[ovs-dev,08/13] log: New function ovsdb_log_compose_record().

diff --git a/ovsdb/log.c b/ovsdb/log.c
index 22cd9fe89599..1769e12266d1 100644
--- a/ovsdb/log.c
+++ b/ovsdb/log.c
@@ -24,6 +24,7 @@ 
 #include <sys/stat.h>
 #include <unistd.h>
 
+#include "openvswitch/dynamic-string.h"
 #include "openvswitch/json.h"
 #include "openvswitch/vlog.h"
 #include "lockfile.h"
@@ -419,18 +420,32 @@  ovsdb_log_unread(struct ovsdb_log *file)
     file->offset = file->prev_offset;
 }
 
+void
+ovsdb_log_compose_record(const struct json *json,
+                         const char *magic, struct ds *header, struct ds *data)
+{
+    ovs_assert(json->type == JSON_OBJECT || json->type == JSON_ARRAY);
+    ovs_assert(!header->length);
+    ovs_assert(!data->length);
+
+    /* Compose content.  Add a new-line (replacing the null terminator) to make
+     * the file easier to read, even though it has no semantic value.  */
+    json_to_ds(json, 0, data);
+    ds_put_char(data, '\n');
+
+    /* Compose header. */
+    uint8_t sha1[SHA1_DIGEST_SIZE];
+    sha1_bytes(data->string, data->length, sha1);
+    ds_put_format(header, "%s %"PRIuSIZE" "SHA1_FMT"\n",
+                  magic, data->length, SHA1_ARGS(sha1));
+}
+
 /* Writes log record 'json' to 'file'.  Returns NULL if successful or an error
  * (which the caller must eventually destroy) on failure. */
 struct ovsdb_error *
 ovsdb_log_write(struct ovsdb_log *file, const struct json *json)
 {
-    uint8_t sha1[SHA1_DIGEST_SIZE];
     struct ovsdb_error *error;
-    char *json_string;
-    char header[128];
-    size_t length;
-
-    json_string = NULL;
 
     switch (file->state) {
     case OVSDB_LOG_WRITE:
@@ -463,22 +478,18 @@  ovsdb_log_write(struct ovsdb_log *file, const struct json *json)
         goto error;
     }
 
-    /* Compose content.  Add a new-line (replacing the null terminator) to make
-     * the file easier to read, even though it has no semantic value.  */
-    json_string = json_to_string(json, 0);
-    length = strlen(json_string) + 1;
-    json_string[length - 1] = '\n';
-
-    /* Compose header. */
-    sha1_bytes(json_string, length, sha1);
-    snprintf(header, sizeof header, "%s %"PRIuSIZE" "SHA1_FMT"\n",
-             file->magic, length, SHA1_ARGS(sha1));
+    struct ds header = DS_EMPTY_INITIALIZER;
+    struct ds data = DS_EMPTY_INITIALIZER;
+    ovsdb_log_compose_record(json, file->magic, &header, &data);
+    size_t total_length = header.length + data.length;
 
     /* Write. */
-    if (fwrite(header, strlen(header), 1, file->stream) != 1
-        || fwrite(json_string, length, 1, file->stream) != 1
-        || fflush(file->stream))
-    {
+    bool ok = (fwrite(header.string, header.length, 1, file->stream) == 1
+               && fwrite(data.string, data.length, 1, file->stream) == 1
+               && fflush(file->stream) == 0);
+    ds_destroy(&header);
+    ds_destroy(&data);
+    if (!ok) {
         error = ovsdb_io_error(errno, "%s: write failed", file->name);
 
         static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(5, 5);
@@ -492,14 +503,12 @@  ovsdb_log_write(struct ovsdb_log *file, const struct json *json)
         goto error;
     }
 
-    file->offset += strlen(header) + length;
-    free(json_string);
+    file->offset += total_length;
     return NULL;
 
 error:
     file->state = OVSDB_LOG_WRITE_ERROR;
     file->error = ovsdb_error_clone(error);
-    free(json_string);
     return error;
 }
 
diff --git a/ovsdb/log.h b/ovsdb/log.h
index 41734e73b351..cc8469c01e57 100644
--- a/ovsdb/log.h
+++ b/ovsdb/log.h
@@ -19,6 +19,7 @@ 
 #include <sys/types.h>
 #include "compiler.h"
 
+struct ds;
 struct json;
 struct ovsdb_log;
 
@@ -42,6 +43,9 @@  struct ovsdb_error *ovsdb_log_read(struct ovsdb_log *, struct json **)
     OVS_WARN_UNUSED_RESULT;
 void ovsdb_log_unread(struct ovsdb_log *);
 
+void ovsdb_log_compose_record(const struct json *, const char *magic,
+                              struct ds *header, struct ds *data);
+
 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 *)