wallet: handle non-writable db directories

1) For wallet load, this fixes a crash.

We currently allow loading wallets located on non-writable directories.
This is problematic because the node crashes on any subsequent write.
E.g. generating a block is enough to trigger it.

2) For wallet creation, this improves the returned error msg.

Before: creating a wallet would return a generic error:
"SQLiteDatabase: Failed to open database: unable to open database file"

After: creating a wallet returns:
"SQLiteDatabase: Failed to open database in directory <dir_path>: directory
is not writable"

src/util/fs_helpers.cpp

@@ -6,8 +6,9 @@
 #include <bitcoin-build-config.h> // IWYU pragma: keep
 
 #include <util/fs_helpers.h>
-
+#include <random.h>
 #include <sync.h>
+#include <tinyformat.h>
 #include <util/byte_units.h> // IWYU pragma: keep
 #include <util/fs.h>
 #include <util/log.h>
@@ -18,6 +19,7 @@
 #include <map>
 #include <memory>
 #include <optional>
+#include <stdexcept>
 #include <string>
 #include <system_error>
 #include <utility>
@@ -306,6 +308,29 @@ std::optional<fs::perms> InterpretPermString(const std::string& s)
     }
 }
 
+bool IsDirWritable(const fs::path& dir_path)
+{
+    // Attempt to create a tmp file in the directory
+    if (!fs::is_directory(dir_path)) throw std::runtime_error(strprintf("Path %s is not a directory", fs::PathToString(dir_path)));
+    FastRandomContext rng;
+    const auto tmp = dir_path / fs::PathFromString(strprintf(".tmp_%d", rng.rand64()));
+
+    const char* mode;
+#ifdef __MINGW64__
+    mode = "w"; // Temporary workaround for https://github.com/bitcoin/bitcoin/issues/30210
+#else
+    mode = "wx";
+#endif
+
+    if (const auto created{fsbridge::fopen(tmp, mode)}) {
+        std::fclose(created);
+        std::error_code ec;
+        fs::remove(tmp, ec); // clean up, ignore errors
+        return true;
+    }
+    return false;
+}
+
 #ifdef __APPLE__
 FSType GetFilesystemType(const fs::path& path)
 {

src/util/fs_helpers.h

@@ -94,6 +94,14 @@ std::string PermsToSymbolicString(fs::perms p);
  */
 std::optional<fs::perms> InterpretPermString(const std::string& s);
 
+/** Check if a directory is writable by creating a temporary file on it.
+ *
+ * @param[in] dir_path Path of the directory to test
+ * @return true if a temporary file could be created and removed, false otherwise.
+ * @throw std::runtime_error if dir_path is not a directory.
+ */
+bool IsDirWritable(const fs::path& dir_path);
+
 #ifdef WIN32
 fs::path GetSpecialFolderPath(int nFolder, bool fCreate = true);
 #endif

src/wallet/sqlite.cpp

@@ -257,7 +257,11 @@ void SQLiteDatabase::Open(int additional_flags)
     if (m_db == nullptr) {
         if (!(flags & SQLITE_OPEN_MEMORY)) {
             TryCreateDirectories(m_dir_path);
+            if (!IsDirWritable(m_dir_path)) {
+                throw std::runtime_error(strprintf("SQLiteDatabase: Failed to open database in directory '%s': directory is not writable", fs::PathToString(m_dir_path)));
+            }
         }
+
         int ret = sqlite3_open_v2(m_file_path.c_str(), &m_db, flags, nullptr);
         if (ret != SQLITE_OK) {
             throw std::runtime_error(strprintf("SQLiteDatabase: Failed to open database: %s\n", sqlite3_errstr(ret)));