util: detect and warn when using exFAT on macOS

exFAT is known to cause corruption on macOS. See #28552.

Therefore we should warn when using this fs format for either the blocks
or data directories on macOS.

Co-authored-by: l0rinc <pap.lorinc@gmail.com>

doc/files.md

@@ -18,6 +18,8 @@
 
 - [Installed Files](#installed-files)
 
+- [Filesystem recommendations](#filesystem-recommendations)
+
 ## Data directory location
 
 The data directory is the default location where the Bitcoin Core files are stored.
@@ -160,3 +162,8 @@ This table describes the files installed by Bitcoin Core across different platfo
 - *Italicized* files are only installed in source builds if relevant CMake options are enabled. They are not included in binary releases.
 - README and bitcoin.conf files are included in binary releases but not installed in source builds.
 - On Windows, binaries have a `.exe` suffix (e.g., `bitcoin-cli.exe`).
+
+## Filesystem recommendations
+
+When choosing a filesystem for the data directory (`datadir`) or blocks directory (`blocksdir`) on **macOS**,the `exFAT` filesystem should be avoided.
+There have been multiple reports of database corruption and data loss when using this filesystem with Bitcoin Core, see [Issue #31454](https://github.com/bitcoin/bitcoin/issues/31454) for more details.

src/init.cpp

@@ -1866,6 +1866,26 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         }
     }
 
+#ifdef __APPLE__
+    auto check_and_warn_fs{[&](const fs::path& path, std::string_view desc) {
+        const auto path_desc{strprintf("%s (\"%s\")", desc, fs::PathToString(path))};
+        switch (GetFilesystemType(path)) {
+        case FSType::EXFAT:
+            InitWarning(strprintf(_("The %s path uses exFAT, which is known to have intermittent corruption problems on macOS. "
+                "Move this directory to a different filesystem to avoid data loss."), path_desc));
+            break;
+        case FSType::ERROR:
+            LogInfo("Failed to detect filesystem type for %s", path_desc);
+            break;
+        case FSType::OTHER:
+            break;
+        }
+    }};
+
+    check_and_warn_fs(args.GetDataDirNet(), "data directory");
+    check_and_warn_fs(args.GetBlocksDirPath(), "blocks directory");
+#endif
+
 #if HAVE_SYSTEM
     const std::string block_notify = args.GetArg("-blocknotify", "");
     if (!block_notify.empty()) {

src/util/fs_helpers.cpp

@@ -30,6 +30,11 @@
 #include <shlobj.h>
 #endif // WIN32
 
+#ifdef __APPLE__
+#include <sys/mount.h>
+#include <sys/param.h>
+#endif
+
 /** Mutex to protect dir_locks. */
 static GlobalMutex cs_dir_locks;
 /** A map that contains all the currently held directory locks. After
@@ -298,3 +303,15 @@ std::optional<fs::perms> InterpretPermString(const std::string& s)
         return std::nullopt;
     }
 }
+
+#ifdef __APPLE__
+FSType GetFilesystemType(const fs::path& path)
+{
+    if (struct statfs fs_info; statfs(path.c_str(), &fs_info)) {
+        return FSType::ERROR;
+    } else if (std::string_view{fs_info.f_fstypename} == "exfat") {
+        return FSType::EXFAT;
+    }
+    return FSType::OTHER;
+}
+#endif

src/util/fs_helpers.h

@@ -14,6 +14,23 @@
 #include <limits>
 #include <optional>
 
+#ifdef __APPLE__
+enum class FSType {
+    EXFAT,
+    OTHER,
+    ERROR
+};
+
+/**
+ * Detect filesystem type for a given path.
+ * Currently identifies exFAT filesystems which cause issues on macOS.
+ *
+ * @param[in] path The directory path to check
+ * @return FSType enum indicating the filesystem type
+ */
+FSType GetFilesystemType(const fs::path& path);
+#endif
+
 /**
  * Ensure file contents are fully committed to disk, using a platform-specific
  * feature analogous to fsync().