Merge bitcoin/bitcoin#27073: Convert ArgsManager::GetDataDir to a read-only function

64c105442ce8c10900ea6fbecdbcfebe42f2d387 util: make GetDataDir read-only & create datadir.. (willcl-ark)
56e370fbb9413260723c598048392219b1055ad0 util: add ArgsManager datadir helper functions (willcl-ark)

Pull request description:

  Fixes #20070

  Currently `ArgsManager::GetDataDir()` ensures it will always return a datadir by creating one if necessary. The function is shared between `bitcoind` `bitcoin-qt` and `bitcoin-cli` which results in the undesirable behaviour described in #20070.

  This PR splits out the part of the function which creates directories and adds it as a standalone function, only called as part of `bitcoind` and `bitcoin-qt` init, but not `bitcoin-cli`.

  `ReadConfigFiles`' behavior is changed to use the absolute path of the config file in error and warning messages instead of a relative path.

  This was inadvertantly the form being tested [here](https://github.com/bitcoin/bitcoin/blob/73966f75f67fb797163f0a766292a79d4b2c1b70/test/functional/feature_config_args.py#L287), whilst we were _not_ testing that a relative path was returned by the message even though we passed a relative path in as argument.

ACKs for top commit:
  achow101:
    ACK 64c105442ce8c10900ea6fbecdbcfebe42f2d387
  hebasto:
    re-ACK 64c105442ce8c10900ea6fbecdbcfebe42f2d387, only comments have been adjusted as requsted since my previous [review](https://github.com/bitcoin/bitcoin/pull/27073#pullrequestreview-1307435890).
  TheCharlatan:
    Re-ACK 64c105442ce8c10900ea6fbecdbcfebe42f2d387
  ryanofsky:
    Code review ACK 64c105442ce8c10900ea6fbecdbcfebe42f2d387. Only comment changes since last review

Tree-SHA512: b129501346071ad62551c9714492b21536d0558a94117d97218e255ef4e948d00df899a4bc2788faea27d3b1f20fc6136ef9d03e6a08498d926d9ad8688d6c96

3 files changed, 42 insertions, 12 deletions

diff --git a/src/qt/bitcoin.cpp b/src/qt/bitcoin.cpp
index 59f433749d..c383c8bd58 100644
--- a/src/qt/bitcoin.cpp
+++ b/src/qt/bitcoin.cpp
@@ -167,6 +167,7 @@ static void initTranslations(QTranslator &qtTranslatorBase, QTranslator &qtTrans
 
 static bool InitSettings()
 {
+    gArgs.EnsureDataDir();
     if (!gArgs.GetSettingsPath()) {
         return true; // Do nothing if settings file disabled.
     }
diff --git a/src/util/system.cpp b/src/util/system.cpp
index e72c970157..77b659df7e 100644
--- a/src/util/system.cpp
+++ b/src/util/system.cpp
@@ -417,8 +417,7 @@ const fs::path& ArgsManager::GetDataDir(bool net_specific) const
     LOCK(cs_args);
     fs::path& path = net_specific ? m_cached_network_datadir_path : m_cached_datadir_path;
 
-    // Cache the path to avoid calling fs::create_directories on every call of
-    // this function
+    // Used cached path if available
     if (!path.empty()) return path;
 
     const fs::path datadir{GetPathArg("-datadir")};
@@ -432,20 +431,34 @@ const fs::path& ArgsManager::GetDataDir(bool net_specific) const
         path = GetDefaultDataDir();
     }
 
-    if (!fs::exists(path)) {
-        fs::create_directories(path / "wallets");
-    }
-
     if (net_specific && !BaseParams().DataDir().empty()) {
         path /= fs::PathFromString(BaseParams().DataDir());
-        if (!fs::exists(path)) {
-            fs::create_directories(path / "wallets");
-        }
     }
 
     return path;
 }
 
+void ArgsManager::EnsureDataDir() const
+{
+    /**
+     * "/wallets" subdirectories are created in all **new**
+     * datadirs, because wallet code will create new wallets in the "wallets"
+     * subdirectory only if exists already, otherwise it will create them in
+     * the top-level datadir where they could interfere with other files.
+     * Wallet init code currently avoids creating "wallets" directories itself
+     * for backwards compatibility, but this be changed in the future and
+     * wallet code here could go away (#16220).
+     */
+    auto path{GetDataDir(false)};
+    if (!fs::exists(path)) {
+        fs::create_directories(path / "wallets");
+    }
+    path = GetDataDir(true);
+    if (!fs::exists(path)) {
+        fs::create_directories(path / "wallets");
+    }
+}
+
 void ArgsManager::ClearPathCache()
 {
     LOCK(cs_args);
@@ -491,6 +504,7 @@ bool ArgsManager::IsArgSet(const std::string& strArg) const
 
 bool ArgsManager::InitSettings(std::string& error)
 {
+    EnsureDataDir();
     if (!GetSettingsPath()) {
         return true; // Do nothing if settings file disabled.
     }
@@ -965,6 +979,11 @@ bool ArgsManager::ReadConfigStream(std::istream& stream, const std::string& file
     return true;
 }
 
+fs::path ArgsManager::GetConfigFilePath() const
+{
+    return GetConfigFile(GetPathArg("-conf", BITCOIN_CONF_FILENAME));
+}
+
 bool ArgsManager::ReadConfigFiles(std::string& error, bool ignore_invalid_keys)
 {
     {
@@ -973,8 +992,8 @@ bool ArgsManager::ReadConfigFiles(std::string& error, bool ignore_invalid_keys)
         m_config_sections.clear();
     }
 
-    const fs::path conf_path = GetPathArg("-conf", BITCOIN_CONF_FILENAME);
-    std::ifstream stream{GetConfigFile(conf_path)};
+    const auto conf_path{GetConfigFilePath()};
+    std::ifstream stream{conf_path};
 
     // not ok to have a config file specified that cannot be opened
     if (IsArgSet("-conf") && !stream.good()) {
diff --git a/src/util/system.h b/src/util/system.h
index c053adf8c3..14f093501a 100644
--- a/src/util/system.h
+++ b/src/util/system.h
@@ -242,6 +242,11 @@ protected:
     void SelectConfigNetwork(const std::string& network);
 
     [[nodiscard]] bool ParseParameters(int argc, const char* const argv[], std::string& error);
+
+    /**
+     * Return config file path (read-only)
+     */
+    fs::path GetConfigFilePath() const;
     [[nodiscard]] bool ReadConfigFiles(std::string& error, bool ignore_invalid_keys = false);
 
     /**
@@ -475,13 +480,18 @@ protected:
      */
     void LogArgs() const;
 
+    /**
+     * If datadir does not exist, create it along with wallets/
+     * subdirectory(s).
+     */
+    void EnsureDataDir() const;
+
 private:
     /**
      * Get data directory path
      *
      * @param net_specific Append network identifier to the returned path
      * @return Absolute path on success, otherwise an empty path when a non-directory path would be returned
-     * @post Returned directory path is created unless it is empty
      */
     const fs::path& GetDataDir(bool net_specific) const;