src/el-file.c: create symlink file to most recent log file

embedlog creates rotated log files from 0 (oldest) to N (newest),
this leads to situation when name of the most recent log file is
not deterministic (it may be log.1, and some time later log.2).

For this option EL_FROTATE_SYMLINK has been added, and when this
is enabled (default) embedlog will always keep symlink with file
basename to most recent log file like:

  log.0
  log.1
  log.3
  log -> log.3

Signed-off-by: Michał Łyszczek <michal.lyszczek@bofc.pl>

4 files changed, 64 insertions, 3 deletions

diff --git a/configure.ac b/configure.ac
index 9d1a00c..e27ded9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -107,7 +107,7 @@ AS_IF([test "x$enable_out_file" = "xyes"],
 [
     AC_DEFINE([ENABLE_OUT_FILE], [1], [Enable printing to file])
     ENABLE_OUT_FILE=1
-    AC_CHECK_FUNCS(access stat fsync fileno)
+    AC_CHECK_FUNCS(access stat fsync fileno symlink)
 ])
 
 AC_SUBST(ENABLE_OUT_FILE)
diff --git a/include/embedlog.h.in b/include/embedlog.h.in
index bd91415..575094e 100644
--- a/include/embedlog.h.in
+++ b/include/embedlog.h.in
@@ -118,6 +118,7 @@ enum el_option
     EL_FPATH          = 13,
     EL_FROTATE_NUMBER = 14,
     EL_FROTATE_SIZE   = 15,
+    EL_FROTATE_SYMLINK= 19,
     EL_FSYNC_EVERY    = 16,
     EL_FSYNC_LEVEL    = 17,
     EL_FILE_SYNC_EVERY= 16,  /* deprecated */
@@ -130,7 +131,7 @@ enum el_option
      * options you add).
      */
 
-    EL_OPT_ERROR      = 19
+    EL_OPT_ERROR      = 20
 };
 
 enum el_option_timestamp
@@ -209,6 +210,7 @@ struct el
 
 #if @ENABLE_OUT_FILE@
     unsigned int    fsync_level:3;
+    unsigned int    frotate_symlink:1;
     unsigned int    frotate_number;
     unsigned int    fcurrent_rotate;
     unsigned long   frotate_size;
diff --git a/src/el-file.c b/src/el-file.c
index 675b74f..21a8014 100644
--- a/src/el-file.c
+++ b/src/el-file.c
@@ -36,6 +36,7 @@
 
 
 #include "el-private.h"
+#include "el-utils.h"
 
 #if HAVE_UNISTD_H
 #   include <unistd.h>
@@ -161,6 +162,40 @@ static int el_file_exists
 
 
 /* ==========================================================================
+    create symlink without any suffix to newest log file.  Rationale: it's
+    very often that user wants to view newest log file, but with the fact
+    that newest file has non deterministic suffix, it may be hard and
+    unconvenient to first check newest file and then read it, so we create
+    that nice symlink in a way:
+
+        log.0
+        log.1
+        log.5
+        log.7
+        log -> log.7
+
+    Function does not return anything as errors are not checked here.
+    Symlink is optional and if it is not created no data is lost and
+    returning error would confuse caller.
+   ========================================================================== */
+
+
+static void el_symlink_to_newest_log
+(
+    struct el     *el  /* embedlog object to work on */
+)
+{
+#if HAVE_SYMLINK
+    if (el->frotate_symlink)
+    {
+        (void)remove(el->fname);
+        (void)symlink(el_basename(el->fcurrent_log), el->fname);
+    }
+#endif
+}
+
+
+/* ==========================================================================
     function rotates file, meaning if currently opened file has suffix .3,
     function will close that file and will open file with suffix .4. If
     creating new suffix is impossible (we reached frotate_number of
@@ -258,6 +293,7 @@ skip_rotate:
         return -1;
     }
 
+    el_symlink_to_newest_log(el);
     el->fpos = 0;
 
     return 0;
@@ -476,10 +512,12 @@ int el_file_open
 
 #endif /* HAVE_STAT */
 
+
             /* oldest file found, file is already opened so we
-             * simply return from the function
+             * simply update symlink to newest log file and return.
              */
 
+            el_symlink_to_newest_log(el);
             el->fcurrent_rotate = i;
             el->fpos = fsize;
             el->file = f;
diff --git a/src/el-options.c b/src/el-options.c
index b1de908..0694991 100644
--- a/src/el-options.c
+++ b/src/el-options.c
@@ -491,6 +491,25 @@ static int el_vooption
 
 
     /* ==================================================================
+      ___           __         __                          __ _        __
+     / _/____ ___  / /_ ___ _ / /_ ___   ___ __ __ __ _   / /(_)___   / /__
+    / _// __// _ \/ __// _ `// __// -_) (_-</ // //  ' \ / // // _ \ /  '_/
+   /_/ /_/   \___/\__/ \_,_/ \__/ \__/ /___/\_, //_/_/_//_//_//_//_//_/\_\
+                                           /___/
+       ================================================================== */
+
+
+    case EL_FROTATE_SYMLINK:
+        value_int = va_arg(ap, int);
+        VALID(EINVAL, (value_int & ~1) == 0);
+
+        el_lock(el);
+        el->frotate_symlink = value_int;
+        el_unlock(el);
+        return 0;
+
+
+    /* ==================================================================
                ___
               / _/___ __ __ ___  ____  ___  _  __ ___  ____ __ __
              / _/(_-</ // // _ \/ __/ / -_)| |/ // -_)/ __// // /
@@ -507,6 +526,7 @@ static int el_vooption
         el_unlock(el);
         return 0;
 
+
 #   endif  /* ENABLE_OUT_FILE */
 
 
@@ -711,6 +731,7 @@ static int el_vooption
 #if ENABLE_OUT_FILE
     el->fsync_every = 32768;
     el->fsync_level = EL_FATAL;
+    el->frotate_symlink = 1;
 #endif
 
 #if ENABLE_PTHREAD