config/Data: add WithEach(ConfigBlockOption)

To improve error messages without making callers more complex.

src/config/Block.cxx

@@ -143,3 +143,10 @@ ConfigBlock::GetBlockValue(const char *name, bool default_value) const
 
 	return bp->GetBoolValue();
 }
+
+void
+ConfigBlock::ThrowWithNested() const
+{
+	std::throw_with_nested(FormatRuntimeError("Error in block on line %i",
+						  line));
+}

src/config/Block.hxx

@@ -137,6 +137,14 @@ struct ConfigBlock {
 	unsigned GetPositiveValue(const char *name, unsigned default_value) const;
 
 	bool GetBlockValue(const char *name, bool default_value) const;
+
+	/**
+	 * Call this method in a "catch" block to throw a nested
+	 * exception showing the location of this block in the
+	 * configuration file.
+	 */
+	[[noreturn]]
+	void ThrowWithNested() const;
 };
 
 #endif

src/config/Data.hxx

@@ -121,6 +121,26 @@ struct ConfigData {
 
 	ConfigBlock &MakeBlock(ConfigBlockOption option,
 				     const char *key, const char *value);
+
+	/**
+	 * Invoke the given function for each instance of the
+	 * specified block.
+	 *
+	 * Exceptions thrown by the function will be nested in one
+	 * that specifies the location of the block.
+	 */
+	template<typename F>
+	void WithEach(ConfigBlockOption option, F &&f) const {
+		for (const auto &block : GetBlockList(option)) {
+			block.SetUsed();
+
+			try {
+				f(block);
+			} catch (...) {
+				block.ThrowWithNested();
+			}
+		}
+	}
 };
 
 #endif