sync: provide more info when "Failed to sync and interpolate"

src/main/java/me/itzg/helpers/errors/ExceptionDetailer.java

@@ -0,0 +1,41 @@
+package me.itzg.helpers.errors;
+
+import java.util.ArrayList;
+import java.util.Iterator;
+
+public class ExceptionDetailer {
+
+    private static class CauseIterator implements Iterator<Throwable> {
+        private Throwable current;
+
+        CauseIterator(Throwable initial) {
+            this.current = initial;
+        }
+
+        @Override
+        public boolean hasNext() {
+            return current != null;
+        }
+
+        public Throwable next() {
+            if (current == null) {
+                return null;
+            }
+            Throwable result = current;
+            current = current.getCause();
+            return result;
+        }
+    }
+
+    public static String buildCausalMessages(Throwable throwable) {
+        final CauseIterator causeIterator = new CauseIterator(throwable);
+        final ArrayList<String> parts = new ArrayList<>();
+        while (causeIterator.hasNext()) {
+            final Throwable cause = causeIterator.next();
+            if (cause != null) {
+                parts.add(cause.getClass().getSimpleName() + ": " + cause.getMessage());
+            }
+        }
+        return String.join("; ", parts);
+    }
+}

src/main/java/me/itzg/helpers/sync/SynchronizingFileVisitor.java

@@ -9,6 +9,8 @@
 import java.nio.file.attribute.FileTime;
 import java.util.List;
 import lombok.extern.slf4j.Slf4j;
+import me.itzg.helpers.errors.ExceptionDetailer;
+import me.itzg.helpers.errors.InvalidParameterException;
 
 @Slf4j
 class SynchronizingFileVisitor implements FileVisitor<Path> {
@@ -24,9 +26,21 @@ public SynchronizingFileVisitor(Path src, Path dest, boolean skipNewerInDestinat
         this.fileProcessor = fileProcessor;
     }
 
+    /**
+     * Convenience method that processes each source path into the destination path by applying a {@link SynchronizingFileVisitor}
+     * to each source.
+     * @param srcDest source... dest
+     * @param skipNewerInDestination provided to {@link SynchronizingFileVisitor}
+     * @param fileProcessor provided to {@link SynchronizingFileVisitor}
+     * @return exit code style of 1 for failure, 0 for success
+     */
     static int walkDirectories(List<Path> srcDest, boolean skipNewerInDestination,
         FileProcessor fileProcessor
     ) {
+        if (srcDest.size() < 2) {
+            throw new InvalidParameterException("At least one source and destination path is required");
+        }
+
         // TODO can use getLast() with java 21
         final Path dest = srcDest.get(srcDest.size() - 1);
 
@@ -35,7 +49,7 @@ static int walkDirectories(List<Path> srcDest, boolean skipNewerInDestination,
                 try {
                     Files.walkFileTree(src, new SynchronizingFileVisitor(src, dest, skipNewerInDestination, fileProcessor));
                 } catch (IOException e) {
-                    log.error("Failed to sync and interpolate {} into {} : {}", src, dest, e.getMessage());
+                    log.error("Failed to sync and interpolate {} into {}: {}", src, dest, ExceptionDetailer.buildCausalMessages(e));
                     log.debug("Details", e);
                     return 1;
                 }
@@ -104,7 +118,7 @@ private boolean shouldProcessFile(Path srcFile, Path destFile) throws IOExceptio
 
     @Override
     public FileVisitResult visitFileFailed(Path file, IOException e) {
-        log.warn("Failed to visit file {} due to {}:{}", file, e.getClass(), e.getMessage());
+        log.warn("Failed to visit file {} due to {}", file, ExceptionDetailer.buildCausalMessages(e));
         log.debug("Details", e);
         return FileVisitResult.CONTINUE;
     }