api: add glfs_get_volfile

From the API-header description:

> Sometimes it's useful e.g. for scripts to see the volfile, so that
> they can parse it and find subvolumes to do things like split-brain
> resolution or custom layouts.  The API here was specifically intended
> to make access e.g. from Python as simple as possible.
>
> Note that the volume must be started (not necessarily mounted) for
> this to work.

Change-Id: If3f55ee9611cdf4b77aa53659f0af0d21957817d
Signed-off-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-on: http://review.gluster.org/7183
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>

api/examples/getvolfile.py

@@ -0,0 +1,43 @@
+#!/usr/bin/python
+
+import ctypes
+import ctypes.util
+
+api = ctypes.CDLL(ctypes.util.find_library("gfapi"))
+api.glfs_get_volfile.argtypes = [ctypes.c_void_p,
+							     ctypes.c_void_p,
+								 ctypes.c_ulong]
+api.glfs_get_volfile.restype = ctypes.c_long;
+
+def get_volfile (host, volume):
+	# This is set to a large value to exercise the "buffer not big enough"
+	# path.  More realistically, you'd just start with a huge buffer.
+	BUF_LEN = 0
+	fs = api.glfs_new(volume)
+	#api.glfs_set_logging(fs,"/dev/stderr",7)
+	api.glfs_set_volfile_server(fs,"tcp",host,24007)
+	api.glfs_init(fs)
+	vbuf = ctypes.create_string_buffer(BUF_LEN)
+	vlen = api.glfs_get_volfile(fs,vbuf,BUF_LEN)
+	if vlen < 0:
+		vlen = BUF_LEN - vlen
+		vbuf = ctypes.create_string_buffer(vlen)
+		vlen = api.glfs_get_volfile(fs,vbuf,vlen)
+	api.glfs_fini(fs)
+	if vlen <= 0:
+		return vlen
+	return vbuf.value[:vlen]
+
+if __name__ == "__main__":
+	import sys
+
+	try:
+		res = apply(get_volfile,sys.argv[1:3])
+	except:
+		print "fetching volfile failed (volume not started?)"
+
+	try:
+		for line in res.split('\n'):
+			print line
+	except:
+		print "bad return value %s" % res

api/src/glfs-mgmt.c

@@ -207,6 +207,8 @@ glusterfs_oldvolfile_update (struct glfs *fs, char *volfile, ssize_t size)
 {
 	int ret = -1;
 
+        pthread_mutex_lock (&fs->mutex);
+
 	fs->oldvollen = size;
 	if (!fs->oldvolfile) {
 		fs->oldvolfile = GF_CALLOC (1, size+1, glfs_mt_volfile_t);
@@ -222,6 +224,8 @@ glusterfs_oldvolfile_update (struct glfs *fs, char *volfile, ssize_t size)
 		ret = 0;
 	}
 
+        pthread_mutex_unlock (&fs->mutex);
+
 	return ret;
 }
 

api/src/glfs.c

@@ -680,3 +680,23 @@ glfs_fini (struct glfs *fs)
 
         return ret;
 }
+
+ssize_t
+glfs_get_volfile (struct glfs *fs, void *buf, size_t len)
+{
+        ssize_t         res;
+
+        glfs_lock(fs);
+        if (len >= fs->oldvollen) {
+                gf_log ("glfs", GF_LOG_TRACE, "copying %lu to %p", len, buf);
+                memcpy(buf,fs->oldvolfile,len);
+                res = len;
+        }
+        else {
+                res = len - fs->oldvollen;
+                gf_log ("glfs", GF_LOG_TRACE, "buffer is %ld too short", -res);
+        }
+        glfs_unlock(fs);
+
+        return res;
+}

api/src/glfs.h

@@ -252,6 +252,36 @@ int glfs_init (glfs_t *fs) __THROW;
 
 int glfs_fini (glfs_t *fs) __THROW;
 
+/*
+  SYNOPSIS
+
+      glfs_getvol: Get the volfile associated with a 'virtual mount'
+
+  DESCRIPTION
+
+      Sometimes it's useful e.g. for scripts to see the volfile, so that they
+      can parse it and find subvolumes to do things like split-brain resolution
+      or custom layouts.  The API here was specifically intended to make access
+      e.g. from Python as simple as possible.
+
+      Note that the volume must be started (not necessarily mounted) for this
+      to work.
+
+  PARAMETERS
+
+      @fs:  The 'virtual mount' object for which a volfile is desired
+      @buf: Pointer to a place for the volfile length to be stored
+      @len: Length of @buf
+
+  RETURN VALUES
+
+      >0: filled N bytes of buffer
+       0: no volfile available
+      <0: volfile length exceeds @len by N bytes (@buf unchanged)
+*/
+
+ssize_t glfs_get_volfile (glfs_t *fs, void *buf, size_t len) __THROW;
+
 /*
  * FILE OPERATION
  *