Implement forcing CLI colour on, and document it better

This is necessary to make some old tests work when testing colour
against non-interactive outputs.

Change-Id: Id89f8a1f45c587fede35a69db85f7a52f2c0a981

doc/manual/src/contributing/testing.md

@@ -427,6 +427,7 @@ I grepped `src/` for `get[eE]nv\("` to find the mentions in Lix code.
 - `NIX_SHOW_STATS_PATH` - Writes those statistics into a file at the given path instead of stdout. Undocumented.
 - `NIX_SHOW_SYMBOLS` - Dumps the symbol table into the show-stats json output.
 - `TERM` - If `dumb` or unset, disables ANSI colour output.
+- `FORCE_COLOR`, `CLICOLOR_FORCE` - Enables ANSI colour output if `NO_COLOR`/`NOCOLOR` not set.
 - `NO_COLOR`, `NOCOLOR` - Disables ANSI colour output.
 - `_NIX_DEVELOPER_SHOW_UNKNOWN_LOCATIONS` - Highlights unknown locations in errors.
 - `NIX_PROFILE` - Selects which profile `nix-env` will operate on. Documented elsewhere.

src/libutil/terminal.cc

@@ -9,9 +9,21 @@ namespace nix {
 
 bool shouldANSI()
 {
-    return isatty(STDERR_FILENO)
-        && getEnv("TERM").value_or("dumb") != "dumb"
-        && !(getEnv("NO_COLOR").has_value() || getEnv("NOCOLOR").has_value());
+    // Implements the behaviour described by https://bixense.com/clicolors/
+    // As well as https://force-color.org/ for compatibility, since it fits in the same shape.
+    // NO_COLOR CLICOLOR CLICOLOR_FORCE Colours?
+    // set      x        x              No
+    // unset    x        set            Yes
+    // unset    x        unset          If attached to a terminal
+    //                                  [we choose the "modern" approach of colour-by-default]
+    auto compute = []() -> bool {
+        bool mustNotColour = getEnv("NO_COLOR").has_value() || getEnv("NOCOLOR").has_value();
+        bool shouldForce = getEnv("CLICOLOR_FORCE").has_value() || getEnv("FORCE_COLOR").has_value();
+        bool isTerminal = isatty(STDERR_FILENO) && getEnv("TERM").value_or("dumb") != "dumb";
+        return !mustNotColour && (shouldForce || isTerminal);
+    };
+    static bool cached = compute();
+    return cached;
 }
 
 std::string filterANSIEscapes(std::string_view s, bool filterAll, unsigned int width)

src/libutil/terminal.hh

@@ -9,6 +9,15 @@ namespace nix {
 /**
  * Determine whether ANSI escape sequences are appropriate for the
  * present output.
+ *
+ * This follows the rules described on https://bixense.com/clicolors/
+ * with CLICOLOR defaulted to enabled (and thus ignored).
+ *
+ * That is to say, the following procedure is followed in order:
+ * - NO_COLOR or NOCOLOR set             -> always disable colour
+ * - CLICOLOR_FORCE or FORCE_COLOR set   -> enable colour
+ * - The output is a tty; TERM != "dumb" -> enable colour
+ * - Otherwise                           -> disable colour
  */
 bool shouldANSI();