Stabilize unresolved symbolic links (--experimental_allow_unresolved_symlinks)

Description of the problem / feature request:

https://github.com/bazelbuild/bazel/issues/9075 added support for unresolved symlinks, via the actions.declare_symlink() and actions.symlink() functions. That functionality is currently gated by --experimental_allow_unresolved_symlinks.

I’ve discovered that unresolved symlinks are critical for supporting hermetic Node.JS builds. Specifically, Node’s use of symlinks for module resolution semantics mean that we need to be able to create symlink artifacts that reference dependencies and can be used as runfiles, but exist outside the runfiles root.

What are the current blockers to stabilizing unresolved symlinks? Is there a list of known remaining work to get them enabled by default?

cc @lberki @laszlocsomor

[pauldraper]

AFAIK this has been experimental since Bazel 1.0.

Is progress likely?

[AlessandroPatti]

This seems to be failing on macOS if paired with remote caching;

java.lang.IllegalStateException: Encountered symlink input '<some nice path here>', but all symlinks should have been resolved by SkyFrame. This is a bug.
	at com.google.devtools.build.lib.remote.merkletree.DirectoryTreeBuilder.fromActionInputs(DirectoryTreeBuilder.java:106)
	at com.google.devtools.build.lib.remote.merkletree.DirectoryTreeBuilder.fromActionInputs(DirectoryTreeBuilder.java:47)
	at com.google.devtools.build.lib.remote.merkletree.MerkleTree.build(MerkleTree.java:123)
	at com.google.devtools.build.lib.remote.RemoteSpawnCache.lookup(RemoteSpawnCache.java:121)
	at com.google.devtools.build.lib.exec.AbstractSpawnStrategy.exec(AbstractSpawnStrategy.java:119)
	at com.google.devtools.build.lib.exec.AbstractSpawnStrategy.exec(AbstractSpawnStrategy.java:94)
	at com.google.devtools.build.lib.actions.SpawnStrategy.beginExecution(SpawnStrategy.java:39)
	at com.google.devtools.build.lib.exec.ProxySpawnActionContext.beginExecution(ProxySpawnActionContext.java:60)
	at com.google.devtools.build.lib.rules.cpp.CppCompileAction.beginExecution(CppCompileAction.java:1381)
	at com.google.devtools.build.lib.actions.Action.execute(Action.java:124)
	at com.google.devtools.build.lib.skyframe.SkyframeActionExecutor$4.execute(SkyframeActionExecutor.java:966)
	at com.google.devtools.build.lib.skyframe.SkyframeActionExecutor$ActionRunner.continueAction(SkyframeActionExecutor.java:1114)
	at com.google.devtools.build.lib.skyframe.SkyframeActionExecutor$ActionRunner.run(SkyframeActionExecutor.java:1085)
	at com.google.devtools.build.lib.skyframe.ActionExecutionState.runStateMachine(ActionExecutionState.java:136)
	at com.google.devtools.build.lib.skyframe.ActionExecutionState.getResultOrDependOnFuture(ActionExecutionState.java:80)
	at com.google.devtools.build.lib.skyframe.SkyframeActionExecutor.executeAction(SkyframeActionExecutor.java:612)
	at com.google.devtools.build.lib.skyframe.ActionExecutionFunction.checkCacheAndExecuteIfNeeded(ActionExecutionFunction.java:907)
	at com.google.devtools.build.lib.skyframe.ActionExecutionFunction.compute(ActionExecutionFunction.java:297)
	at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:438)