradleSyncState.syncEnded(GradleSyncState.java:309)
        at com.android.tools.idea.gradle.project.sync.setup.post.PostSyncProjectSetup.notifySyncFinished(PostSyncProjectSetup.java:259)
        at com.android.tools.idea.gradle.project.sync.setup.post.PostSyncProjectSetup.setUpProject(PostSyncProjectSetup.java:207)
        at com.android.tools.idea.gradle.variant.view.BuildVariantUpdater.lambda$updateSelectedVariant$0(BuildVariantUpdater.java:99)
        at com.android.tools.idea.gradle.variant.view.BuildVariantUpdater$$Lambda$3924/1021669240.run(Unknown Source)
        at com.intellij.openapi.roots.impl.ProjectRootManagerImpl.mergeRootsChangesDuring(ProjectRootManagerImpl.java:298)
        at com.android.tools.idea.gradle.util.GradleProjects.lambda$null$0(GradleProjects.java:70)
        at com.android.tools.idea.gradle.util.GradleProjects$$Lambda$2602/491861947.run(Unknown Source)
        at com.intellij.openapi.application.impl.ApplicationImpl.runWriteAction(ApplicationImpl.java:1003)
        at com.android.tools.idea.gradle.util.GradleProjects.lambda$executeProjectChanges$1(GradleProjects.java:68)
        at com.android.tools.idea.gradle.util.GradleProjects$$Lambda$2588/1161340492.run(Unknown Source)
        at com.intellij.openapi.application.impl.ApplicationImpl.invokeAndWait(ApplicationImpl.java:653)
        at com.intellij.openapi.application.impl.ApplicationImpl.invokeAndWait(ApplicationImpl.java:666)
        at com.android.tools.idea.gradle.util.GradleProjects.executeProjectChanges(GradleProjects.java:68)
        at com.android.tools.idea.gradle.variant.view.BuildVariantUpdater.updateSelectedVariant(BuildVariantUpdater.java:93)
        at com.android.tools.idea.gradle.variant.view.BuildVariantView.buildVariantSelected(BuildVariantView.java:527)
        at com.android.tools.idea.gradle.variant.view.BuildVariantView$BuildVariantTable.lambda$addBuildVariants$0(BuildVariantView.java:493)
        at com.android.tools.idea.gradle.variant.view.BuildVariantView$BuildVariantTable$$Lambda$2261/314803153.itemStateChanged(Unknown Source)


"ApplicationImpl pooled thread 819" prio=0 tid=0x0 nid=0x0 waiting on condition
     java.lang.Thread.State: TIMED_WAITING
 on com.intellij.openapi.application.impl.ReadMostlyRWLock@7ada3390
        at sun.misc.Unsafe.park(Native Method)
        at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215)
        at com.intellij.openapi.application.impl.ReadMostlyRWLock.waitABit(ReadMostlyRWLock.java:113)
        at com.intellij.openapi.application.impl.ReadMostlyRWLock.readLock(ReadMostlyRWLock.java:104)
        at com.intellij.openapi.application.impl.ApplicationImpl.startRead(ApplicationImpl.java:949)
        at com.intellij.openapi.application.impl.ApplicationImpl.runReadAction(ApplicationImpl.java:925)
        at com.android.tools.idea.gradle.project.sync.GradleFiles.computeHash(GradleFiles.java:209)
        at com.android.tools.idea.gradle.project.sync.GradleFiles.areHashesEqual(GradleFiles.java:222)
        at com.android.tools.idea.gradle.project.sync.GradleFiles.filterHashes(GradleFiles.java:244)
        at com.android.tools.idea.gradle.project.sync.GradleFiles.checkHashesOfChangedFiles(GradleFiles.java:232)
        at com.android.tools.idea.gradle.project.sync.GradleFiles.areGradleFilesModified(GradleFiles.java:335)
        at com.android.tools.idea.gradle.project.sync.GradleSyncState.isSyncNeeded(GradleSyncState.java:419)
        at com.android.tools.idea.gradle.util.GradleVersions.getGradleVersion(GradleVersions.java:57)
        at com.android.tools.idea.gradle.project.sync.setup.post.ProjectStructureUsageTracker.trackProjectStructure(ProjectStructureUsageTracker.java:118)
        at com.android.tools.idea.gradle.project.sync.setup.post.ProjectStructureUsageTracker.lambda$trackProjectStructure$0(ProjectStructureUsageTracker.java:73)
        at com.android.tools.idea.gradle.project.sync.setup.post.ProjectStructureUsageTracker$$Lambda$2240/831390881.run(Unknown Source)
        at com.intellij.openapi.application.impl.ApplicationImpl$1.run(ApplicationImpl.java:315)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
        at java.util.concurrent.FutureTask.run(FutureTask.java:266)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:745)

One more thread dump (looks like related)

Sorry, can't provide additional info,
reproduced on big corp project only

It looks like the thread trace should be enough. Thanks for picking this up :)

Still reproduced in 3.1 RC2

my steps:
1) Multi module project with several branches
2) Cange active branch from on to another in two project modules (switch workspace disabled)
3) Click on Build Variant, to change active build type
4) IDE is completely stuck, but not used CPU

Freeze Logs

Thanks for pinging.

The fix this isn't included in the 3.1 builds.
It should however be in the 3.2 canaries (the later ones at least).

I'll close this bug for now, if you can still reproduce this issue on the latest canary please feel free to reopen and I'll take another look.

Reproduced in 3.1 final