mxqkill: Add cancel_job

To cancel a single job:

- Verify the owner uid in advance, so that we don't need a join with
  mxq_group during a later update statement. The join during update
  doesn't work well with the triggers, which want to modify mxq_group.
  By avoiding this, we can later dispose the CANCELLING state which is
  also just a workaround.

- Set job_cancelled to true for the job (don't mind job_status)

job_status transitions will be done by following commits.

mxqkill.c

@@ -33,6 +33,16 @@
 #define UINT64_SPECIAL_MIN (uint64_t)(-2)
 #define UINT64_HASVALUE(x) ((x) < UINT64_SPECIAL_MIN)
 
+__attribute__ ((noreturn))
+__attribute__ ((format (printf, 1, 2)))
+static void die(const char *restrict fmt, ...) {
+    va_list ap;
+    va_start(ap, fmt);
+    vfprintf(stderr, fmt, ap);
+    va_end(ap);
+    _exit(1);
+}
+
 static void print_usage(void)
 {
     mxq_print_generic_version();
@@ -148,6 +158,65 @@ static int update_job_status_cancelled_by_group(struct mx_mysql *mysql, struct m
     return (int)num_rows;
 }
 
+static void verify_job_owner(struct mx_mysql *mysql, uint64_t job_id, uint64_t user_uid) {
+    struct mx_mysql_stmt *stmt = mx_mysql_statement_prepare(mysql,
+        "SELECT user_uid FROM mxq_job, mxq_group"
+        " WHERE mxq_job.group_id = mxq_group.group_id"
+        " AND job_id = ?"
+    );
+    if (!stmt)
+        die("mx_mysql_statement_prepare(): %s\n", mx_mysql_error());
+
+    mx_mysql_statement_param_bind(stmt, 0, uint64, &job_id);
+
+    unsigned long long num_rows;
+    int res = mx_mysql_statement_execute(stmt, &num_rows);
+    if (res < 0)
+        die("mx_mysql_statement_execute(): %s\n", mx_mysql_error());
+
+    if (num_rows == 0)
+        die("no such job_id %lu\n", job_id);
+
+    uint64_t uid;
+    mx_mysql_statement_result_bind(stmt, 0, uint64, &uid);
+
+    res = mx_mysql_statement_fetch(stmt);
+    if (res < 0)
+        die("mx_mysql_statement_fetch: %s\n", mx_mysql_error());
+
+    if (uid != user_uid)
+        die("job %lu: permission denied\n", job_id);
+
+    mx_mysql_statement_close(&stmt);
+}
+
+static void set_job_cancelled(struct mx_mysql *mysql, uint64_t job_id) {
+    struct mx_mysql_stmt *stmt = mx_mysql_statement_prepare(mysql,
+            "UPDATE mxq_job SET job_cancelled = TRUE"
+            " WHERE job_id = ?"
+    );
+    if (!stmt)
+        die("mx_mysql_statement_prepare(): %s\n", mx_mysql_error());
+
+    mx_mysql_statement_param_bind(stmt, 0, uint64, &job_id);
+
+    unsigned long long num_rows;
+    int res = mx_mysql_statement_execute(stmt, &num_rows);
+    if (res < 0)
+        die("mx_mysql_statement_execute(): %s\n", mx_mysql_error());
+    if (num_rows == 0)
+        die("no such job_id %lu\n", job_id);
+
+    mx_mysql_statement_close(&stmt);
+}
+
+__attribute__ ((unused))
+static void cancel_job(struct mx_mysql *mysql, uint64_t job_id, uint64_t user_uid) {
+
+    verify_job_owner(mysql, job_id, user_uid);
+    set_job_cancelled(mysql, job_id);
+}
+
 static int update_job_status_cancelling_by_job_id_for_user(struct mx_mysql *mysql, uint64_t job_id, uint64_t user_uid)
 {
     struct mx_mysql_stmt *stmt = NULL;