Sporadic failures with -w/--watch

[lihaoyi]

Started seeing this when working on a new project. Pretty trivial single-module Scala build. Mill 0.11.x. Not sure why I've never seen it before elsewhere, but it seems pretty frequent (happens several times a day). Using Intellij BSP.

Not sure what the ~ is at the end of /Users/lihaoyi/Github/usql/test/src/MainTests.scala~. Maybe some kind of temporary file IntelliJ/IntelliJ-BSP is creating and deleting quickly, cause a TOCTTOU race condition?

Watching for changes to 11 paths and 4 other values... (Enter to re-run, Ctrl-C to exit)
Exception in thread "main" java.lang.reflect.InvocationTargetException
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at mill.main.client.IsolatedMillMainLoader.runMain(IsolatedMillMainLoader.java:58)
	at mill.main.client.MillClientMain.main(MillClientMain.java:64)
Caused by: java.nio.file.NoSuchFileException: /Users/lihaoyi/Github/usql/test/src/MainTests.scala~
	at java.base/sun.nio.fs.UnixException.translateToIOException(UnixException.java:92)
	at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:106)
	at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:111)
	at java.base/sun.nio.fs.UnixFileAttributeViews$Posix.readAttributes(UnixFileAttributeViews.java:257)
	at java.base/sun.nio.fs.UnixFileAttributeViews$Posix.readAttributes(UnixFileAttributeViews.java:168)
	at java.base/sun.nio.fs.UnixFileSystemProvider.readAttributes(UnixFileSystemProvider.java:148)
	at java.base/java.nio.file.Files.readAttributes(Files.java:1851)
	at java.base/java.nio.file.Files.getPosixFilePermissions(Files.java:2125)
	at os.perms$.apply(PermsOps.scala:13)
	at mill.api.PathRef$.$anonfun$apply$3(PathRef.scala:131)
	at mill.api.PathRef$.$anonfun$apply$3$adapted(PathRef.scala:124)
	at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:576)
	at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:574)
	at scala.collection.AbstractIterable.foreach(Iterable.scala:933)
	at scala.collection.IterableOps$WithFilter.foreach(Iterable.scala:903)
	at mill.api.PathRef$.apply(PathRef.scala:124)
	at mill.api.PathRef.recomputeSig(PathRef.scala:23)
	at mill.util.Watchable$Path.poll(Watchable.scala:21)
	at mill.util.Watchable.validate(Watchable.scala:15)
	at mill.util.Watchable.validate$(Watchable.scala:15)
	at mill.util.Watchable$Path.validate(Watchable.scala:20)
	at mill.runner.Watching$.$anonfun$statWatchWait$1(Watching.scala:75)
	at mill.runner.Watching$.$anonfun$statWatchWait$1$adapted(Watching.scala:75)
	at scala.collection.immutable.List.forall(List.scala:386)
	at mill.runner.Watching$.statWatchWait0$1(Watching.scala:75)
	at mill.runner.Watching$.statWatchWait(Watching.scala:98)
	at mill.runner.Watching$.watchAndWait(Watching.scala:67)
	at mill.runner.Watching$.watchLoop(Watching.scala:45)
	at mill.runner.MillMain$.$anonfun$main0$1(MillMain.scala:198)
	at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
	at scala.Console$.withErr(Console.scala:193)
	at mill.api.SystemStreams$.$anonfun$withStreams$2(SystemStreams.scala:62)
	at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
	at scala.Console$.withOut(Console.scala:164)
	at mill.api.SystemStreams$.$anonfun$withStreams$1(SystemStreams.scala:61)
	at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
	at scala.Console$.withIn(Console.scala:227)
	at mill.api.SystemStreams$.withStreams(SystemStreams.scala:60)
	at mill.runner.MillMain$.main0(MillMain.scala:80)
	at mill.runner.MillMain$.main(MillMain.scala:58)
	at mill.runner.MillMain.main(MillMain.scala)
	... 6 more

[lefou]

If it's some temporary file, we should ignore it when collecting source files. We already apply a filter to exclude linux hidden files starting with a dot.

[lihaoyi]

Ignoring specific files won't help, because the crash comes when checking the PathRef signature for the entire src/ folder, before we have chance to filter individual files

We could add some smartness to PathRef to provide some way of pre-filtering things, but for now we should just make it robust and fail a bit more elegantly in the face of concurrent updates