features/changelog : historical journal consumption.

Facilitates Glusterfs with the ability to detect file-operations
happened in past by scanning the back-end(brick-level) glusterfs
journal (changelog).

Design:
  * List of changelogs produces in one perfectly running session are
    stored in htime file which also holds necessary information about
    the session start and end time.
  * Involves fixed sized seeks to identify N'th changelog in the list.
  * Requires O(log n), (where n is number of changelogs in the list),
    time to identify the end changelog for the given start-end time
    interval.

Currently the background processing of changelogs is sub optimal. BZ
1097041 tracks the development effort.

For complete design, refer the below link:
http://lists.nongnu.org/archive/html/gluster-devel/2014-02/msg00206.html

Change-Id: I27e49f75e492e843084d0ecaf9130224d08462a0
BUG: 1091961
Signed-off-by: Ajeet Jha <ajha@redhat.com>
Signed-off-by: Venky Shankar <vshankar@redhat.com>
Signed-off-by: Ajeet Jha <ajha@redhat.com>
Reviewed-on: http://review.gluster.org/6930
Reviewed-by: Kotresh HR <khiremat@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>

4 files changed, 873 insertions, 14 deletions

diff --git a/xlators/features/changelog/lib/examples/c/get-history.c b/xlators/features/changelog/lib/examples/c/get-history.c
new file mode 100644
index 00000000000..33eb8c32d4d
--- /dev/null
+++ b/xlators/features/changelog/lib/examples/c/get-history.c
@@ -0,0 +1,109 @@
+/*
+   Copyright (c) 2013 Red Hat, Inc. <http://www.redhat.com>
+   This file is part of GlusterFS.
+
+   This file is licensed to you under your choice of the GNU Lesser
+   General Public License, version 3 or any later version (LGPLv3 or
+   later), or the GNU General Public License, version 2 (GPLv2), in all
+   cases as published by the Free Software Foundation.
+*/
+
+/**
+ * get set of new changes every 10 seconds (just print the file names)
+ *
+ * Compile it using:
+ *  gcc -o gethistory `pkg-config --cflags libgfchangelog` get-history.c \
+ *  `pkg-config --libs libgfchangelog`
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/un.h>
+#include <limits.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <errno.h>
+
+#include "changelog.h"
+
+#define handle_error(fn)                                \
+        printf ("%s (reason: %s)\n", fn, strerror (errno))
+
+int
+main (int argc, char ** argv)
+{
+        int     i            = 0;
+        int     ret          = 0;
+        ssize_t nr_changes   = 0;
+        ssize_t changes      = 0;
+        char fbuf[PATH_MAX]  = {0,};
+        unsigned long end_ts = 0;
+
+        ret = gf_changelog_register ("/export1/v1/b1",
+                                     "/tmp/scratch_v1", "/tmp/scratch_v1/changes.log",
+                                     9, 5);
+        if (ret) {
+                handle_error ("register failed");
+                goto out;
+        }
+
+        int a, b;
+        printf ("give the two numbers start and end\t");
+        scanf ("%d%d", &a, &b);
+        ret = gf_history_changelog ("/export1/v1/b1/.glusterfs/changelogs",a, b, 3, &end_ts);
+        if (ret == -1) {
+                printf ("history failed");
+                goto out;
+        }
+
+        printf ("end time till when changelog available : %d , ret(%d) \t", end_ts, ret);
+        fflush(stdout);
+
+        while (1) {
+                nr_changes = gf_history_changelog_scan ();
+                printf ("scanned, nr_changes : %d\n",nr_changes);
+                if (nr_changes < 0) {
+                        handle_error ("scan(): ");
+                        break;
+                }
+
+                if (nr_changes == 0) {
+                        printf ("done scanning \n");
+                        goto out;
+                }
+
+                printf ("Got %ld changelog files\n", nr_changes);
+
+                while ( (changes =
+                         gf_history_changelog_next_change (fbuf, PATH_MAX)) > 0) {
+                        printf ("changelog file [%d]: %s\n", ++i, fbuf);
+
+                        /* process changelog */
+                        /* ... */
+                        /* ... */
+                        /* ... */
+                        /* done processing */
+
+                        ret = gf_history_changelog_done (fbuf);
+                        if (ret)
+                                handle_error ("gf_changelog_done");
+                }
+                /*
+                if (changes == -1)
+                        handle_error ("gf_changelog_next_change");
+                if (nr_changes ==1){
+                        printf("continue scanning\n");
+                }
+
+                if(nr_changes == 0){
+                        printf("done scanning \n");
+                        goto out;
+                }
+                */
+        }
+
+
+out:
+        return ret;
+}
diff --git a/xlators/features/changelog/lib/src/gf-changelog-helpers.h b/xlators/features/changelog/lib/src/gf-changelog-helpers.h
index 2d545da9e82..218896b86e7 100644
--- a/xlators/features/changelog/lib/src/gf-changelog-helpers.h
+++ b/xlators/features/changelog/lib/src/gf-changelog-helpers.h
@@ -24,6 +24,7 @@
 #define GF_CHANGELOG_PROCESSED_DIR  ".processed"
 #define GF_CHANGELOG_PROCESSING_DIR ".processing"
 #define GF_CHANGELOG_HISTORY_DIR    ".history"
+#define TIMESTAMP_LENGTH 10
 
 #ifndef MAXLINE
 #define MAXLINE 4096
@@ -72,8 +73,45 @@ typedef struct gf_changelog {
 
         /* Holds gfc for History API */
         struct gf_changelog *hist_gfc;
+
+        /* holds 0 done scanning, 1 keep scanning and -1 error */
+        int hist_done;
 } gf_changelog_t;
 
+typedef struct gf_changelog_history_data {
+        int           len;
+
+        int           htime_fd;
+
+        /* parallelism count */
+        int           n_parallel;
+
+        /* history from, to indexes */
+        unsigned long from;
+        unsigned long to;
+} gf_changelog_history_data_t;
+
+typedef struct gf_changelog_consume_data {
+        /** set of inputs */
+
+        /* fd to read from */
+        int             fd;
+
+        /* from @offset */
+        off_t           offset;
+
+        xlator_t       *this;
+        gf_changelog_t *gfc;
+
+        /** set of outputs */
+
+        /* return value */
+        int retval;
+
+        /* journal processed */
+        char changelog[PATH_MAX];
+} gf_changelog_consume_data_t;
+
 int
 gf_changelog_notification_init (xlator_t *this, gf_changelog_t *gfc);
 
@@ -98,4 +136,11 @@ gf_ftruncate (int fd, off_t length);
 off_t
 gf_lseek (int fd, off_t offset, int whence);
 
+int
+gf_changelog_consume (xlator_t *this,
+                      gf_changelog_t *gfc,
+                      char *from_path, gf_boolean_t no_publish);
+int
+gf_changelog_publish (xlator_t *this, gf_changelog_t *gfc, char *from_path);
+
 #endif
diff --git a/xlators/features/changelog/lib/src/gf-changelog-process.c b/xlators/features/changelog/lib/src/gf-changelog-process.c
index 3ea2700c62b..3b8d2683672 100644
--- a/xlators/features/changelog/lib/src/gf-changelog-process.c
+++ b/xlators/features/changelog/lib/src/gf-changelog-process.c
@@ -459,8 +459,49 @@ gf_changelog_decode (xlator_t *this, gf_changelog_t *gfc, int from_fd,
         return ret;
 }
 
-static int
-gf_changelog_consume (xlator_t *this, gf_changelog_t *gfc, char *from_path)
+int
+gf_changelog_publish (xlator_t *this, gf_changelog_t *gfc, char *from_path)
+{
+        int         ret        = 0;
+        char dest[PATH_MAX]    = {0,};
+        char to_path[PATH_MAX] = {0,};
+        struct stat stbuf      = {0,};
+
+        (void) snprintf (to_path, PATH_MAX, "%s%s",
+                         gfc->gfc_current_dir, basename (from_path));
+
+        /* handle zerob file that wont exist in current */
+        ret = stat (from_path, &stbuf);
+        if (ret)
+                goto out;
+
+        if (stbuf.st_size == 0) {
+                ret = unlink (from_path);
+                if (ret)
+                        gf_log (this->name, GF_LOG_ERROR,
+                                "could not unlink %s (reason %s)",
+                                from_path, strerror (errno));
+                goto out;
+        }
+
+        (void) snprintf (dest, PATH_MAX, "%s%s",
+                         gfc->gfc_processing_dir, basename (from_path));
+
+        ret = rename (to_path, dest);
+        if (ret){
+                gf_log (this->name, GF_LOG_ERROR,
+                        "error moving %s to processing dir"
+                        " (reason: %s)", to_path, strerror (errno));
+        }
+
+out:
+        return ret;
+}
+
+int
+gf_changelog_consume (xlator_t *this,
+                      gf_changelog_t *gfc,
+                      char *from_path, gf_boolean_t no_publish)
 {
         int         ret        = -1;
         int         fd1        = 0;
@@ -472,6 +513,7 @@ gf_changelog_consume (xlator_t *this, gf_changelog_t *gfc, char *from_path)
 
         ret = stat (from_path, &stbuf);
         if (ret || !S_ISREG(stbuf.st_mode)) {
+                ret = -1;
                 gf_log (this->name, GF_LOG_ERROR,
                         "stat failed on changelog file: %s", from_path);
                 goto out;
@@ -506,6 +548,8 @@ gf_changelog_consume (xlator_t *this, gf_changelog_t *gfc, char *from_path)
                 if (!ret) {
                         /* move it to processing on a successfull
                            decode */
+                        if (no_publish == _gf_true)
+                                goto close_fd;
                         ret = rename (to_path, dest);
                         if (ret)
                                 gf_log (this->name, GF_LOG_ERROR,
@@ -516,10 +560,15 @@ gf_changelog_consume (xlator_t *this, gf_changelog_t *gfc, char *from_path)
 
                 /* remove it from .current if it's an empty file */
                 if (zerob) {
+                        if (no_publish == _gf_true) {
+                                ret = 0;
+                                goto close_fd;
+                        }
+
                         ret = unlink (to_path);
                         if (ret)
                                 gf_log (this->name, GF_LOG_ERROR,
-                                        "could not unlink %s (reason: %s",
+                                        "could not unlink %s (reason: %s)",
                                         to_path, strerror (errno));
                 }
         }
@@ -546,7 +595,7 @@ gf_changelog_ext_change (xlator_t *this,
                         alo = 1;
                         gf_log (this->name, GF_LOG_DEBUG,
                                 "processing changelog: %s", path);
-                        ret = gf_changelog_consume (this, gfc, path);
+                        ret = gf_changelog_consume (this, gfc, path, _gf_false);
                 }
 
                 if (ret)
diff --git a/xlators/features/changelog/lib/src/gf-history-changelog.c b/xlators/features/changelog/lib/src/gf-history-changelog.c
index bfc4cd37dc3..a895037eeca 100644
--- a/xlators/features/changelog/lib/src/gf-history-changelog.c
+++ b/xlators/features/changelog/lib/src/gf-history-changelog.c
@@ -11,6 +11,7 @@
 #include "globals.h"
 #include "glusterfs.h"
 #include "logging.h"
+#include "syscall.h"
 
 #include "gf-changelog-helpers.h"
 
@@ -18,7 +19,8 @@
 #include "changelog-misc.h"
 #include "changelog-mem-types.h"
 
-/*@API
+/**
+ * @API
  * gf_history_changelog_done:
  *    Move processed history changelog file from .processing
  *    to .processed
@@ -86,6 +88,7 @@ gf_history_changelog_done (char *file)
                 free (buffer); /* allocated by realpath() */
         return ret;
 }
+
 /**
  * @API
  *  gf_history_changelog_start_fresh:
@@ -126,7 +129,7 @@ gf_history_changelog_start_fresh ()
         return -1;
 }
 
-/*
+/**
  * @API
  *  gf_history_changelog_next_change:
  *     Return the next history changelog file entry. Zero means all
@@ -182,7 +185,7 @@ gf_history_changelog_next_change (char *bufptr, size_t maxlen)
         return -1;
 }
 
-/*
+/**
  * @API
  *  gf_history_changelog_scan:
  *     Scan and generate a list of change entries.
@@ -191,8 +194,13 @@ gf_history_changelog_next_change (char *bufptr, size_t maxlen)
  *     This call also acts as a cancellation point for the consumer.
  *
  *  RETURN VALUES:
- *     nr_entries: On success.
- *     -1        : On error.
+ *      +ve integer : success and keep scanning.(count of changelogs)
+ *      0           : success and done scanning.
+ *     -1           : error.
+ *
+ *  NOTE: After first 0 return call_get_next change for once more time
+ *        to empty the tracker
+ *
  */
 ssize_t
 gf_history_changelog_scan ()
@@ -204,10 +212,11 @@ gf_history_changelog_scan ()
         xlator_t       *this       = NULL;
         size_t          nr_entries = 0;
         gf_changelog_t *gfc        = NULL;
-        gf_changelog_t *hist_gfc        = NULL;
+        gf_changelog_t *hist_gfc   = NULL;
         struct dirent  *entryp     = NULL;
         struct dirent  *result     = NULL;
         char buffer[PATH_MAX]      = {0,};
+        static int    is_last_scan = 0;
 
         this = THIS;
         if (!this)
@@ -221,15 +230,23 @@ gf_history_changelog_scan ()
         if (!hist_gfc)
                 goto out;
 
+ retry:
+        if (is_last_scan == 1)
+                return 0;
+        if (hist_gfc->hist_done == 0)
+                is_last_scan = 1;
+
         errno = EINVAL;
+        if (hist_gfc->hist_done == -1)
+                goto out;
 
         tracker_fd = hist_gfc->gfc_fd;
 
         if (gf_ftruncate (tracker_fd, 0))
                 goto out;
 
-        len = offsetof(struct dirent, d_name)
-                + pathconf(hist_gfc->gfc_processing_dir, _PC_NAME_MAX) + 1;
+        len = offsetof (struct dirent, d_name)
+                + pathconf (hist_gfc->gfc_processing_dir, _PC_NAME_MAX) + 1;
         entryp = GF_CALLOC (1, len,
                             gf_changelog_mt_libgfchangelog_dirent_t);
         if (!entryp)
@@ -265,10 +282,649 @@ gf_history_changelog_scan ()
 
         GF_FREE (entryp);
 
+        gf_log (this->name, GF_LOG_DEBUG,
+                "hist_done %d, is_last_scan: %d", hist_gfc->hist_done, is_last_scan);
+
         if (!result) {
-                if (gf_lseek (tracker_fd, 0, SEEK_SET) != -1)
-                        return nr_entries;
+                if (gf_lseek (tracker_fd, 0, SEEK_SET) != -1) {
+                        if (nr_entries > 0)
+                                return nr_entries;
+                        else {
+                                sleep(1);
+                                goto retry;
+                        }
+                }
         }
  out:
         return -1;
 }
+
+/*
+ * Gets timestamp value at the changelog path at index.
+ * Returns 0 on success(updates given time-stamp), -1 on failure.
+ */
+int
+gf_history_get_timestamp (int fd, int index, int len,
+                          unsigned long *ts)
+{
+        xlator_t        *this             = NULL;
+        int             n_read            = -1;
+        char            path_buf[PATH_MAX]= {0,};
+        char            *iter             = path_buf;
+        size_t          offset            = index * (len+1);
+        unsigned long   value             = 0;
+        int             ret               = 0;
+
+        this = THIS;
+        if (!this) {
+                return -1;
+        }
+
+        n_read = pread (fd, path_buf, len, offset);
+        if (n_read < 0 ) {
+                ret = -1;
+                gf_log ( this->name, GF_LOG_ERROR,
+                         "could not read from htime file");
+                goto out;
+        }
+        iter+= len - TIMESTAMP_LENGTH;
+        sscanf (iter, "%lu",&value);
+out:
+        if(ret == 0)
+                *ts = value;
+        return ret;
+}
+
+/*
+ * Function to ensure correctness of search
+ * Checks whether @value is there next to @target_index or not
+ */
+int
+gf_history_check ( int fd, int target_index, unsigned long value, int len)
+{
+        int             ret = 0;
+        unsigned long   ts1 = 0;
+        unsigned long   ts2 = 0;
+
+        if (target_index == 0) {
+                ret = gf_history_get_timestamp (fd, target_index, len, &ts1);
+                if (ret == -1)
+                        goto out;
+                if (value <= ts1)
+                        goto out;
+                else {
+                        ret = -1;
+                        goto out;
+                }
+        }
+
+        ret = gf_history_get_timestamp (fd, target_index, len, &ts1);
+        if (ret ==-1)
+                goto out;
+        ret = gf_history_get_timestamp (fd, target_index -1, len, &ts2);
+        if (ret ==-1)
+                goto out;
+
+        if ( (value <= ts1) && (value > ts2) ) {
+                goto out;
+        }
+        else
+                ret = -1;
+out:
+        return ret;
+}
+
+/*
+ * This is a "binary search" based search function which checks neighbours
+ * for in-range availability of the value to be searched and provides the
+ * index at which the changelog file nearest to the requested timestamp(value)
+ * can be read from.
+ *
+ * Actual offset can be calculated as (index* (len+1) ).
+ * "1" is because the changelog paths are null terminated.
+ *
+ * @path        : Htime file to search in
+ * @value       : time stamp to search
+ * @from        : start index to search
+ * @to          : end index to search
+ * @len         : length of fixes length strings seperated by null
+ */
+
+int
+gf_history_b_search (int fd, unsigned long value,
+                     unsigned long from, unsigned long to, int len)
+{
+        int             m_index   = -1;
+        unsigned long   cur_value = 0;
+        unsigned long   ts1       = 0;
+        int             ret       = 0;
+
+        m_index = (from + to)/2;
+
+        if ( (to - from) <=1 ) {
+                /* either one or 2 changelogs left */
+                if ( to != from ) {
+                        /* check if value is less or greater than to
+                         * return accordingly
+                         */
+                        ret = gf_history_get_timestamp (fd, from, len, &ts1);
+                        if (ret ==-1)
+                                goto out;
+                        if ( ts1 >= value) {
+                                /* actually compatision should be
+                                 * exactly == but considering
+                                 *
+                                 * case of only 2 changelogs in htime file
+                                 */
+                                return from;
+                        }
+                        else
+                                return to;
+                }
+                else
+                        return to;
+        }
+
+        ret = gf_history_get_timestamp (fd, m_index, len, &cur_value);
+        if (ret == -1)
+                goto out;
+        if (cur_value == value) {
+                return m_index;
+        }
+        else if (value > cur_value) {
+                ret = gf_history_get_timestamp (fd, m_index+1, len, &cur_value);
+                if (ret == -1)
+                        goto out;
+                if (value < cur_value)
+                        return m_index + 1;
+                else
+                        return gf_history_b_search (fd, value,
+                                                    m_index+1, to, len);
+        }
+        else {
+                if (m_index ==0) {
+                       /*  we are sure that values exists
+                        *  in this htime file
+                        */
+                        return 0;
+                }
+                else {
+                        ret = gf_history_get_timestamp (fd, m_index-1, len,
+                                                        &cur_value);
+                        if (ret == -1)
+                                goto out;
+                        if (value > cur_value) {
+                                return m_index;
+                        }
+                        else
+                                return gf_history_b_search (fd, value, from,
+                                                            m_index-1, len);
+                }
+        }
+out:
+        return -1;
+}
+
+void *
+gf_changelog_consume_wrap (void* data)
+{
+        int                          ret   = -1;
+        ssize_t                      nread = 0;
+        xlator_t                    *this  = NULL;
+        gf_changelog_consume_data_t *ccd   = NULL;
+
+        ccd = (gf_changelog_consume_data_t *) data;
+        this = ccd->this;
+
+        ccd->retval = -1;
+
+        nread = pread (ccd->fd, ccd->changelog, PATH_MAX, ccd->offset);
+        if (nread < 0) {
+                gf_log (this->name, GF_LOG_ERROR,
+                        "cannot read from history metadata file (reason %s)",
+                        strerror (errno));
+                goto out;
+        }
+
+        /* TODO: handle short reads and EOF. */
+
+        ret = gf_changelog_consume (ccd->this,
+                                    ccd->gfc, ccd->changelog, _gf_true);
+        if (ret) {
+                gf_log (this->name, GF_LOG_ERROR,
+                        "could not parse changelog: %s", ccd->changelog);
+                goto out;
+        }
+
+        ccd->retval = 0;
+
+ out:
+        return NULL;
+}
+
+/**
+ * "gf_history_consume" is a worker function for history.
+ * parses and moves changelogs files from index "from"
+ * to index "to" in open htime file whose fd is "fd".
+ */
+
+#define MAX_PARALLELS  10
+
+void *
+gf_history_consume (void * data)
+{
+        xlator_t                    *this              = NULL;
+        gf_changelog_t              *gfc               = NULL;
+        gf_changelog_t              *hist_gfc          = NULL;
+        int                          ret               = 0;
+        int                          iter              = 0;
+        int                          fd                = -1;
+        int                          from              = -1;
+        int                          to                = -1;
+        int                          len               = -1;
+        int                          n_parallel        = 0;
+        int                          n_envoked         = 0;
+        gf_boolean_t                 publish           = _gf_true;
+        pthread_t th_id[MAX_PARALLELS]                 = {0,};
+        gf_changelog_history_data_t *hist_data         = NULL;
+        gf_changelog_consume_data_t ccd[MAX_PARALLELS] = {{0},};
+        gf_changelog_consume_data_t *curr              = NULL;
+
+        hist_data = (gf_changelog_history_data_t *) data;
+        if (hist_data == NULL) {
+                ret = -1;
+                goto out;
+        }
+
+        fd         = hist_data->htime_fd;
+        from       = hist_data->from;
+        to         = hist_data->to;
+        len        = hist_data->len;
+        n_parallel = hist_data->n_parallel;
+
+        this = THIS;
+        if (!this) {
+                ret = -1;
+                goto out;
+        }
+
+        gfc = (gf_changelog_t *) this->private;
+        if (!gfc) {
+                ret = -1;
+                goto out;
+        }
+
+        hist_gfc = gfc->hist_gfc;
+        if (!hist_gfc) {
+                ret = -1;
+                goto out;
+        }
+
+        while (from <= to) {
+                n_envoked = 0;
+
+                for (iter = 0 ; (iter < n_parallel) && (from <= to); iter++) {
+                        curr = &ccd[iter];
+
+                        curr->this   = this;
+                        curr->gfc    = hist_gfc;
+                        curr->fd     = fd;
+                        curr->offset = from * (len + 1);
+
+                        curr->retval = 0;
+                        memset (curr->changelog, '\0', PATH_MAX);
+
+                        ret = pthread_create (&th_id[iter], NULL,
+                                              gf_changelog_consume_wrap, curr);
+                        if (ret) {
+                                gf_log ( this->name, GF_LOG_ERROR,
+                                        "could not create consume-thread"
+                                        " reason (%s)", strerror (ret));
+                                ret = -1;
+                                goto sync;
+                        } else
+                                n_envoked++;
+
+                        from++;
+                }
+
+        sync:
+                for (iter = 0; iter < n_envoked; iter++) {
+                        ret = pthread_join (th_id[iter], NULL);
+                        if (ret) {
+                                publish = _gf_false;
+                                gf_log (this->name, GF_LOG_ERROR,
+                                        "pthread_join() error %s",
+                                        strerror (ret));
+                                /* try to join the rest */
+                                continue;
+                        }
+
+                        if (publish == _gf_false)
+                                continue;
+
+                        curr = &ccd[iter];
+                        if (ccd->retval) {
+                                publish = _gf_false;
+                                gf_log (this->name, GF_LOG_ERROR,
+                                        "parsing error, ceased publishing...");
+                                continue;
+                        }
+
+                        ret = gf_changelog_publish (curr->this,
+                                                    curr->gfc, curr->changelog);
+                        if (ret) {
+                                publish = _gf_false;
+                                gf_log (this->name, GF_LOG_ERROR,
+                                        "publish error, ceased publishing...");
+                        }
+                }
+        }
+
+       /* informing "parsing done". */
+        hist_gfc->hist_done = (publish == _gf_true) ? 0 : -1;
+
+out:
+        if (fd != -1)
+                close (fd);
+        GF_FREE (hist_data);
+        return NULL;
+}
+
+/**
+ * @API
+ * gf_history_changelog() : Get/parse historical changelogs and get them ready
+ * for consumption.
+ *
+ * Arguments:
+ * @changelog_dir : Directory location from where history changelogs are
+ * supposed to be consumed.
+ * @start: Unix timestamp FROM where changelogs should be consumed.
+ * @end: Unix timestamp TO where changelogsshould be consumed.
+ * @n_parallel : degree of parallelism while changelog parsing.
+ * @actual_end : the end time till where changelogs are available.
+ *
+ * Return:
+ * Returns <timestamp> on success, the last time till where changelogs are
+ *      available.
+ * Returns -1 on failure(error).
+ */
+
+#define MAKE_HTIME_FILE_PATH(htime_file, htime_dir, htime_bname) do {   \
+                strcpy (htime_file, htime_dir);                         \
+                strcat (htime_file, "/");                               \
+                strcat (htime_file, htime_bname);                       \
+        } while (0)
+
+/**
+ * Extract timestamp range from a historical metadata file
+ * Returns:
+ *    0 : Success ({min,max}_ts with the appropriate values)
+ *   -1 : Failure
+ *   -2 : Ignore this metadata file and process next
+ */
+int
+gf_changelog_extract_min_max (const char *dname, const char *htime_dir,
+                              int *fd, unsigned long *total,
+                              unsigned long *min_ts, unsigned long *max_ts)
+{
+        int          ret          = -1;
+        xlator_t    *this         = NULL;
+        char htime_file[PATH_MAX] = {0,};
+        struct stat  stbuf        = {0,};
+        char        *iter         = NULL;
+        char x_value[30]          = {0,};
+
+        this = THIS;
+
+        MAKE_HTIME_FILE_PATH (htime_file, htime_dir, dname);
+
+        iter = (htime_file + strlen (htime_file) - TIMESTAMP_LENGTH);
+        sscanf (iter ,"%lu",min_ts);
+
+        ret = stat (htime_file, &stbuf);
+        if (ret) {
+                ret = -1;
+                gf_log (this->name, GF_LOG_ERROR,
+                        "stat() failed on htime file %s (reason %s)",
+                        htime_file, strerror (errno));
+                goto out;
+        }
+
+        /* ignore everything except regular files */
+        if (!S_ISREG (stbuf.st_mode)) {
+                ret = -2;
+                goto out;
+        }
+
+        *fd = open (htime_file, O_RDONLY);
+        if (*fd < 0) {
+                ret = -1;
+                gf_log (this->name, GF_LOG_ERROR,
+                        "open() failed for htime %s (reasong %s)",
+                        htime_file, strerror (errno));
+                goto out;
+        }
+
+        /* Looks good, extract max timestamp */
+        ret = sys_fgetxattr (*fd, HTIME_KEY, x_value, sizeof (x_value));
+        if (ret < 0) {
+                ret = -1;
+                gf_log (this->name, GF_LOG_ERROR,
+                        "error extracting max timstamp from htime file"
+                        " %s (reason %s)", htime_file, strerror (errno));
+                goto out;
+        }
+
+        sscanf (x_value, "%lu:%lu", max_ts, total);
+        gf_log (this->name, GF_LOG_INFO,
+                "MIN: %lu, MAX: %lu, TOTAL CHANGELOGS: %lu",
+                *min_ts, *max_ts, *total);
+
+        ret = 0;
+
+ out:
+        return ret;
+}
+
+int
+gf_history_changelog (char* changelog_dir, unsigned long start,
+                      unsigned long end, int n_parallel,
+                      unsigned long *actual_end)
+{
+        int                          ret             = 0;
+        int                          len             = -1;
+        int                          fd              = -1;
+        int                          n_read          = -1;
+        unsigned long                min_ts          = 0;
+        unsigned long                max_ts          = 0;
+        unsigned long                end2            = 0;
+        unsigned long                ts1             = 0;
+        unsigned long                ts2             = 0;
+        unsigned long                to              = 0;
+        unsigned long                from            = 0;
+        unsigned long                total_changelog = 0;
+        xlator_t                    *this            = NULL;
+        gf_changelog_t              *gfc             = NULL;
+        gf_changelog_t              *hist_gfc        = NULL;
+        gf_changelog_history_data_t *hist_data       = NULL;
+        DIR                         *dirp            = NULL;
+        struct dirent               *dp              = NULL;
+        pthread_t                    consume_th      = 0;
+        char            htime_dir[PATH_MAX]          = {0,};
+        char buffer[PATH_MAX]                        = {0,};
+
+        pthread_attr_t attr;
+
+        this = THIS;
+        if (!this) {
+                ret = -1;
+                goto out;
+        }
+
+        gfc = (gf_changelog_t *) this->private;
+        if (!gfc) {
+                ret = -1;
+                goto out;
+        }
+
+        hist_gfc = (gf_changelog_t *) gfc->hist_gfc;
+        if (!hist_gfc) {
+                ret = -1;
+                goto out;
+        }
+
+        /* basic sanity check */
+        if (start > end || n_parallel <= 0) {
+                ret = -1;
+                goto out;
+        }
+
+        /* cap parallelism count */
+        if (n_parallel > MAX_PARALLELS)
+                n_parallel = MAX_PARALLELS;
+
+        CHANGELOG_FILL_HTIME_DIR (changelog_dir, htime_dir);
+
+        dirp = opendir (htime_dir);
+        if (dirp == NULL) {
+                gf_log (this->name, GF_LOG_ERROR,
+                        "open dir on htime failed : %s (reason: %s)",
+                        htime_dir, strerror (errno));
+                ret = -1;
+                goto out;
+        }
+
+        while ((dp = readdir (dirp)) != NULL) {
+                ret = gf_changelog_extract_min_max (dp->d_name, htime_dir,
+                                                    &fd, &total_changelog,
+                                                    &min_ts, &max_ts);
+                if (ret) {
+                        if (-2 == ret)
+                                continue;
+                        goto out;
+                }
+
+                if (start >= min_ts && start < max_ts) {
+                        /**
+                         * TODO: handle short reads later...
+                         */
+                        n_read = read (fd, buffer, PATH_MAX);
+                        if (n_read < 0) {
+                                ret = -1;
+                                gf_log ( this->name, GF_LOG_ERROR,
+                                        "unable to read htime file");
+                                goto out;
+                        }
+
+                        len = strlen (buffer);
+
+                        /**
+                         * search @start in the htime file returning it's index
+                         * (@from)
+                         */
+                        from = gf_history_b_search (fd, start, 0,
+                                                   total_changelog - 1, len);
+
+                        /* ensuring correctness of gf_b_search */
+                        if (gf_history_check (fd, from, start, len) != 0) {
+                                ret = -1;
+                                gf_log (this->name, GF_LOG_ERROR,
+                                        "wrong result for start: %lu idx: %lu",
+                                        start, from);
+                                goto out;
+                        }
+
+                        end2 = (end <= max_ts) ? end : max_ts;
+
+                        /**
+                         * search @end2 in htime file returning it's index (@to)
+                         */
+                        to = gf_history_b_search (fd, end2,
+                                                  0, total_changelog - 1, len);
+
+                        if (gf_history_check (fd, to, end2, len) != 0) {
+                                ret = -1;
+                                gf_log (this->name, GF_LOG_ERROR,
+                                        "wrong result for start: %lu idx: %lu",
+                                        end2, to);
+                                goto out;
+                        }
+
+                        ret = gf_history_get_timestamp (fd, from, len, &ts1);
+                        if (ret == -1)
+                                goto out;
+
+                        ret = gf_history_get_timestamp (fd, to, len, &ts2);
+                        if (ret == -1)
+                                goto out;
+
+                        gf_log (this->name, GF_LOG_INFO,
+                                "FINAL: from: %lu, to: %lu, changes: %lu",
+                                ts1, ts2, (to - from + 1));
+
+                        hist_data =  GF_CALLOC (1,
+                                     sizeof (gf_changelog_history_data_t),
+                                     gf_changelog_mt_history_data_t);
+
+                        hist_data->htime_fd   = fd;
+                        hist_data->from       = from;
+                        hist_data->to         = to;
+                        hist_data->len        = len;
+                        hist_data->n_parallel = n_parallel;
+
+                        ret = pthread_attr_init (&attr);
+                        if (ret != 0) {
+                                ret = -1;
+                                goto out;
+                        }
+
+                        ret = pthread_attr_setdetachstate
+                                (&attr, PTHREAD_CREATE_DETACHED);
+                        if (ret != 0) {
+                                gf_log (this->name, GF_LOG_ERROR,
+                                        "unable to sets the detach"
+                                        " state attribute, reason(%s)",
+                                        strerror (ret));
+                                ret = -1;
+                                goto out;
+                        }
+
+                        /* spawn a thread for background parsing & publishing */
+                        ret = pthread_create (&consume_th, &attr,
+                                              gf_history_consume, hist_data);
+                        if (ret) {
+                                gf_log (this->name, GF_LOG_ERROR,
+                                        "creation of consume parent-thread"
+                                        " failed. reason(%s)", strerror (ret));
+                                ret = -1;
+                                goto out;
+                        }
+
+                        goto out;
+
+                } /* end of range check */
+
+        } /* end of readdir() */
+
+        if (!from || !to)
+                ret = -1;
+
+out:
+        if (dirp != NULL)
+                closedir (dirp);
+
+        if (ret < 0) {
+                if (fd != -1)
+                        close (fd);
+                GF_FREE (hist_data);
+                (void) pthread_attr_destroy (&attr);
+
+                return ret;
+        }
+
+        hist_gfc->hist_done = 1;
+        *actual_end = ts2;
+
+        return ret;
+}