[PATCH v1 02/10] libxl_internal: functions to lock / unlock domain configuration

[Wei Liu <wei.liu2@citrix.com>]

Simple file lock taken from xl to serialise access to "libxl-json" file.
If a thread cannot get hold of the lock it waits due to F_SETLKW.

In order to generate lock file name, rename userdata_path to
libxl__userdata_path and declare it in libxl_internal.h

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
---
 tools/libxl/libxl_dom.c      |   14 ++++----
 tools/libxl/libxl_internal.c |   76 ++++++++++++++++++++++++++++++++++++++++++
 tools/libxl/libxl_internal.h |   11 ++++++
 3 files changed, 94 insertions(+), 7 deletions(-)

diff --git a/tools/libxl/libxl_dom.c b/tools/libxl/libxl_dom.c
index 83eb29a..a41e8fd 100644
--- a/tools/libxl/libxl_dom.c
+++ b/tools/libxl/libxl_dom.c
@@ -1762,9 +1762,9 @@ char *libxl__uuid2string(libxl__gc *gc, const libxl_uuid uuid)
     return GCSPRINTF(LIBXL_UUID_FMT, LIBXL_UUID_BYTES(uuid));
 }
 
-static const char *userdata_path(libxl__gc *gc, uint32_t domid,
-                                      const char *userdata_userid,
-                                      const char *wh)
+const char *libxl__userdata_path(libxl__gc *gc, uint32_t domid,
+                                 const char *userdata_userid,
+                                 const char *wh)
 {
     libxl_ctx *ctx = libxl__gc_owner(gc);
     char *uuid_string;
@@ -1799,7 +1799,7 @@ void libxl__userdata_destroyall(libxl__gc *gc, uint32_t domid)
     glob_t gl;
     int r, i;
 
-    pattern = userdata_path(gc, domid, "*", "?");
+    pattern = libxl__userdata_path(gc, domid, "*", "?");
     if (!pattern)
         goto out;
 
@@ -1830,7 +1830,7 @@ int libxl_userdata_store(libxl_ctx *ctx, uint32_t domid,
     int e, rc;
     int fd = -1;
 
-    filename = userdata_path(gc, domid, userdata_userid, "d");
+    filename = libxl__userdata_path(gc, domid, userdata_userid, "d");
     if (!filename) {
         rc = ERROR_NOMEM;
         goto out;
@@ -1841,7 +1841,7 @@ int libxl_userdata_store(libxl_ctx *ctx, uint32_t domid,
         goto out;
     }
 
-    newfilename = userdata_path(gc, domid, userdata_userid, "n");
+    newfilename = libxl__userdata_path(gc, domid, userdata_userid, "n");
     if (!newfilename) {
         rc = ERROR_NOMEM;
         goto out;
@@ -1892,7 +1892,7 @@ int libxl_userdata_retrieve(libxl_ctx *ctx, uint32_t domid,
     int datalen = 0;
     void *data = 0;
 
-    filename = userdata_path(gc, domid, userdata_userid, "d");
+    filename = libxl__userdata_path(gc, domid, userdata_userid, "d");
     if (!filename) {
         rc = ERROR_NOMEM;
         goto out;
diff --git a/tools/libxl/libxl_internal.c b/tools/libxl/libxl_internal.c
index dc47177..9cc60e8 100644
--- a/tools/libxl/libxl_internal.c
+++ b/tools/libxl/libxl_internal.c
@@ -436,6 +436,82 @@ out:
     return rc;
 }
 
+int libxl__lock_domain_configuration(libxl__gc *gc, uint32_t domid,
+                                     int *fd_lock)
+{
+    int rc;
+    struct flock fl;
+    const char *lockfile;
+
+    if (*fd_lock >= 0)
+        return ERROR_INVAL;
+
+    lockfile = libxl__userdata_path(gc, domid, "libxl-json.lock", "d");
+    if (!lockfile)
+        return ERROR_FAIL;
+
+    *fd_lock = open(lockfile, O_WRONLY|O_CREAT, S_IWUSR);
+    if (*fd_lock < 0) {
+        LOGE(ERROR, "cannot open lockfile %s errno=%d\n", lockfile, errno);
+        return ERROR_FAIL;
+    }
+
+    if (fcntl(*fd_lock, F_SETFD, FD_CLOEXEC) < 0) {
+        close(*fd_lock);
+        LOGE(ERROR, "cannot set cloexec to lockfile %s errno=%d\n",
+             lockfile, errno);
+        return ERROR_FAIL;
+    }
+
+get_lock:
+    fl.l_type = F_WRLCK;
+    fl.l_whence = SEEK_SET;
+    fl.l_start = 0;
+    fl.l_len = 0;
+    rc = fcntl(*fd_lock, F_SETLKW, &fl);
+    if (rc < 0 && errno == EINTR)
+        goto get_lock;
+
+    if (rc < 0) {
+        LOGE(ERROR, "cannot acquire lock %s errno=%d\n", lockfile, errno);
+        rc = ERROR_FAIL;
+    } else
+        rc = 0;
+
+    return rc;
+}
+
+int libxl__unlock_domain_configuration(libxl__gc *gc, uint32_t domid,
+                                       int *fd_lock)
+{
+    int rc;
+    struct flock fl;
+
+    if (*fd_lock < 0)
+        return ERROR_INVAL;
+
+release_lock:
+    fl.l_type = F_UNLCK;
+    fl.l_whence = SEEK_SET;
+    fl.l_start = 0;
+    fl.l_len = 0;
+    rc = fcntl(*fd_lock, F_SETLKW, &fl);
+    if (rc < 0 && errno == EINTR)
+        goto release_lock;
+
+    if (rc < 0) {
+        const char *lockfile;
+        lockfile = libxl__userdata_path(gc, domid, "libxl-json.lock", "d");
+        LOGE(ERROR, "cannot release lock %s, errno=%d\n", lockfile, errno);
+        rc = ERROR_FAIL;
+    } else
+        rc = 0;
+
+    close(*fd_lock);
+    *fd_lock = -1;
+
+    return rc;
+}
 
 /*
  * Local variables:
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index ef2111b..e9d93ac 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -991,6 +991,9 @@ _hidden int libxl__toolstack_restore(uint32_t domid, const uint8_t *buf,
                                      uint32_t size, void *data);
 _hidden int libxl__domain_resume_device_model(libxl__gc *gc, uint32_t domid);
 
+_hidden const char *libxl__userdata_path(libxl__gc *gc, uint32_t domid,
+                                         const char *userdata_userid,
+                                         const char *wh);
 _hidden void libxl__userdata_destroyall(libxl__gc *gc, uint32_t domid);
 
 _hidden int libxl__domain_resume(libxl__gc *gc, uint32_t domid,
@@ -3228,6 +3231,14 @@ int libxl__get_domain_configuration(libxl__gc *gc, uint32_t domid,
                                     libxl_domain_config *d_config);
 int libxl__set_domain_configuration(libxl__gc *gc, uint32_t domid,
                                     libxl_domain_config *d_config);
+/*
+ * Lock / unlock domain configuration in libxl private data store.
+ * fd_lock contains the file descriptor pointing to the lock file.
+ */
+int libxl__lock_domain_configuration(libxl__gc *gc, uint32_t domid,
+                                     int *fd_lock);
+int libxl__unlock_domain_configuration(libxl__gc *gc, uint32_t domid,
+                                       int *fd_lock);
 
 #endif
 
-- 
1.7.10.4