[libvirt] [PATCH 2/3] 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.

This is a prerequisite for
https://bugzilla.redhat.com/show_bug.cgi?id=1463285.

Signed-off-by: Erik Skultety <eskultet at redhat.com>
---
 src/libvirt_private.syms |  1 +
 src/util/virfile.c       | 36 ++++++++++++++++++++++++++++++++++++
 src/util/virfile.h       |  2 ++
 3 files changed, 39 insertions(+)

diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
index c1e9471c5..53878a30f 100644
--- a/src/libvirt_private.syms
+++ b/src/libvirt_private.syms
@@ -1698,6 +1698,7 @@ virFileStripSuffix;
 virFileTouch;
 virFileUnlock;
 virFileUpdatePerm;
+virFileWaitForAccess;
 virFileWrapperFdClose;
 virFileWrapperFdFree;
 virFileWrapperFdNew;
diff --git a/src/util/virfile.c b/src/util/virfile.c
index 6bbcc3d15..0b1a91699 100644
--- a/src/util/virfile.c
+++ b/src/util/virfile.c
@@ -4164,3 +4164,39 @@ 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 100ms (for up to @tries times).
+ *
+ * Returns 0 on success, -1 on error (ENOENT is fine here).
+ */
+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 (errno != ENOENT) {
+            virReportSystemError(errno, "%s", path);
+            return -1;
+        } else if (tries == 10) {
+            virReportError(VIR_ERR_INTERNAL_ERROR,
+                           _("Failed to access '%s' after %zu tries"),
+                           path, tries);
+            return -1;
+        } else {
+            VIR_DEBUG("Failed to access '%s', re-try in %zu ms", path, ms);
+            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,
-- 
2.13.1