better detached part name parsing

dbms/src/Storages/MergeTree/MergeTreeData.cpp

@@ -2579,22 +2579,7 @@ MergeTreeData::getDetachedParts() const
         res.emplace_back();
         auto & part = res.back();
 
-        /// First, try to parse as <part_name>.
+        DetachedPartInfo::tryParseDetachedPartName(dir_name, &part, format_version);
-        if (MergeTreePartInfo::tryParsePartName(dir_name, &part, format_version))
-            continue;
-
-        /// Next, as <prefix>_<partname>. Use entire name as prefix if it fails.
-        part.prefix = dir_name;
-        const auto first_separator = dir_name.find_first_of('_');
-        if (first_separator == String::npos)
-            continue;
-
-        const auto part_name = dir_name.substr(first_separator + 1,
-            dir_name.size() - first_separator - 1);
-        if (!MergeTreePartInfo::tryParsePartName(part_name, &part, format_version))
-           continue;
-
-        part.prefix = dir_name.substr(0, first_separator);
     }
     return res;
 }

dbms/src/Storages/MergeTree/MergeTreePartInfo.cpp

@@ -188,4 +188,26 @@ String MergeTreePartInfo::getPartNameV0(DayNum left_date, DayNum right_date) con
     return wb.str();
 }
 
+bool DetachedPartInfo::tryParseDetachedPartName(const String & dir_name, DetachedPartInfo * part_info,
+                                                MergeTreeDataFormatVersion format_version)
+{
+    /// First, try to parse as <part_name>.
+    if (MergeTreePartInfo::tryParsePartName(dir_name, part_info, format_version))
+        return part_info->valid_name = true;
+
+    /// Next, as <prefix>_<partname>. Use entire name as prefix if it fails.
+    part_info->prefix = dir_name;
+    const auto first_separator = dir_name.find_first_of('_');
+    if (first_separator == String::npos)
+        return part_info->valid_name = false;
+
+    // TODO what if <prefix> contains '_'?
+    const auto part_name = dir_name.substr(first_separator + 1,
+                                           dir_name.size() - first_separator - 1);
+    if (!MergeTreePartInfo::tryParsePartName(part_name, part_info, format_version))
+        return part_info->valid_name = false;
+
+    part_info->prefix = dir_name.substr(0, first_separator);
+    return part_info->valid_name = true;
+}
 }

dbms/src/Storages/MergeTree/MergeTreePartInfo.h

@@ -93,6 +93,12 @@ struct MergeTreePartInfo
 struct DetachedPartInfo : public MergeTreePartInfo
 {
     String prefix;
+
+    /// If false, prefix contains full directory name and MergeTreePartInfo may be in invalid state
+    /// (directory name was not successfully parsed).
+    bool valid_name;
+
+    static bool tryParseDetachedPartName(const String & dir_name, DetachedPartInfo * part_info, MergeTreeDataFormatVersion format_version);
 };
 
 }

dbms/src/Storages/System/StorageSystemDetachedParts.cpp

@@ -28,6 +28,7 @@ public:
 protected:
     explicit StorageSystemDetachedParts()
     {
+        // TODO add column "directory_name" or "is_valid_name"
         setColumns(ColumnsDescription{{
             {"database", std::make_shared<DataTypeString>()},
             {"table", std::make_shared<DataTypeString>()},