Print DLite errors in color by default (#748)

# Description
By default, print DLite errors in color if the error stream is a
terminal.

The default can be overridden with the ERR_COLOR environment variable.
Like the `--color` option to `ls`, ERR_COLOR can be "always" or "never".
All other values correspond to the default.

## Type of change
- [ ] Bug fix & code cleanup
- [x] New feature
- [ ] Documentation update
- [ ] Test update

## Checklist for the reviewer
This checklist should be used as a help for the reviewer.

- [ ] Is the change limited to one issue?
- [ ] Does this PR close the issue?
- [ ] Is the code easy to read and understand?
- [ ] Do all new feature have an accompanying new test?
- [ ] Has the documentation been updated as necessary?

doc/user_guide/environment_variables.md

@@ -121,8 +121,11 @@ Environment variables for controlling error handling
       - "4" | "ignore-new"  : ignore new error message
       - otherwise           : append new error message to the old one
 
-  - **ERR_COLOR**: If defined, error messages to stdout and stderr will be
-    written in colour.
+  - **ERR_COLOR**: Whether to print error messages in colour. May be:
+      - "0" | "never"       : print errors colour-coded
+      - "1" | "always"      : print errors not colour-coded
+      - otherwise           : only print errors colour-coded if the error
+                              stream is a terminal
 
 
 Path handling when using the pre-packaged wheel (Linux, Windows)

src/utils/CMakeLists.txt

@@ -21,6 +21,7 @@ check_include_file(endian.h     HAVE_ENDIAN_H)
 check_include_file(byteswap.h   HAVE_BYTESWAP_H)
 check_include_file(locale.h     HAVE_LOCALE_H)
 check_include_file(time.h       HAVE_TIME_H)
+check_include_file(unistd.h     HAVE_UNISTD_H)
 check_include_file(windows.h    HAVE_WINDOWS_H)
 
 # -- check for symbols
@@ -48,6 +49,9 @@ check_symbol_exists(stat                sys/stat.h   HAVE_STAT)
 check_symbol_exists(exec                unistd.h     HAVE_EXEC)
 check_symbol_exists(clock               time.h       HAVE_CLOCK)
 check_symbol_exists(P_tmpdir            stdio.h      HAVE_P_TMPDIR)
+check_symbol_exists(_fileno             stdio.h      HAVE__FILENO)
+check_symbol_exists(isatty              unistd.h     HAVE_ISATTY)
+check_symbol_exists(_isatty             io.h         HAVE__ISATTY)
 
 if(HAVE_WINDOWS_H)
 check_symbol_exists(GetFullPathNameW    windows.h    HAVE_GetFullPathNameW)

src/utils/config.h.in

@@ -35,6 +35,7 @@
 #cmakedefine HAVE_INTTYPES_H
 #cmakedefine HAVE_LOCALE_H
 #cmakedefine HAVE_TIME_H
+#cmakedefine HAVE_UNISTD_H
 #cmakedefine HAVE_WINDOWS_H
 
 /* Whether symbols exists. If not, they are defined in compat.c or compat/ */
@@ -55,6 +56,9 @@
 #cmakedefine HAVE_EXEC
 #cmakedefine HAVE_CLOCK
 #cmakedefine HAVE_P_TMPDIR
+#cmakedefine HAVE__FILENO
+#cmakedefine HAVE_ISATTY
+#cmakedefine HAVE__ISATTY
 
 #cmakedefine HAVE_GetFullPathNameW
 #cmakedefine HAVE_GetFullPathName
@@ -66,6 +70,8 @@
 #cmakedefine HAVE_CreateProcessA
 #cmakedefine HAVE_GetSystemTime
 #cmakedefine HAVE_BCryptGenRandom
+// #cmakedefine HAVE_GetConsoleMode
+// #cmakedefine HAVE_GetStdHandle
 #cmakedefine HAVE_MBSTOWCS
 #cmakedefine HAVE_WCSTOMBS
 #cmakedefine HAVE_MBSTOWCS_S

src/utils/err.c

@@ -8,7 +8,6 @@
 /*
  * TODO: Consider to reuse good ideas from https://github.com/rxi/log.c
  */
-
 #include <assert.h>
 #include <stdio.h>
 #include <stdarg.h>
@@ -17,6 +16,10 @@
 #include "compat.h"
 #include "err.h"
 
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+
 /* Thread local storage
  * https://stackoverflow.com/questions/18298280/how-to-declare-a-variable-as-thread-local-portab
  */
@@ -68,6 +71,9 @@ typedef struct {
    * If negative (default), check the environment. */
   ErrDebugMode err_debug_mode;
 
+  /* Whether to */
+  ErrColorMode err_color_mode;
+
   /* How to handle overridden errors in  ErrTry clauses.
    * If negative (default), check the environment. */
   ErrOverrideMode err_override;
@@ -109,6 +115,7 @@ static void reset_tls(void)
   _tls.err_abort_mode = -1;
   _tls.err_warn_mode = -1;
   _tls.err_debug_mode = -1;
+  _tls.err_color_mode = -1;
   _tls.err_override = -1;
   _tls.err_record = &_tls.err_root_record;
   _tls.globals = &_globals;
@@ -123,6 +130,7 @@ static ThreadLocals *get_tls(void)
     _tls.err_abort_mode = -1;
     _tls.err_warn_mode = -1;
     _tls.err_debug_mode = -1;
+    _tls.err_color_mode = -1;
     _tls.err_override = -1;
     _tls.err_record = &_tls.err_root_record;
     _tls.globals = &_globals;
@@ -619,11 +627,54 @@ ErrOverrideMode err_get_override_mode()
   return tls->err_override;
 }
 
+ErrColorMode err_set_color_mode(ErrColorMode mode)
+{
+  ThreadLocals *tls = get_tls();
+  ErrColorMode prev = tls->err_color_mode;
+  tls->err_color_mode = mode;
+  return prev;
+}
+
+int err_get_color_coded()
+{
+  ThreadLocals *tls = get_tls();
+  if (tls->err_color_mode < 0) {
+    char *mode = getenv("ERR_COLOR");
+    tls->err_color_mode =
+      (!mode || !*mode)                                      ? errColorAuto :
+      (strcmp(mode, "never") == 0 || strcmp(mode, "0") == 0) ? errColorNever :
+      (strcmp(mode, "always") == 0 || strcmp(mode,"1") == 0) ? errColorAlways :
+      errColorAuto;
+  }
+
+  switch (tls->err_color_mode) {
+  case errColorAuto:
+    {
+      int terminal = 0;
+#if defined(HAVE_UNISTD_H) && defined(HAVE_ISATTY)
+      int fno = (tls->globals->err_stream) ?
+        fileno(tls->globals->err_stream) : -1;
+      if (fno >= 0) terminal = (isatty(fno) == 1);
+#elif defined(HAVE__FILENO) && defined(HAVE__ISATTY)
+      int fno = (tls->globals->err_stream) ?
+        _fileno(tls->globals->err_stream) : -1;
+      if (fno >= 0) terminal = _isatty(fno);
+#endif
+      return (terminal) ? 1 : 0;
+    }
+  case errColorAlways:
+    return 1;
+  default:
+    return 0;
+  }
+}
+
 /* Default error handler. */
 void err_default_handler(const ErrRecord *record)
 {
   FILE *stream = err_get_stream();
-  if ((stream == stderr || stream == stdout) && getenv("ERR_COLOR")) {
+  if (err_get_color_coded()) {
+    //if ((stream == stderr || stream == stdout) && getenv("ERR_COLOR")) {
     /* Output the first word of the error message in red and the remaining
        of it in magenta. */
     int n = strcspn(record->msg, ": ");

src/utils/err.h

@@ -56,8 +56,11 @@
  *       - "4" | "ignore-new"  : ignore new error message
  *       - otherwise           : append new error message to the old one
  *
- *   * `ERR_COLOR`: If defined, error messages to stdout and stderr will be
- *     written in colour.
+ *   * `ERR_COLOR`: Whether to write error messages colour-coded.
+ *       - unset | "auto"   : colour-coded only if connected to a terminal
+ *       - "no"  | "never"  : not colour-coded
+ *       - "yes" | "always" : colour-coded
+ *
  */
 
 /** @cond private */
@@ -131,6 +134,14 @@ typedef enum {
   errDebugFull             /*!< add also function name to error messages */
 } ErrDebugMode;
 
+/** Error color mode */
+typedef enum {
+  errColorEnv=-1,       /*!< set from ERR_COLOR environment variable */
+  errColorAuto=0,       /*!< only color-coded if connected to terminal */
+  errColorAlways,       /*!< always color-coded */
+  errColorNever,        /*!< never color-coded */
+} ErrColorMode;
+
 /** Error override mode */
 typedef enum {
   errOverrideEnv=-1,       /*!< set from ERR_OVERRIDE environment variable */
@@ -465,6 +476,18 @@ ErrOverrideMode err_set_override_mode(int mode);
  */
 ErrOverrideMode err_get_override_mode(void);
 
+
+/**
+ * @brief Sets whether to print error messages color-coded.
+ */
+ErrColorMode err_set_color_mode(ErrColorMode mode);
+
+/**
+ * @brief Returns whether error messages are printed color-coded.
+ */
+int err_get_color_coded();
+
+
 /* Where in an ErrTry.. ErrEnd clause we are */
 typedef enum {
   errTryNormal,