util: add perm string helper functions

PermsToSymbolicString will convert from fs::perms to string type
'rwxrwxrwx'.

InterpretPermString will convert from a user-supplied "perm string" such
as 'owner', 'group' or 'all, into appropriate fs::perms.

src/util/fs_helpers.cpp

@@ -16,6 +16,7 @@
 #include <fstream>
 #include <map>
 #include <memory>
+#include <optional>
 #include <string>
 #include <system_error>
 #include <utility>
@@ -269,3 +270,42 @@ bool TryCreateDirectories(const fs::path& p)
     // create_directories didn't create the directory, it had to have existed already
     return false;
 }
+
+std::string PermsToSymbolicString(fs::perms p)
+{
+    std::string perm_str(9, '-');
+
+    auto set_perm = [&](size_t pos, fs::perms required_perm, char letter) {
+        if ((p & required_perm) != fs::perms::none) {
+            perm_str[pos] = letter;
+        }
+    };
+
+    set_perm(0, fs::perms::owner_read,   'r');
+    set_perm(1, fs::perms::owner_write,  'w');
+    set_perm(2, fs::perms::owner_exec,   'x');
+    set_perm(3, fs::perms::group_read,   'r');
+    set_perm(4, fs::perms::group_write,  'w');
+    set_perm(5, fs::perms::group_exec,   'x');
+    set_perm(6, fs::perms::others_read,  'r');
+    set_perm(7, fs::perms::others_write, 'w');
+    set_perm(8, fs::perms::others_exec,  'x');
+
+    return perm_str;
+}
+
+std::optional<fs::perms> InterpretPermString(const std::string& s)
+{
+    if (s == "owner") {
+        return fs::perms::owner_read | fs::perms::owner_write;
+    } else if (s == "group") {
+        return fs::perms::owner_read | fs::perms::owner_write |
+               fs::perms::group_read;
+    } else if (s == "all") {
+        return fs::perms::owner_read | fs::perms::owner_write |
+               fs::perms::group_read |
+               fs::perms::others_read;
+    } else {
+        return std::nullopt;
+    }
+}

src/util/fs_helpers.h

@@ -12,6 +12,7 @@
 #include <cstdio>
 #include <iosfwd>
 #include <limits>
+#include <optional>
 
 /**
  * Ensure file contents are fully committed to disk, using a platform-specific
@@ -62,6 +63,19 @@ void ReleaseDirectoryLocks();
 bool TryCreateDirectories(const fs::path& p);
 fs::path GetDefaultDataDir();
 
+/** Convert fs::perms to symbolic string of the form 'rwxrwxrwx'
+ *
+ * @param[in] p the perms to be converted
+ * @return Symbolic permissions string
+ */
+std::string PermsToSymbolicString(fs::perms p);
+/** Interpret a custom permissions level string as fs::perms
+ *
+ * @param[in] s Permission level string
+ * @return Permissions as fs::perms
+ */
+std::optional<fs::perms> InterpretPermString(const std::string& s);
+
 #ifdef WIN32
 fs::path GetSpecialFolderPath(int nFolder, bool fCreate = true);
 #endif