[libvirt] [PATCH v3 5/6] util: Introduce virFileWaitForAccess

[Erik Skultety]

Since we have a number of places where we workaround timing issues with
devices, attributes (files in general) not being available at the time
of processing them by calling usleep in a loop for a fixed number of
tries, we could as well have a utility function that would do that.
Therefore we won't have to duplicate this ugly workaround even more.

Signed-off-by: Erik Skultety <eskultet at redhat.com>
---
 src/libvirt_private.syms |  1 +
 src/util/virfile.c       | 29 +++++++++++++++++++++++++++++
 src/util/virfile.h       |  2 ++
 3 files changed, 32 insertions(+)

diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
index 2149b11b7..b9fb54a79 100644
--- a/src/libvirt_private.syms
+++ b/src/libvirt_private.syms
@@ -1724,6 +1724,7 @@ virFileStripSuffix;
 virFileTouch;
 virFileUnlock;
 virFileUpdatePerm;
+virFileWaitForAccess;
 virFileWrapperFdClose;
 virFileWrapperFdFree;
 virFileWrapperFdNew;
diff --git a/src/util/virfile.c b/src/util/virfile.c
index 2f28e83f4..15b46bcdd 100644
--- a/src/util/virfile.c
+++ b/src/util/virfile.c
@@ -4166,3 +4166,32 @@ virFileReadValueString(char **value, const char *format, ...)
     VIR_FREE(str);
     return ret;
 }
+
+
+/**
+ * virFileWaitForAccess:
+ * @path: absolute path to a sysfs attribute (can be a symlink)
+ * @ms: how long to wait (in milliseconds)
+ * @tries: how many times should we try to wait for @path to become accessible
+ *
+ * Checks the existence of @path. In case the file defined by @path
+ * doesn't exist, we wait for it to appear in @ms milliseconds (for up to
+ * @tries attempts).
+ *
+ * Returns 0 on success, -1 on error, setting errno appropriately.
+ */
+int
+virFileWaitForAccess(const char *path, size_t ms, size_t tries)
+{
+    errno = 0;
+
+    /* wait for @path to be accessible in @ms milliseconds, up to @tries */
+    while (tries-- > 0 && !virFileExists(path)) {
+        if (tries == 0 || errno != ENOENT)
+            return -1;
+
+        usleep(ms * 1000);
+    }
+
+    return 0;
+}
diff --git a/src/util/virfile.h b/src/util/virfile.h
index 57ceb8072..42630ebb5 100644
--- a/src/util/virfile.h
+++ b/src/util/virfile.h
@@ -347,6 +347,8 @@ int virFileReadValueScaledInt(unsigned long long *value, const char *format, ...
 int virFileReadValueString(char **value, const char *format, ...)
  ATTRIBUTE_FMT_PRINTF(2, 3);
 
+int virFileWaitForAccess(const char *path, size_t ms, size_t tries);
+
 
 int virFileInData(int fd,
                   int *inData,
-- 
2.13.3