exec: export matched container id if present

Provides matched container id to each command execution via
handler_context. If there is no matched node, the handler_context has
the focused node. The idea is that this can simplify scripting.

Relates to PR 6160 at i3

Author: RobertMueller2

sway/commands/exec_always.c

@@ -63,6 +63,26 @@ struct cmd_results *cmd_exec_process(int argc, char **argv) {
 			}
 		}
 
+		if (config->handler_context.node != NULL) {
+			size_t con_id = config->handler_context.node->id;
+			int con_id_len = snprintf(NULL, 0, "%zu", con_id);
+			if (con_id_len < 0) {
+				sway_log(SWAY_ERROR, "Unable to determine buffer length for SWAY_EXEC_CON_ID");
+				goto no_con_id_export;
+			}
+			// accommodate \0
+			con_id_len++;
+			char* con_id_str = malloc(con_id_len);
+			if (!con_id_str) {
+				sway_log(SWAY_ERROR, "Unable to allocate buffer for SWAY_EXEC_CON_ID");
+				goto no_con_id_export;
+			}
+			snprintf(con_id_str, con_id_len, "%zu", con_id);
+			setenv("SWAY_EXEC_CON_ID", con_id_str, 1);
+			free(con_id_str);
+		}
+no_con_id_export:
+
 		execlp("sh", "sh", "-c", cmd, (void*)NULL);
 		sway_log_errno(SWAY_ERROR, "execve failed");
 		_exit(0); // Close child process

sway/sway.5.scd

@@ -698,7 +698,9 @@ The default colors are:
 	afterwards.
 
 *exec* <shell command>
-	Executes _shell command_ with sh.
+	Executes _shell command_ with sh. If following a criteria match, the matched
+	node's id is made available in env variable SWAY_EXEC_CON_ID, otherwise
+	the focused node.
 
 *exec_always* <shell command>
 	Like *exec*, but the shell command will be executed _again_ after *reload*.