put curses/terminfo vars into the environment

We need to actually export the curses/terminfo env vars in order for
`setupterm()` to be able to use them. While fixing this I reworked the
fallback logic implemented by @zanchey in response to issue #1060 in
order to simplify the logic and clarify the error messages.

This does not allow someone to change the curses/terminfo env vars after
the first prompt is displayed (you can but it won't affect the current
fish process). It only makes it possible to set `TERM`, `TERMINFO`, and
`TERMINFO_DIRS` in *config.fish* or similar config file and have them be
honored by fish.

src/input.cpp

@@ -6,7 +6,6 @@
 
 #include "config.h"
 
-#include <assert.h>
 #include <errno.h>
 #include <unistd.h>
 #include <wchar.h>
@@ -457,43 +456,27 @@ void update_fish_color_support(void)
     output_set_color_support(support);
 }
 
-int input_init()
-{
-    if (is_init)
-        return 1;
-
+int input_init() {
+    if (is_init) return 1;
     is_init = true;
-
     input_common_init(&interrupt_handler);
 
-    const env_var_t term = env_get_string(L"TERM");
-    int errret;
-    if (setupterm(const_cast<char *>(wcs2string(term).c_str()), STDOUT_FILENO, &errret) == ERR)
-    {
-        debug(0, _(L"Could not set up terminal"));
-        if (errret == 0)
-        {
-            debug(0, _(L"Check that your terminal type, '%ls', is supported on this system"),
-                  term.c_str());
-            debug(0, _(L"Attempting to use '%ls' instead"), DEFAULT_TERM);
-            env_set(L"TERM", DEFAULT_TERM, ENV_GLOBAL | ENV_EXPORT);
-            const std::string default_term = wcs2string(DEFAULT_TERM);
-            if (setupterm(const_cast<char *>(default_term.c_str()), STDOUT_FILENO, &errret) == ERR)
-            {
-                debug(0, _(L"Could not set up terminal"));
-                exit_without_destructors(1);
-            }
-        }
-        else
-        {
+    int err_ret;
+    if (setupterm(NULL, STDOUT_FILENO, &err_ret) == ERR) {
+        env_var_t term = env_get_string(L"TERM");
+        debug(0, _(L"Your TERM value of '%ls' is not valid"), term.c_str());
+        debug(0, _(L"Check that your terminal type is supported on this system"));
+        env_set(L"TERM", DEFAULT_TERM, ENV_GLOBAL | ENV_EXPORT);
+        if (setupterm(NULL, STDOUT_FILENO, &err_ret) == ERR) {
+            debug(0, _(L"Unable to setup terminal using your TERM or the '%ls' fallback"),
+                  DEFAULT_TERM);
             exit_without_destructors(1);
+        } else {
+            debug(0, _(L"Using fallback terminal type '%ls' instead"), DEFAULT_TERM);
         }
     }
-    assert(! term.missing());
-    output_set_term(term);
 
     input_terminfo_init();
-
     update_fish_color_support();
 
     /* If we have no keybindings, add a few simple defaults */

src/output.cpp

@@ -29,6 +29,7 @@
 #include "fallback.h"
 #include "wutil.h" // IWYU pragma: keep - needed for wgettext
 #include "common.h"
+#include "env.h"
 #include "output.h"
 
 static int writeb_internal(char c);
@@ -37,14 +38,8 @@ static int writeb_internal(char c);
 /**
  The function used for output
  */
-
 static int (*out)(char c) = &writeb_internal;
 
-/**
- Name of terminal
- */
-static wcstring current_term;
-
 /* Whether term256 and term24bit are supported */
 static color_support_t color_support = 0;
 
@@ -554,29 +549,13 @@ rgb_color_t parse_color(const wcstring &val, bool is_background)
     return result;
 }
 
-void output_set_term(const wcstring &term)
-{
-    current_term.assign(term);
-}
-
-const wchar_t *output_get_term()
-{
-    return current_term.empty() ? L"<unknown>" : current_term.c_str();
-}
-
-void writembs_check(char *mbs, const char *mbs_name, const char *file, long line)
-{
-    if (mbs != NULL)
-    {
+void writembs_check(char *mbs, const char *mbs_name, const char *file, long line) {
+    if (mbs != NULL) {
         tputs(mbs, 1, &writeb);
-    }
-    else
-    {
-        debug( 0, _(L"Tried to use terminfo string %s on line %ld of %s, which is undefined in terminal of type \"%ls\". Please report this error to %s"),
-              mbs_name,
-              line,
-              file,
-              output_get_term(),
-              PACKAGE_BUGREPORT);
+    } else {
+        env_var_t term = env_get_string(L"TERM");
+        debug(0, _(L"Tried to use terminfo string %s on line %ld of %s, which is undefined in "
+                   L"terminal of type \"%ls\". Please report this error to %s"),
+              mbs_name, line, file, term.c_str(), PACKAGE_BUGREPORT);
     }
 }

src/output.h

@@ -105,11 +105,6 @@ void output_set_writer(int (*writer)(char));
  */
 int (*output_get_writer())(char) ;
 
-/** Set the terminal name */
-void output_set_term(const wcstring &term);
-
-/** Return the terminal name */
-const wchar_t *output_get_term();
 
 /** Sets what colors are supported */
 enum