i3bar: Print error message when status_command fails

Add a function to i3bar to print an error message in the status line
when the child process invoked by status_command fails to provide
input that can be displayed as a statusline.

When the child provides JSON that cannot be parsed, alert the user and
convey a short message provided by yajl communicating the specific
problem.

When the child (or the shell executing the status command) exits
unexpectedly, alert the user and display the exit code. The cases where
the status command is not executable or not found in the user's PATH are
treated specially.

fixes #1130

i3bar/src/child.c

@@ -13,6 +13,7 @@
 #include <sys/wait.h>
 #include <signal.h>
 #include <stdio.h>
+#include <stdarg.h>
 #include <fcntl.h>
 #include <string.h>
 #include <errno.h>
@@ -60,6 +61,58 @@ char *statusline_buffer = NULL;
 
 int child_stdin;
 
+/*
+ * Clears all blocks from the statusline structure in memory and frees their
+ * associated resources.
+ */
+static void clear_status_blocks() {
+    struct status_block *first;
+    while (!TAILQ_EMPTY(&statusline_head)) {
+        first = TAILQ_FIRST(&statusline_head);
+        I3STRING_FREE(first->full_text);
+        TAILQ_REMOVE(&statusline_head, first, blocks);
+        free(first);
+    }
+}
+
+/*
+ * Replaces the statusline in memory with an error message. Pass a format
+ * string and format parameters as you would in `printf'. The next time
+ * `draw_bars' is called, the error message text will be drawn on the bar in
+ * the space allocated for the statusline.
+ */
+
+/* forward function declaration is needed to add __attribute__ mechanism which
+ * helps the compiler understand we are defining a printf wrapper */
+static void set_statusline_error(const char *format, ...) __attribute__ ((format (printf, 1, 2)));
+
+static void set_statusline_error(const char *format, ...) {
+    clear_status_blocks();
+
+    char *message;
+    va_list args;
+    va_start(args, format);
+    vasprintf(&message, format, args);
+
+    struct status_block *err_block = scalloc(sizeof(struct status_block));
+    err_block->full_text = i3string_from_utf8("Error: ");
+    err_block->name = "error";
+    err_block->color = "red";
+    err_block->no_separator = true;
+
+    struct status_block *message_block = scalloc(sizeof(struct status_block));
+    message_block->full_text = i3string_from_utf8(message);
+    message_block->name = "error_message";
+    message_block->color = "red";
+    message_block->no_separator = true;
+
+    TAILQ_INSERT_HEAD(&statusline_head, err_block, blocks);
+    TAILQ_INSERT_TAIL(&statusline_head, message_block, blocks);
+
+    FREE(message);
+    va_end(args);
+}
+
 /*
  * Stop and free() the stdin- and sigchild-watchers
  *
@@ -241,6 +294,7 @@ static unsigned char *get_buffer(ev_io *watcher, int *ret_buffer_len) {
             /* end of file, kill the watcher */
             ELOG("stdin: received EOF\n");
             cleanup();
+            set_statusline_error("Received EOF from statusline process");
             draw_bars(false);
             *ret_buffer_len = -1;
             return NULL;
@@ -280,8 +334,18 @@ static bool read_json_input(unsigned char *input, int length) {
 #else
     if (status != yajl_status_ok && status != yajl_status_insufficient_data) {
 #endif
-        fprintf(stderr, "[i3bar] Could not parse JSON input (code %d): %.*s\n",
-                status, length, input);
+        char *message = (char *)yajl_get_error(parser, 0, input, length);
+
+        /* strip the newline yajl adds to the error message */
+        if (message[strlen(message) - 1] == '\n')
+            message[strlen(message) - 1] = '\0';
+
+        fprintf(stderr, "[i3bar] Could not parse JSON input (code = %d, message = %s): %.*s\n",
+                status, message, length, input);
+
+        set_statusline_error("Could not parse JSON (%s)", message);
+        yajl_free_error(parser, (unsigned char*)message);
+        draw_bars(false);
     } else if (parser_context.has_urgent) {
         has_urgent = true;
     }
@@ -351,10 +415,23 @@ void stdin_io_first_line_cb(struct ev_loop *loop, ev_io *watcher, int revents) {
  *
  */
 void child_sig_cb(struct ev_loop *loop, ev_child *watcher, int revents) {
+    int exit_status = WEXITSTATUS(watcher->rstatus);
+
     ELOG("Child (pid: %d) unexpectedly exited with status %d\n",
            child.pid,
-           watcher->rstatus);
+           exit_status);
+
+    /* this error is most likely caused by a user giving a nonexecutable or
+     * nonexistent file, so we will handle those cases separately. */
+    if (exit_status == 126)
+        set_statusline_error("status_command is not executable (exit %d)", exit_status);
+    else if (exit_status == 127)
+        set_statusline_error("status_command not found (exit %d)", exit_status);
+    else
+        set_statusline_error("status_command process exited unexpectedly (exit %d)", exit_status);
+
     cleanup();
+    draw_bars(false);
 }
 
 void child_write_output(void) {