Fix Process::Status for unknown signals (#15280)

Using `Signal.from_value` limits the result to named members of the enum `Signal`. That means it only accepts signals that are explicitly defined in the Crystal bindings. While we generally strive for completeness, there's no guarantee that the operating system might indicate a signal that is not in the bindings. And in fact there are no mappings for real-time signals (`SIGRTMIN`..`SIGRTMAX`), for example (not sure if they're particularly relevant as exit signals, but it shows incompleteness).

This mechanism should be fault tolerant and be able to represent _any_ signal value, regardless of whether it's mapped in the Crystal bindings or not. If it's missing we cannot associate a name, but we know it's a signal and can express that in the type `Signal` (enums can take any value, not just those of named members).

spec/std/process/status_spec.cr

@@ -105,6 +105,9 @@ describe Process::Status do
       Process::Status.new(Signal::INT.value).exit_signal.should eq Signal::INT
       last_signal = Signal.values[-1]
       Process::Status.new(last_signal.value).exit_signal.should eq last_signal
+
+      unknown_signal = Signal.new(126)
+      Process::Status.new(unknown_signal.value).exit_signal.should eq unknown_signal
     end
 
     it "#normal_exit? with signal code" do
@@ -249,6 +252,8 @@ describe Process::Status do
         assert_prints Process::Status.new(Signal::HUP.value).to_s, "HUP"
         last_signal = Signal.values[-1]
         assert_prints Process::Status.new(last_signal.value).to_s, last_signal.to_s
+
+        assert_prints Process::Status.new(Signal.new(126).value).to_s, "Signal[126]"
       end
     {% end %}
   end
@@ -275,6 +280,9 @@ describe Process::Status do
         assert_prints Process::Status.new(Signal::HUP.value).inspect, "Process::Status[Signal::HUP]"
         last_signal = Signal.values[-1]
         assert_prints Process::Status.new(last_signal.value).inspect, "Process::Status[#{last_signal.inspect}]"
+
+        unknown_signal = Signal.new(126)
+        assert_prints Process::Status.new(unknown_signal.value).inspect, "Process::Status[Signal[126]]"
       end
     {% end %}
   end

src/enum.cr

@@ -225,7 +225,8 @@ abstract struct Enum
     end
   end
 
-  private def member_name
+  # :nodoc:
+  def member_name : String?
     # Can't use `case` here because case with duplicate values do
     # not compile, but enums can have duplicates (such as `enum Foo; FOO = 1; BAR = 1; end`).
     {% for member in @type.constants %}

src/process/status.cr

@@ -217,7 +217,7 @@ class Process::Status
   # which also works on Windows.
   def exit_signal : Signal
     {% if flag?(:unix) && !flag?(:wasm32) %}
-      Signal.from_value(signal_code)
+      Signal.new(signal_code)
     {% else %}
       raise NotImplementedError.new("Process::Status#exit_signal")
     {% end %}
@@ -298,7 +298,12 @@ class Process::Status
       if normal_exit?
         io << exit_code
       else
-        io << exit_signal
+        signal = exit_signal
+        if name = signal.member_name
+          io << name
+        else
+          signal.inspect(io)
+        end
       end
     {% end %}
   end
@@ -314,7 +319,8 @@ class Process::Status
       if normal_exit?
         exit_code.to_s
       else
-        exit_signal.to_s
+        signal = exit_signal
+        signal.member_name || signal.inspect
       end
     {% end %}
   end