[PATCH 08/21] libxl: provide libxl__xs_*_checked and libxl__xs_transaction_*

[Ian Jackson <ian.jackson@eu.citrix.com>]

These useful utility functions make dealing with xenstore a little
less painful.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

Changes in v4:
 * Fixed typo `commited' in comment.

Changes in v3:
 * Fixed typo `transacton' in log messages.
---
 tools/libxl/libxl_internal.h |   38 +++++++++++++++++++++
 tools/libxl/libxl_xshelp.c   |   76 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 114 insertions(+), 0 deletions(-)

diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 1a7b526..b108d00 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -498,6 +498,44 @@ _hidden bool libxl__xs_mkdir(libxl__gc *gc, xs_transaction_t t,
 
 _hidden char *libxl__xs_libxl_path(libxl__gc *gc, uint32_t domid);
 
+
+/*----- "checked" xenstore access functions -----*/
+/* Each of these functions will check that it succeeded; if it
+ * fails it logs and returns ERROR_FAIL.
+ */
+
+/* On success, *result_out came from the gc.
+ * On error, *result_out is undefined.
+ * ENOENT counts as success but sets *result_out=0
+ */
+int libxl__xs_read_checked(libxl__gc *gc, xs_transaction_t t,
+                           const char *path, const char **result_out);
+
+/* Does not include a trailing null.
+ * May usefully be combined with GCSPRINTF if the format string
+ * behaviour of libxl__xs_write is desirable. */
+int libxl__xs_write_checked(libxl__gc *gc, xs_transaction_t t,
+                            const char *path, const char *string);
+
+/* ENOENT is not an error (even if the parent directories don't exist) */
+int libxl__xs_rm_checked(libxl__gc *gc, xs_transaction_t t, const char *path);
+
+/* Transaction functions, best used together.
+ * The caller should initialise *t to 0 (XBT_NULL) before calling start.
+ * Each function leaves *t!=0 iff the transaction needs cleaning up.
+ *
+ * libxl__xs_transaction_commit returns:
+ *   <0  failure - a libxl error code
+ *   +1  commit conflict; transaction has been destroyed and caller
+ *        must go round again (call _start again and retry)
+ *    0  committed successfully
+ */
+int libxl__xs_transaction_start(libxl__gc *gc, xs_transaction_t *t);
+int libxl__xs_transaction_commit(libxl__gc *gc, xs_transaction_t *t);
+void libxl__xs_transaction_abort(libxl__gc *gc, xs_transaction_t *t);
+
+
+
 /*
  * This is a recursive delete, from top to bottom. What this function does
  * is remove empty folders that contained the deleted entry.
diff --git a/tools/libxl/libxl_xshelp.c b/tools/libxl/libxl_xshelp.c
index c5b5364..993f527 100644
--- a/tools/libxl/libxl_xshelp.c
+++ b/tools/libxl/libxl_xshelp.c
@@ -135,6 +135,82 @@ char *libxl__xs_libxl_path(libxl__gc *gc, uint32_t domid)
     return s;
 }
 
+int libxl__xs_read_checked(libxl__gc *gc, xs_transaction_t t,
+                           const char *path, const char **result_out)
+{
+    char *result = libxl__xs_read(gc, t, path);
+    if (!result) {
+        if (errno != ENOENT) {
+            LOGE(ERROR, "xenstore read failed: `%s'", path);
+            return ERROR_FAIL;
+        }
+    }
+    *result_out = result;
+    return 0;
+}
+
+int libxl__xs_write_checked(libxl__gc *gc, xs_transaction_t t,
+                            const char *path, const char *string)
+{
+    size_t length = strlen(string);
+    if (!xs_write(CTX->xsh, t, path, string, length)) {
+        LOGE(ERROR, "xenstore write failed: `%s' = `%s'", path, string);
+        return ERROR_FAIL;
+    }
+    return 0;
+}
+
+int libxl__xs_rm_checked(libxl__gc *gc, xs_transaction_t t, const char *path)
+{
+    if (!xs_rm(CTX->xsh, t, path)) {
+        if (errno == ENOENT)
+            return 0;
+
+        LOGE(ERROR, "xenstore rm failed: `%s'", path);
+        return ERROR_FAIL;
+    }
+    return 0;
+}
+
+int libxl__xs_transaction_start(libxl__gc *gc, xs_transaction_t *t)
+{
+    assert(!*t);
+    *t = xs_transaction_start(CTX->xsh);
+    if (!*t) {
+        LOGE(ERROR, "could not create xenstore transaction");
+        return ERROR_FAIL;
+    }
+    return 0;
+}
+
+int libxl__xs_transaction_commit(libxl__gc *gc, xs_transaction_t *t)
+{
+    assert(*t);
+
+    if (!xs_transaction_end(CTX->xsh, *t, 0)) {
+        if (errno == EAGAIN)
+            return +1;
+
+        *t = 0;
+        LOGE(ERROR, "could not commit xenstore transaction");
+        return ERROR_FAIL;
+    }
+
+    *t = 0;
+    return 0;
+}
+
+void libxl__xs_transaction_abort(libxl__gc *gc, xs_transaction_t *t)
+{
+    if (!*t)
+        return;
+
+    if (!xs_transaction_end(CTX->xsh, *t, 1))
+        LOGE(ERROR, "could not abort xenstore transaction");
+
+    *t = 0;
+}
+
 int libxl__xs_path_cleanup(libxl__gc *gc, xs_transaction_t t, char *user_path)
 {
     unsigned int nb = 0;
-- 
1.7.2.5