[Xen-devel] [PATCH 2 of 7 v4] blktap3/tapback: Introduces functionality required to access XenStore

[Thanos Makatos <thanos.makatos@xxxxxxxxxx>]

This patch introduces convenience functions that read/write values from/to
XenStore.

Signed-off-by: Thanos Makatos <thanos.makatos@xxxxxxxxxx>

---
Changed since v2:
  * Introduced makefile to facilitate development.
  * Removed functions (v)mprintf as (v)asprintf suffice.
  * Ensure tapback_xs_vread (a) returns a NULL terminated string, and (b) the
    returned string doesn't contain NULL characters, apart from the
    NULL-terminating one.
  * Removed unnecessary variable initialisations.
  * Minor whitespace clean up.
  * Fixed typo in tapback_xs_vread.
  * function tapback_xs_vread: handle corner case where the value returned by
    xs_read is a zero-length string.

diff --git a/tools/blktap3/tapback/Makefile b/tools/blktap3/tapback/Makefile
new file mode 100644
--- /dev/null
+++ b/tools/blktap3/tapback/Makefile
@@ -0,0 +1,31 @@
+XEN_ROOT := $(CURDIR)/../../../
+include $(XEN_ROOT)/tools/Rules.mk
+
+BLKTAP_ROOT := ..
+
+# -D_GNU_SOURCE is required by vasprintf.
+override CFLAGS += \
+    -I$(BLKTAP_ROOT)/include \
+    -I$(BLKTAP_ROOT)/control \
+    -D_GNU_SOURCE \
+    $(CFLAGS_libxenstore) \
+    $(CFLAGS_libxenctrl) \
+    $(CFLAGS_xeninclude) \
+    -Wall \
+    -Wextra \
+    -Werror
+
+# FIXME cause trouble
+override CFLAGS += \
+    -Wno-old-style-declaration \
+    -Wno-sign-compare \
+    -Wno-type-limits
+
+override LDFLAGS += \
+    $(LDLIBS_libxenstore) \
+    $(LDFLAGS_libxenctrl)
+
+clean:
+       rm -f *.o *.o.d .*.o.d
+
+.PHONY: clean install
diff --git a/tools/blktap3/tapback/xenstore.c b/tools/blktap3/tapback/xenstore.c
new file mode 100644
--- /dev/null
+++ b/tools/blktap3/tapback/xenstore.c
@@ -0,0 +1,195 @@
+/*
+ * Copyright (C) 2012      Citrix Ltd.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301,
+ * USA.
+ */
+
+#include <stdarg.h>
+#include <stdio.h>
+#include <xenstore.h>
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "blktap3.h"
+#include "tapback.h"
+#include "xenstore.h"
+
+char *
+tapback_xs_vread(struct xs_handle * const xs, xs_transaction_t xst,
+        const char * const fmt, va_list ap)
+{
+    char *path, *data = NULL;
+    unsigned int len = 0;
+
+    assert(xs);
+
+    if (vasprintf(&path, fmt, ap) == -1)
+        goto fail;
+    assert(path);
+
+    data = xs_read(xs, xst, path, &len);
+    free(path);
+
+    if (!data)
+        return NULL;
+
+    assert(len >= 0);
+
+    /*
+     * Make sure the returned string is NULL-terminated.
+     */
+    if ((len > 0 && data[len - 1] != '\0') || (len == 0 && data[0] != '\0')) {
+        char *_data = strndup(data, len);
+        if (!_data)
+            /* TODO log error */
+            goto fail;
+        free(data);
+        data = _data;
+    }
+
+    /*
+     * Make sure the returned string does not containing NULL characters, apart
+     * from the NULL-terminating one.
+     *
+     * We should be checking for extraneous NULLs before duplicating the
+     * buffer, but this way logic is simplified.
+     */
+    if (strrchr(data, '\0') - data != len)
+        /* TODO log error */
+        goto fail;
+
+    return data;
+fail:
+    free(data);
+    return NULL;
+}
+
+__printf(3, 4)
+char *
+tapback_xs_read(struct xs_handle * const xs, xs_transaction_t xst,
+        const char * const fmt, ...)
+{
+    va_list ap;
+    char *s;
+
+    assert(xs);
+
+    va_start(ap, fmt);
+    s = tapback_xs_vread(xs, xst, fmt, ap);
+    va_end(ap);
+
+    return s;
+}
+
+char *
+tapback_device_read(const vbd_t * const device, const char * const path)
+{
+    assert(device);
+    assert(path);
+
+    return tapback_xs_read(blktap3_daemon.xs, blktap3_daemon.xst,
+            "%s/%d/%s/%s", BLKTAP3_BACKEND_PATH, device->domid, device->name,
+            path);
+}
+
+char *
+tapback_device_read_otherend(vbd_t * const device,
+        const char * const path)
+{
+    assert(device);
+    assert(path);
+
+    return tapback_xs_read(blktap3_daemon.xs, blktap3_daemon.xst, "%s/%s",
+            device->frontend_path, path);
+}
+
+__scanf(3, 4)
+int
+tapback_device_scanf_otherend(vbd_t * const device,
+        const char * const path, const char * const fmt, ...)
+{
+    va_list ap;
+    int n = 0;
+    char *s = NULL;
+
+    assert(device);
+    assert(path);
+
+    if (!(s = tapback_device_read_otherend(device, path)))
+        return -1;
+    va_start(ap, fmt);
+    n = vsscanf(s, fmt, ap);
+    free(s);
+    va_end(ap);
+
+    return n;
+}
+
+__printf(4, 5)
+int
+tapback_device_printf(vbd_t * const device, const char * const key,
+        const bool mkread, const char * const fmt, ...)
+{
+    va_list ap;
+    int err = 0;
+    char *path = NULL, *val = NULL;
+    bool nerr = false;
+
+    assert(device);
+    assert(key);
+
+    if (-1 == asprintf(&path, "%s/%d/%s/%s", BLKTAP3_BACKEND_PATH,
+                device->domid, device->name, key)) {
+        err = -errno;
+        goto fail;
+    }
+
+    va_start(ap, fmt);
+    if (-1 == vasprintf(&val, fmt, ap))
+        val = NULL;
+    va_end(ap);
+
+    if (!val) {
+        err = -errno;
+        goto fail;
+    }
+
+    if (!(nerr = xs_write(blktap3_daemon.xs, blktap3_daemon.xst, path, val,
+                    strlen(val)))) {
+        err = -errno;
+        goto fail;
+    }
+
+    if (mkread) {
+        struct xs_permissions perms = {
+            device->domid,
+            XS_PERM_READ
+        };
+
+        if (!(nerr = xs_set_permissions(blktap3_daemon.xs, blktap3_daemon.xst,
+                        path, &perms, 1))) {
+            err = -errno;
+            goto fail;
+        }
+    }
+
+fail:
+    free(path);
+    free(val);
+
+    return err;
+}
diff --git a/tools/blktap3/tapback/xenstore.h b/tools/blktap3/tapback/xenstore.h
new file mode 100644
--- /dev/null
+++ b/tools/blktap3/tapback/xenstore.h
@@ -0,0 +1,95 @@
+/*
+ * Copyright (C) 2012      Citrix Ltd.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ * 
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301,
+ * USA.
+ */
+
+/**
+ * Retrieves the XenStore value of the specified key of the VBD's front-end.
+ * The caller must free the returned buffer.
+ *
+ * @param device the VBD
+ * @param path key under the front-end directory
+ * @returns a buffer containing the value, or NULL on error
+ */
+char *
+tapback_device_read_otherend(vbd_t * const device,
+        const char * const path);
+
+/**
+ * Writes to XenStore backened/tapback/<domid>/<devname>/@key = @fmt.
+ *
+ * @param device the VBD
+ * @param key the key to write to
+ * @param mkread TODO
+ * @param fmt format
+ * @returns 0 on success, an error code otherwise
+ */
+__printf(4, 5)
+int
+tapback_device_printf(vbd_t * const device, const char * const key,
+        const bool mkread, const char * const fmt, ...);
+
+/**
+ * Reads the specified XenStore path under the front-end directory in a
+ * scanf-like manner.
+ *
+ * @param device the VBD
+ * @param path XenStore path to read
+ * @param fmt format
+ */
+__scanf(3, 4)
+int
+tapback_device_scanf_otherend(vbd_t * const device,
+        const char * const path, const char * const fmt, ...);
+
+/**
+ * Retrieves the value of the specified of the device from XenStore,
+ * i.e. backend/tapback/<domid>/<devname>/@path
+ * The caller must free the returned buffer.
+ *
+ * @param device the VBD 
+ * @param path the XenStore key
+ * @returns a buffer containing the value, or NULL on error
+ */
+char *
+tapback_device_read(const vbd_t * const device, const char * const path);
+
+/**
+ * Reads the specified XenStore path. The caller must free the returned buffer.
+ *
+ * @param xs handle to XenStore
+ * @param xst XenStore transaction 
+ * @param fmt format
+ * @param ap arguments
+ * @returns a buffer containing the value, or NULL on error
+ */
+char *
+tapback_xs_vread(struct xs_handle * const xs, xs_transaction_t xst,
+        const char * const fmt, va_list ap);
+
+/**
+ * Reads the specified XenStore path. The caller must free the returned buffer.
+ *
+ * @param xs handle to XenStore
+ * @param xst XenStore transaction
+ * @param fmt format
+ * @returns a buffer containing the value, or NULL on error
+ */
+__printf(3, 4)
+char *
+tapback_xs_read(struct xs_handle * const xs, xs_transaction_t xst,
+        const char * const fmt, ...);

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