NoSuchFileException after removing `target/classes` externally using `rm -rf`

[jurgenvinju]

This happened in the VScode extension JVM of Rascal, after I did rm -rf target/classes and saved a Rascal file in an open editor:

ERROR BaseFileVisitor Could not walk regular file: /Users/jurgenv/git/rascal/target/classes/rascal (java.nio.file.NoSuchFileException: /Users/jurgenv/git/rascal/target/classes/rascal)
 java.nio.file.NoSuchFileException: /Users/jurgenv/git/rascal/target/classes/rascal
        at java.base/sun.nio.fs.UnixException.translateToIOException(UnixException.java:92)
        at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:111)
        at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:116)
        at java.base/sun.nio.fs.UnixFileAttributeViews$Basic.readAttributes(UnixFileAttributeViews.java:55)
        at java.base/sun.nio.fs.UnixFileSystemProvider.readAttributes(UnixFileSystemProvider.java:149)
        at java.base/java.nio.file.Files.readAttributes(Files.java:1764)
        at java.base/java.nio.file.FileTreeWalker.getAttributes(FileTreeWalker.java:219)
        at java.base/java.nio.file.FileTreeWalker.visit(FileTreeWalker.java:276)
        at java.base/java.nio.file.FileTreeWalker.next(FileTreeWalker.java:373)
        at java.base/java.nio.file.Files.walkFileTree(Files.java:2761)
        at engineering.swat.watch.impl.overflows.BaseFileVisitor.walkFileTree(BaseFileVisitor.java:66)
        at engineering.swat.watch.impl.overflows.MemorylessRescanner.rescan(MemorylessRescanner.java:57)
        at engineering.swat.watch.impl.overflows.MemorylessRescanner.lambda$accept$0(MemorylessRescanner.java:111)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
        at java.base/java.lang.Thread.run(Thread.java:829)

[sungshik]

Thanks for reporting, @jurgenvinju. I could reproduce the issue as follows:

1. Create a temp dir
2. Start a watch on the temp dir
3. Delete the temp dir
4. Artificially trigger an overflow event on the temp dir

Step 4 causes an internal rescanner to explicitly check the file system if the watch might have missed events due to the overflow. However, since the temp dir doesn't exist anymore, Java's file tree walker (java.nio.file.Files.walkFileTree) fails to walk it.

In the specific reported scenario, I think the following happened:

1. Earlier, a watch was started for target/classes
2. target/classes was removed
3. Either just before or maybe during (caused by) the removal, the file system reported an overflow event
4. Only after the removal, the overflow event made its way through the watch code to activate the rescanner, which failed, as the directory no longer existed by that time.

In principle, triggering a rescan for a non-existent directory---and immediately failing---is fine. In particular, as the directory no longer exists, the number of overflow events can only decrease from that point onward, until none are left forever. So, the issue here is that we report it as an error. Logging it at DEBUG level would be more suitable and less confusing.