Do not attempt to create already existing appDataDir

This change fixes #3998 by avoiding the attempt to create a user's
appDataDir if it already exists. Normally this attempt is not a problem,
as Files#createDirectories does nothing if the target is a directory
that already exists, but in the case that the target is a symbolic link,
Files#createDirectories throws a FileAlreadyExistsException. This change
prevents the call to Files#createDirectories if the target already
exists, and if in fact the target is a symbolic link pointing to an
existing directory, this is not a problem and everything proceeds as per
usual. This mimics the behavior in Bisq v1.2.5 and prior, where
File#mkdirs was used instead of Files#createDirectories. File#mkdirs
does not throw an exception when the target directory is a symlink, it
just returns false.

common/src/main/java/bisq/common/config/Config.java

@@ -798,10 +798,12 @@ private static File tempUserDataDir() {
      * @return the given directory, now guaranteed to exist
      */
     private static File mkAppDataDir(File dir) {
-        try {
-            Files.createDirectories(dir.toPath());
-        } catch (IOException ex) {
-            throw new UncheckedIOException(format("Application data directory '%s' could not be created", dir), ex);
+        if (!dir.exists()) {
+            try {
+                Files.createDirectories(dir.toPath());
+            } catch (IOException ex) {
+                throw new UncheckedIOException(format("Application data directory '%s' could not be created", dir), ex);
+            }
         }
         return dir;
     }

common/src/test/java/bisq/common/config/ConfigTests.java

@@ -1,6 +1,7 @@
 package bisq.common.config;
 
 import java.nio.file.Files;
+import java.nio.file.Path;
 
 import java.io.ByteArrayOutputStream;
 import java.io.File;
@@ -238,6 +239,22 @@ public void whenAppDataDirCannotBeCreated_thenUncheckedIoExceptionIsThrown() thr
         configWithOpts(opt(USER_DATA_DIR, aFile));
     }
 
+    @Test
+    public void whenAppDataDirIsSymbolicLink_thenAppDataDirCreationIsNoOp() throws IOException {
+        Path parentDir = Files.createTempDirectory("parent");
+        Path targetDir = parentDir.resolve("target");
+        Path symlink = parentDir.resolve("symlink");
+        Files.createDirectory(targetDir);
+        try {
+            Files.createSymbolicLink(symlink, targetDir);
+        } catch (Throwable ex) {
+            // An error occurred trying to create a symbolic link, likely because the
+            // operating system (e.g. Windows) does not support it, so we abort the test.
+            return;
+        }
+        configWithOpts(opt(APP_DATA_DIR, symlink));
+    }
+
 
     // == TEST SUPPORT FACILITIES ========================================================