fix: check if it is a Real terminal, not just if it is a terminal

This will stop printing stuff to dumb terminals that they don't support.

I've overall audited usage of isatty and replaced the ones with intent
to mean "is a Real terminal" with checking for that. I've also caught a
case of carelessly assuming "is a tty" means "should be colour" in
nix-env.

Change-Id: I6d83725d9a2d932ac94ff2294f92c0a1100d23c9

1 files changed, 23 insertions, 1 deletions

diff --git a/src/libutil/terminal.hh b/src/libutil/terminal.hh
index 2c422ecff..28c96c780 100644
--- a/src/libutil/terminal.hh
+++ b/src/libutil/terminal.hh
@@ -6,6 +6,26 @@
 
 namespace nix {
 
+enum class StandardOutputStream {
+    Stdout = 1,
+    Stderr = 2,
+};
+
+/**
+ * Determine whether the output is a real terminal (i.e. not dumb, not a pipe).
+ *
+ * This is probably not what you want, you may want shouldANSI() or something
+ * more specific. Think about how the output should work with a pager or
+ * entirely non-interactive scripting use.
+ *
+ * The user may be redirecting the Lix output to a pager, but have stderr
+ * connected to a terminal. Think about where you are outputting the text when
+ * deciding whether to use STDERR_FILENO or STDOUT_FILENO.
+ *
+ * \param fileno file descriptor number to check if it is a tty
+ */
+bool isOutputARealTerminal(StandardOutputStream fileno);
+
 /**
  * Determine whether ANSI escape sequences are appropriate for the
  * present output.
@@ -18,8 +38,10 @@ namespace nix {
  * - CLICOLOR_FORCE or FORCE_COLOR set   -> enable colour
  * - The output is a tty; TERM != "dumb" -> enable colour
  * - Otherwise                           -> disable colour
+ *
+ * \param fileno which file descriptor number to consider. Use the one you are outputting to
  */
-bool shouldANSI();
+bool shouldANSI(StandardOutputStream fileno = StandardOutputStream::Stderr);
 
 /**
  * Truncate a string to 'width' printable characters. If 'filterAll'