log: Add async commit support.

The OVSDB log code has always had the ability to commit the log to disk and
wait for the commit to finish.  This patch introduces a new feature that
allows the client to start a commit in the background and then to determine
asynchronously that the commit has completed.  This will be especially
useful later for the distributed database feature.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>

ovsdb/file.c

@@ -670,7 +670,7 @@ ovsdb_file_compact(struct ovsdb_file *file)
 
     /* Commit the old version, so that we can be assured that we'll eventually
      * have either the old or the new version. */
-    error = ovsdb_log_commit(file->log);
+    error = ovsdb_log_commit_block(file->log);
     if (error) {
         goto exit;
     }
@@ -866,7 +866,7 @@ ovsdb_file_txn_commit(struct json *json, const char *comment,
     }
 
     if (durable) {
-        error = ovsdb_log_commit(log);
+        error = ovsdb_log_commit_block(log);
         if (error) {
             return ovsdb_wrap_error(error, "committing transaction failed");
         }

ovsdb/log.c

@@ -24,12 +24,17 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
+#include "lockfile.h"
 #include "openvswitch/dynamic-string.h"
 #include "openvswitch/json.h"
 #include "openvswitch/vlog.h"
-#include "lockfile.h"
-#include "ovsdb.h"
+#include "ovs-atomic.h"
+#include "ovs-rcu.h"
+#include "ovs-thread.h"
 #include "ovsdb-error.h"
+#include "ovsdb.h"
+#include "openvswitch/poll-loop.h"
+#include "seq.h"
 #include "sha1.h"
 #include "socket-util.h"
 #include "transaction.h"
@@ -78,6 +83,7 @@ struct ovsdb_log {
     struct lockfile *lockfile;
     FILE *stream;
     off_t base;
+    struct afsync *afsync;
 };
 
 /* Whether the OS supports renaming open files.
@@ -95,6 +101,9 @@ static bool parse_header(char *header, const char **magicp,
                          uint8_t sha1[SHA1_DIGEST_SIZE]);
 static bool is_magic_ok(const char *needle, const char *haystack);
 
+static struct afsync *afsync_create(int fd, uint64_t initial_ticket);
+static uint64_t afsync_destroy(struct afsync *);
+
 /* Attempts to open 'name' with the specified 'open_mode'.  On success, stores
  * the new log into '*filep' and returns NULL; otherwise returns NULL and
  * stores NULL into '*filep'.
@@ -269,6 +278,7 @@ ovsdb_log_open(const char *name, const char *magic,
     file->prev_offset = 0;
     file->offset = 0;
     file->base = 0;
+    file->afsync = NULL;
     *filep = file;
     return NULL;
 
@@ -308,6 +318,7 @@ ovsdb_log_close(struct ovsdb_log *file)
 {
     if (file) {
         ovsdb_error_destroy(file->error);
+        afsync_destroy(file->afsync);
         free(file->name);
         free(file->display_name);
         free(file->magic);
@@ -634,8 +645,10 @@ ovsdb_log_write(struct ovsdb_log *file, const struct json *json)
     return NULL;
 }
 
+/* Attempts to commit 'file' to disk.  Waits for the commit to succeed or fail.
+ * Returns NULL if successful, otherwise the error that occurred. */
 struct ovsdb_error *
-ovsdb_log_commit(struct ovsdb_log *file)
+ovsdb_log_commit_block(struct ovsdb_log *file)
 {
     if (file->stream && fsync(fileno(file->stream))) {
         return ovsdb_io_error(errno, "%s: fsync failed", file->display_name);
@@ -740,7 +753,7 @@ ovsdb_rename(const char *old, const char *new)
 struct ovsdb_error * OVS_WARN_UNUSED_RESULT
 ovsdb_log_replace_commit(struct ovsdb_log *old, struct ovsdb_log *new)
 {
-    struct ovsdb_error *error = ovsdb_log_commit(new);
+    struct ovsdb_error *error = ovsdb_log_commit_block(new);
     if (error) {
         ovsdb_log_replace_abort(new);
         return error;
@@ -812,6 +825,10 @@ ovsdb_log_replace_commit(struct ovsdb_log *old, struct ovsdb_log *new)
     ovsdb_error_destroy(old->error);
     old->error = NULL;
     /* prev_offset only matters for OVSDB_LOG_READ. */
+    if (old->afsync) {
+        uint64_t ticket = afsync_destroy(old->afsync);
+        old->afsync = afsync_create(fileno(old->stream), ticket + 1);
+    }
     old->offset = new->offset;
     /* Keep old->name. */
     free(old->magic);
@@ -844,3 +861,130 @@ ovsdb_log_disable_renaming_open_files(void)
 {
     rename_open_files = false;
 }
+
+struct afsync {
+    pthread_t thread;
+    atomic_uint64_t cur, next;
+    struct seq *request, *complete;
+    int fd;
+};
+
+static void *
+afsync_thread(void *afsync_)
+{
+    struct afsync *afsync = afsync_;
+    uint64_t cur = 0;
+    for (;;) {
+        ovsrcu_quiesce_start();
+
+        uint64_t request_seq = seq_read(afsync->request);
+
+        uint64_t next;
+        atomic_read_explicit(&afsync->next, &next, memory_order_acquire);
+        if (next == UINT64_MAX) {
+            break;
+        }
+
+        if (cur != next && afsync->fd != -1) {
+            int error = fsync(afsync->fd) ? errno : 0;
+            if (!error) {
+                cur = next;
+                atomic_store_explicit(&afsync->cur, cur, memory_order_release);
+                seq_change(afsync->complete);
+            } else {
+                VLOG_WARN("fsync failed (%s)", ovs_strerror(error));
+            }
+        }
+
+        seq_wait(afsync->request, request_seq);
+        poll_block();
+    }
+    return NULL;
+}
+
+static struct afsync *
+afsync_create(int fd, uint64_t initial_ticket)
+{
+    struct afsync *afsync = xzalloc(sizeof *afsync);
+    atomic_init(&afsync->cur, initial_ticket);
+    atomic_init(&afsync->next, initial_ticket);
+    afsync->request = seq_create();
+    afsync->complete = seq_create();
+    afsync->thread = ovs_thread_create("log_fsync", afsync_thread, afsync);
+    afsync->fd = fd;
+    return afsync;
+}
+
+static uint64_t
+afsync_destroy(struct afsync *afsync)
+{
+    if (!afsync) {
+        return 0;
+    }
+
+    uint64_t next;
+    atomic_read(&afsync->next, &next);
+    atomic_store(&afsync->next, UINT64_MAX);
+    seq_change(afsync->request);
+    xpthread_join(afsync->thread, NULL);
+
+    seq_destroy(afsync->request);
+    seq_destroy(afsync->complete);
+
+    free(afsync);
+
+    return next;
+}
+
+static struct afsync *
+ovsdb_log_get_afsync(struct ovsdb_log *log)
+{
+    if (!log->afsync) {
+        log->afsync = afsync_create(log->stream ? fileno(log->stream) : -1, 0);
+    }
+    return log->afsync;
+}
+
+/* Starts committing 'log' to disk.  Returns a ticket that can be passed to
+ * ovsdb_log_commit_wait() or compared against the return value of
+ * ovsdb_log_commit_progress() later. */
+uint64_t
+ovsdb_log_commit_start(struct ovsdb_log *log)
+{
+    struct afsync *afsync = ovsdb_log_get_afsync(log);
+
+    uint64_t orig;
+    atomic_add_explicit(&afsync->next, 1, &orig, memory_order_acq_rel);
+
+    seq_change(afsync->request);
+
+    return orig + 1;
+}
+
+/* Returns a ticket value that represents the current progress of commits to
+ * 'log'.  Suppose that some call to ovsdb_log_commit_start() returns X and any
+ * call ovsdb_log_commit_progress() returns Y, for the same 'log'.  Then commit
+ * X is complete if and only if X <= Y. */
+uint64_t
+ovsdb_log_commit_progress(struct ovsdb_log *log)
+{
+    struct afsync *afsync = ovsdb_log_get_afsync(log);
+    uint64_t cur;
+    atomic_read_explicit(&afsync->cur, &cur, memory_order_acquire);
+    return cur;
+}
+
+/* Causes poll_block() to wake up if and when ovsdb_log_commit_progress(log)
+ * would return at least 'goal'. */
+void
+ovsdb_log_commit_wait(struct ovsdb_log *log, uint64_t goal)
+{
+    struct afsync *afsync = ovsdb_log_get_afsync(log);
+    uint64_t complete = seq_read(afsync->complete);
+    uint64_t cur = ovsdb_log_commit_progress(log);
+    if (cur < goal) {
+        seq_wait(afsync->complete, complete);
+    } else {
+        poll_immediate_wake();
+    }
+}

ovsdb/log.h

@@ -35,6 +35,7 @@
  * that compacting is advised.
  */
 
+#include <stdint.h>
 #include <sys/types.h>
 #include "compiler.h"
 
@@ -70,7 +71,11 @@ void ovsdb_log_compose_record(const struct json *, const char *magic,
 
 struct ovsdb_error *ovsdb_log_write(struct ovsdb_log *, const struct json *)
     OVS_WARN_UNUSED_RESULT;
-struct ovsdb_error *ovsdb_log_commit(struct ovsdb_log *)
+
+uint64_t ovsdb_log_commit_start(struct ovsdb_log *);
+uint64_t ovsdb_log_commit_progress(struct ovsdb_log *);
+void ovsdb_log_commit_wait(struct ovsdb_log *, uint64_t);
+struct ovsdb_error *ovsdb_log_commit_block(struct ovsdb_log *)
     OVS_WARN_UNUSED_RESULT;
 
 void ovsdb_log_mark_base(struct ovsdb_log *);

ovsdb/ovsdb-tool.c

@@ -222,7 +222,7 @@ do_create(struct ovs_cmdl_context *ctx)
     check_ovsdb_error(ovsdb_log_open(db_file_name, OVSDB_MAGIC,
                                      OVSDB_LOG_CREATE_EXCL, -1, &log));
     check_ovsdb_error(ovsdb_log_write(log, json));
-    check_ovsdb_error(ovsdb_log_commit(log));
+    check_ovsdb_error(ovsdb_log_commit_block(log));
     ovsdb_log_close(log);
 
     json_destroy(json);

tests/test-ovsdb.c

@@ -380,7 +380,7 @@ do_log_io(struct ovs_cmdl_context *ctx)
             error = ovsdb_log_write(target, json);
             json_destroy(json);
         } else if (!strcmp(command, "commit")) {
-            error = ovsdb_log_commit(target);
+            error = ovsdb_log_commit_block(target);
         } else if (!strcmp(command, "replace_start")) {
             ovs_assert(!replacement);
             error = ovsdb_log_replace_start(log, &replacement);