core: inherit TERM from PID 1 for all services started on /dev/console

This way, invoking nspawn from a shell in the best case inherits the TERM
setting all the way down into the login shell spawned in the container.

Fixes: #3697

src/basic/terminal-util.c

@@ -785,7 +785,7 @@ bool tty_is_vc_resolve(const char *tty) {
 }
 
 const char *default_term_for_tty(const char *tty) {
-        return tty && tty_is_vc_resolve(tty) ? "TERM=linux" : "TERM=vt220";
+        return tty && tty_is_vc_resolve(tty) ? "linux" : "vt220";
 }
 
 int fd_columns(int fd) {

src/core/execute.c

@@ -219,12 +219,36 @@ static void exec_context_tty_reset(const ExecContext *context, const ExecParamet
                 (void) vt_disallocate(path);
 }
 
+static bool is_terminal_input(ExecInput i) {
+        return IN_SET(i,
+                      EXEC_INPUT_TTY,
+                      EXEC_INPUT_TTY_FORCE,
+                      EXEC_INPUT_TTY_FAIL);
+}
+
 static bool is_terminal_output(ExecOutput o) {
-        return
-                o == EXEC_OUTPUT_TTY ||
-                o == EXEC_OUTPUT_SYSLOG_AND_CONSOLE ||
-                o == EXEC_OUTPUT_KMSG_AND_CONSOLE ||
-                o == EXEC_OUTPUT_JOURNAL_AND_CONSOLE;
+        return IN_SET(o,
+                      EXEC_OUTPUT_TTY,
+                      EXEC_OUTPUT_SYSLOG_AND_CONSOLE,
+                      EXEC_OUTPUT_KMSG_AND_CONSOLE,
+                      EXEC_OUTPUT_JOURNAL_AND_CONSOLE);
+}
+
+static bool exec_context_needs_term(const ExecContext *c) {
+        assert(c);
+
+        /* Return true if the execution context suggests we should set $TERM to something useful. */
+
+        if (is_terminal_input(c->std_input))
+                return true;
+
+        if (is_terminal_output(c->std_output))
+                return true;
+
+        if (is_terminal_output(c->std_error))
+                return true;
+
+        return !!c->tty_path;
 }
 
 static int open_null_as(int flags, int nfd) {
@@ -363,13 +387,6 @@ static int open_terminal_as(const char *path, mode_t mode, int nfd) {
         return r;
 }
 
-static bool is_terminal_input(ExecInput i) {
-        return
-                i == EXEC_INPUT_TTY ||
-                i == EXEC_INPUT_TTY_FORCE ||
-                i == EXEC_INPUT_TTY_FAIL;
-}
-
 static int fixup_input(ExecInput std_input, int socket_fd, bool apply_tty_stdin) {
 
         if (is_terminal_input(std_input) && !apply_tty_stdin)
@@ -1444,12 +1461,21 @@ static int build_environment(
                 our_env[n_env++] = x;
         }
 
-        if (is_terminal_input(c->std_input) ||
-            c->std_output == EXEC_OUTPUT_TTY ||
-            c->std_error == EXEC_OUTPUT_TTY ||
-            c->tty_path) {
+        if (exec_context_needs_term(c)) {
+                const char *tty_path, *term = NULL;
 
-                x = strdup(default_term_for_tty(exec_context_tty_path(c)));
+                tty_path = exec_context_tty_path(c);
+
+                /* If we are forked off PID 1 and we are supposed to operate on /dev/console, then let's try to inherit
+                 * the $TERM set for PID 1. This is useful for containers so that the $TERM the container manager
+                 * passes to PID 1 ends up all the way in the console login shown. */
+
+                if (path_equal(tty_path, "/dev/console") && getppid() == 1)
+                        term = getenv("TERM");
+                if (!term)
+                        term = default_term_for_tty(tty_path);
+
+                x = strappend("TERM=", term);
                 if (!x)
                         return -ENOMEM;
                 our_env[n_env++] = x;

src/core/main.c

@@ -1318,7 +1318,7 @@ static int fixup_environment(void) {
                 return r;
 
         if (r == 0) {
-                term = strdup(default_term_for_tty("/dev/console") + 5);
+                term = strdup(default_term_for_tty("/dev/console"));
                 if (!term)
                         return -ENOMEM;
         }