[llvm/Support] Fallback to $TERM if terminfo has no "colors" capability

[mib]

Summary

It can happen on macOS that the terminal doesn't report the "colors"
capability in the terminfo database, in which case tigetnum returns -1.

This doesn't mean however that the terminal doesn't supports color, it
just means that the capability is absent from the terminal description.

In that case, we should still fallback to the checking the $TERM
environment variable to see if it supports ANSI escapes codes.

Signed-off-by: Med Ismail Bennani <[e-mail]>

[JDevlieghere]

LGTM

llvm/lib/Support/Unix/Process.inc

 extern "C" int del_curterm(struct term *termp);
 extern "C" int tigetnum(char *capname);
 #endif
 
 #ifdef LLVM_ENABLE_TERMINFO
 static ManagedStatic<std::mutex> TermColorMutex;
 #endif
 
+bool checkTerminalEnvironmentForColors() {
+  if (const char *TermStr = std::getenv("TERM")) {
+    return StringSwitch<bool>(TermStr)
+      .Case("ansi", true)
+      .Case("cygwin", true)
+      .Case("linux", true)
+      .StartsWith("screen", true)
+      .StartsWith("xterm", true)
+      .StartsWith("vt100", true)
+      .StartsWith("rxvt", true)
+      .EndsWith("color", true)
+      .Default(false);
+  }
+
+  return false;
+}
+
 static bool terminalHasColors(int fd) {
 #ifdef LLVM_ENABLE_TERMINFO
   // First, acquire a global lock because these C routines are thread hostile.
   std::lock_guard<std::mutex> G(*TermColorMutex);
 
   struct term *previous_term = set_curterm(nullptr);
   int errret = 0;
   if (setupterm(nullptr, fd, &errret) != 0)
#ifdef LLVM_ENABLE_TERMINFO
   // if it will interpret ANSI color escape codes in a reasonable way. Thus, the
   // strategy here is just to query the baseline colors capability and if it
   // supports colors at all to assume it will translate the escape codes into
   // whatever range of colors it does support. We can add more detailed tests
   // here if users report them as necessary.
   //
   // The 'tigetnum' routine returns -2 or -1 on errors, and might return 0 if
   // the terminfo says that no colors are supported.
-  bool HasColors = tigetnum(const_cast<char *>("colors")) > 0;
+  int colors_ti = tigetnum(const_cast<char *>("colors"));
+  bool HasColors = colors_ti >= 0 ? colors_ti : checkTerminalEnvironmentForColors();
 
   // Now extract the structure allocated by setupterm and free its memory
   // through a really silly dance.
   struct term *termp = set_curterm(previous_term);
   (void)del_curterm(termp); // Drop any errors here.
 
   // Return true if we found a color capabilities for the current terminal.
-  if (HasColors)
-    return true;
+  return HasColors;
 #else
   // When the terminfo database is not available, check if the current terminal
   // is one of terminals that are known to support ANSI color escape codes.
-  if (const char *TermStr = std::getenv("TERM")) {
-    return StringSwitch<bool>(TermStr)
-      .Case("ansi", true)
-      .Case("cygwin", true)
-      .Case("linux", true)
-      .StartsWith("screen", true)
-      .StartsWith("xterm", true)
-      .StartsWith("vt100", true)
-      .StartsWith("rxvt", true)
-      .EndsWith("color", true)
-      .Default(false);
-  }
+  return checkTerminalEnvironmentForColors();
 #endif
-
-  // Otherwise, be conservative.
-  return false;
 }
 
 bool Process::FileDescriptorHasColors(int fd) {
   // A file descriptor has colors if it is displayed and the terminal has
   // colors.
   return FileDescriptorIsDisplayed(fd) && terminalHasColors(fd);
 }