Merge bitcoin/bitcoin#28075: util: Remove DirIsWritable, GetUniquePath

fa3da629a1 Remove DirIsWritable, GetUniquePath (MarcoFalke)
fad3a9793b Return LockResult::ErrorWrite in LockDirectory (MarcoFalke)
fa0afe7408 refactor: Return enum in LockDirectory (MarcoFalke)

Pull request description:

  `GetUniquePath` is only used in tests and in `DirIsWritable`. The check by `DirIsWritable` is redundant with the check done in `LockDirectory`.

  Fix the redundancy by removing everything, except `LockDirectory`.

ACKs for top commit:
  TheCharlatan:
    Re-ACK fa3da629a1
  hebasto:
    ACK fa3da629a1, I have reviewed the code and it looks OK.

Tree-SHA512: e95f18cd586de7582e9c08ac7ddb860bfcfcbc8963804f45c5784c5e4c0598dc59ae7e45dd4daf30a5020dbf8433f5db2ad06e46a8676371982003790043c6c9

src/init.cpp

@@ -1047,13 +1047,14 @@ static bool LockDataDirectory(bool probeOnly)
 {
     // Make sure only a single Bitcoin process is using the data directory.
     const fs::path& datadir = gArgs.GetDataDirNet();
-    if (!DirIsWritable(datadir)) {
+    switch (util::LockDirectory(datadir, ".lock", probeOnly)) {
+    case util::LockResult::ErrorWrite:
         return InitError(strprintf(_("Cannot write to data directory '%s'; check permissions."), fs::PathToString(datadir)));
-    }
-    if (!LockDirectory(datadir, ".lock", probeOnly)) {
+    case util::LockResult::ErrorLock:
         return InitError(strprintf(_("Cannot obtain a lock on data directory %s. %s is probably already running."), fs::PathToString(datadir), PACKAGE_NAME));
-    }
-    return true;
+    case util::LockResult::Success: return true;
+    } // no default case, so the compiler can warn about missing cases
+    assert(false);
 }
 
 bool AppInitSanityChecks(const kernel::Context& kernel)