Merge #12630: Provide useful error message if datadir is not writable.

8674e74 Provide relevant error message if datadir is not writable. (murrayn)

Pull request description:

  If the --datadir exists, but is not writable, the current error message on startup is 'Cannot obtain a lock on data directory foo. Bitcoin Core is probably already running.' This is misleading.

  I believe this PR addresses #11668, although the issue is not Windows-specific.

Tree-SHA512: 10cbbaea433072aee4fb3e8938a72073c7a5c841f7a7685c9e12549c322b2925c7d34bac254ac33021b23132bfc352c058712bc9542298cf86f8fd9757f528b2

src/util.cpp

@@ -419,6 +419,19 @@ void ReleaseDirectoryLocks()
     dir_locks.clear();
 }
 
+bool DirIsWritable(const fs::path& directory)
+{
+    fs::path tmpFile = directory / fs::unique_path();
+
+    FILE* file = fsbridge::fopen(tmpFile, "a");
+    if (!file) return false;
+
+    fclose(file);
+    remove(tmpFile);
+
+    return true;
+}
+
 /** Interpret string as boolean, for argument parsing */
 static bool InterpretBool(const std::string& strValue)
 {