Fix some issues with experimental config settings

Issues:

1. Features gated on disabled experimental settings should warn and be
   ignored, not silently succeed.

2. Experimental settings in the same config "batch" (file or env var)
   as the enabling of the experimental feature should work.

3. For (2), the order should not matter.

These are analogous to the issues @roberth caught with my changes for
arg handling, but they are instead for config handling.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

1 files changed, 41 insertions, 5 deletions

diff --git a/src/libutil/config.hh b/src/libutil/config.hh
index 162626791..2675baed7 100644
--- a/src/libutil/config.hh
+++ b/src/libutil/config.hh
@@ -215,8 +215,11 @@ protected:
 
     virtual void set(const std::string & value, bool append = false) = 0;
 
-    virtual bool isAppendable()
-    { return false; }
+    /**
+     * Whether the type is appendable; i.e. whether the `append`
+     * parameter to `set()` is allowed to be `true`.
+     */
+    virtual bool isAppendable() = 0;
 
     virtual std::string to_string() const = 0;
 
@@ -241,6 +244,23 @@ protected:
     const T defaultValue;
     const bool documentDefault;
 
+    /**
+     * Parse the string into a `T`.
+     *
+     * Used by `set()`.
+     */
+    virtual T parse(const std::string & str) const;
+
+    /**
+     * Append or overwrite `value` with `newValue`.
+     *
+     * Some types to do not support appending in which case `append`
+     * should never be passed. The default handles this case.
+     *
+     * @param append Whether to append or overwrite.
+     */
+    virtual void appendOrSet(T && newValue, bool append);
+
 public:
 
     BaseSetting(const T & def,
@@ -268,9 +288,25 @@ public:
     template<typename U>
     void setDefault(const U & v) { if (!overridden) value = v; }
 
-    void set(const std::string & str, bool append = false) override;
+    /**
+     * Require any experimental feature the setting depends on
+     *
+     * Uses `parse()` to get the value from `str`, and `appendOrSet()`
+     * to set it.
+     */
+    void set(const std::string & str, bool append = false) override final;
 
-    bool isAppendable() override;
+    /**
+     * C++ trick; This is template-specialized to compile-time indicate whether
+     * the type is appendable.
+     */
+    struct trait;
+
+    /**
+     * Always defined based on the C++ magic
+     * with `trait` above.
+     */
+    bool isAppendable() override final;
 
     virtual void override(const T & v)
     {
@@ -336,7 +372,7 @@ public:
         options->addSetting(this);
     }
 
-    void set(const std::string & str, bool append = false) override;
+    Path parse(const std::string & str) const override;
 
     Path operator +(const char * p) const { return value + p; }