MXS-1663 Detect conflicting configurations and refuse to start

Use locks with the datadir and cachedir to detect if another MaxScale
instance attempts to use the same directories as the already running
MaxScale.

server/core/gateway.cc

@@ -27,6 +27,8 @@
 #include <time.h>
 #include <unistd.h>
 #include <getopt.h>
+#include <set>
+#include <map>
 
 #include <ini.h>
 #include <openssl/opensslconf.h>
@@ -90,6 +92,8 @@ static bool     datadir_defined = false; /*< If the datadir was already set */
 /* The data directory we created for this gateway instance */
 static char     pidfile[PATH_MAX + 1] = "";
 static int pidfd = PIDFD_CLOSED;
+/* Map containing paths to directory locks with their fds */
+static std::map<std::string, int> directory_locks;
 
 /**
  * If MaxScale is started to run in daemon process the value is true.
@@ -145,6 +149,9 @@ static std::string redirect_output_to;
 
 static int cnf_preparser(void* data, const char* section, const char* name, const char* value);
 static int write_pid_file(); /* write MaxScale pidfile */
+static bool lock_dir(const std::string& path);
+static bool lock_directories();
+static void unlock_directories();
 static void unlink_pidfile(void); /* remove pidfile */
 static void unlock_pidfile();
 static bool file_write_header(FILE* outfile);
@@ -1328,7 +1335,7 @@ int main(int argc, char **argv)
     sigset_t sigpipe_mask;
     sigset_t saved_mask;
     bool to_stdout = false;
-    void   (*exitfunp[4])(void) = { mxs_log_finish, cleanup_process_datadir, write_footer, NULL };
+    void   (*exitfunp[5])(void) = { mxs_log_finish, cleanup_process_datadir, write_footer, unlock_directories, NULL };
     int numlocks = 0;
     bool pid_file_created = false;
     Worker* worker;
@@ -2000,6 +2007,12 @@ int main(int argc, char **argv)
         }
 
         pid_file_created = true;
+
+        if (!lock_directories())
+        {
+            rc = MAXSCALE_ALREADYRUNNING;
+            goto return_main;
+        }
     }
 
     if (!redirect_output_to.empty())
@@ -2209,6 +2222,7 @@ int main(int argc, char **argv)
 
     utils_end();
     cleanup_process_datadir();
+    unlock_directories();
     MXS_NOTICE("MaxScale shutdown completed.");
 
     if (unload_modules_at_exit)
@@ -3216,3 +3230,68 @@ static bool init_sqlite3()
 
     return rv;
 }
+
+static bool lock_dir(const std::string& path)
+{
+    std::string lock = path + "/maxscale.lock";
+    int fd = open(lock.c_str(), O_WRONLY | O_CREAT, 0777);
+    std::string pid = std::to_string(getpid());
+
+    if (fd == -1)
+    {
+        MXS_ERROR("Failed to open lock file %s: %s", lock.c_str(), mxs_strerror(errno));
+        return false;
+    }
+
+    if (lockf(fd, F_TLOCK, 0) == -1)
+    {
+        if (errno == EACCES || errno == EAGAIN)
+        {
+            MXS_ERROR("Failed to lock directory with file '%s', another process is holding a lock on it. "
+                      "Please confirm that no other MaxScale process is using the "
+                      "directory %s", lock.c_str(), path.c_str());
+        }
+        else
+        {
+            MXS_ERROR("Failed to lock file %s. %s", lock.c_str(), mxs_strerror(errno));
+        }
+        close(fd);
+        return false;
+    }
+
+    if (ftruncate(fd, 0) == -1)
+    {
+        MXS_ERROR("Failed to truncate lock file %s: %s", lock.c_str(), mxs_strerror(errno));
+        close(fd);
+        unlink(lock.c_str());
+        return false;
+    }
+
+    if (write(fd, pid.c_str(), pid.length()) == -1)
+    {
+        MXS_ERROR("Failed to write into lock file %s: %s", lock.c_str(), mxs_strerror(errno));
+        close(fd);
+        unlink(lock.c_str());
+        return false;
+    }
+
+    directory_locks.insert(std::pair<std::string, int>(lock, fd));
+
+    return true;
+}
+
+bool lock_directories()
+{
+    std::set<std::string> paths{get_cachedir(), get_datadir()};
+    return std::all_of(paths.begin(), paths.end(), lock_dir);
+}
+
+static void unlock_directories()
+{
+    std::for_each(directory_locks.begin(), directory_locks.end(),
+            [&](std::pair<std::string, int> pair) {
+            close(pair.second);
+            unlink(pair.first.c_str());
+    });
+}
+