nspawn, vmspawn, run0: add env var for turning off background tinting

Some people are just sad, sad lost souls who don't like even the tiniest
ray of color in their life. Let's add an env var knob for allowing them
to turn the background tinting off, to drive the last bit of color from
their life so that they can stay in their grey grey life.

docs/ENVIRONMENT.md

@@ -705,3 +705,11 @@ Tools using the Varlink protocol (such as `varlinkctl`) or sd-bus (such as
   placed in a trusted disk image directory (see above), or if suitable polkit
   authentication was acquired. See `systemd.image-policy(7)` for the valid
   syntax for image policy strings.
+
+`systemd-run`, `run0`, `systemd-nspawn`, `systemd-vmspawn`:
+
+* `$SYSTEMD_TINT_BACKGROUND` – Takes a boolean. When false the automatic
+  tinting of the background for containers, VMs, and interactive `systemd-run`
+  and `run0` invocations is turned off. Note that this environment variable has
+  no effect if the background color is explicitly selected via the relevant
+  `--background=` switch of the tool.

src/nspawn/nspawn.c

@@ -5556,7 +5556,7 @@ static int run_container(
                                                 arg_console_width,
                                                 arg_console_height);
 
-                        if (!arg_background) {
+                        if (!arg_background && shall_tint_background()) {
                                 _cleanup_free_ char *bg = NULL;
 
                                 r = terminal_tint_color(220 /* blue */, &bg);

src/run/run.c

@@ -963,7 +963,7 @@ static int parse_argv_sudo_mode(int argc, char *argv[]) {
         if (strv_extend(&arg_property, "PAMName=systemd-run0") < 0)
                 return log_oom();
 
-        if (!arg_background && arg_stdio == ARG_STDIO_PTY) {
+        if (!arg_background && arg_stdio == ARG_STDIO_PTY && shall_tint_background()) {
                 double hue;
 
                 if (privileged_execution())

src/shared/pretty-print.c

@@ -444,6 +444,21 @@ int terminal_tint_color(double hue, char **ret) {
         return 0;
 }
 
+bool shall_tint_background(void) {
+        static int cache = -1;
+
+        if (cache >= 0)
+                return cache;
+
+        cache = getenv_bool("SYSTEMD_TINT_BACKGROUND");
+        if (cache == -ENXIO)
+                return (cache = true);
+        if (cache < 0)
+                log_debug_errno(cache, "Failed to parse $SYSTEMD_TINT_BACKGROUND, leaving background tinting enabled: %m");
+
+        return cache != 0;
+}
+
 void draw_progress_bar(const char *prefix, double percentage) {
 
         fputc('\r', stderr);

src/shared/pretty-print.h

@@ -50,5 +50,7 @@ static inline const char *green_check_mark_internal(char buffer[static GREEN_CHE
 
 int terminal_tint_color(double hue, char **ret);
 
+bool shall_tint_background(void);
+
 void draw_progress_bar(const char *prefix, double percentage);
 void clear_progress_bar(const char *prefix);

src/vmspawn/vmspawn.c

@@ -1977,7 +1977,7 @@ static int run_virtual_machine(int kvm_device_fd, int vhost_device_fd) {
                 if (r < 0)
                         return log_error_errno(r, "Failed to create PTY forwarder: %m");
 
-                if (!arg_background) {
+                if (!arg_background && shall_tint_background()) {
                         _cleanup_free_ char *bg = NULL;
 
                         r = terminal_tint_color(130 /* green */, &bg);