add_drive: Don't use cache=off if not supported by underlying filesystem.

If you use the guestfs_add_drive function, then currently it
generates a qemu command line element like:

  -drive ...,cache=off,...

This causes qemu to try to open the device with O_DIRECT.
Unfortunately some filesystems don't support this flag, notably tmpfs,
which means you can't use libguestfs in conjunction with tmpfs.  On
some systems /tmp is a tmpfs filesystem.

This patch fixes this so that if the filesystem doesn't support
O_DIRECT, then we omit the cache=off parameter.  This seems reasonable
from a reliability point of view, because if you're using tmpfs then
you probably didn't expect reliability in the case where your system
suddenly powers off.

2 files changed, 25 insertions, 6 deletions

diff --git a/src/generator.ml b/src/generator.ml
index 3a772029..39faffec 100755
--- a/src/generator.ml
+++ b/src/generator.ml
@@ -454,6 +454,8 @@ image).
 
 This is equivalent to the qemu parameter
 C<-drive file=filename,cache=off,if=...>.
+C<cache=off> is omitted in cases where it is not supported by
+the underlying filesystem.
 
 Note that this call checks for the existence of C<filename>.  This
 stops you from specifying other types of drive which are supported
diff --git a/src/guestfs.c b/src/guestfs.c
index 37869e84..ad3980ff 100644
--- a/src/guestfs.c
+++ b/src/guestfs.c
@@ -799,14 +799,31 @@ guestfs_add_drive (guestfs_h *g, const char *filename)
     return -1;
   }
 
-  if (access (filename, F_OK) == -1) {
-    perrorf (g, "%s", filename);
-    return -1;
+  /* cache=off improves reliability in the event of a host crash.
+   *
+   * However this option causes qemu to try to open the file with
+   * O_DIRECT.  This fails on some filesystem types (notably tmpfs).
+   * So we check if we can open the file with or without O_DIRECT,
+   * and use cache=off (or not) accordingly.
+   *
+   * This test also checks for the presence of the file, which
+   * is a documented semantic of this interface.
+   */
+  int fd = open (filename, O_RDONLY|O_DIRECT);
+  if (fd >= 0) {
+    close (fd);
+    snprintf (buf, len, "file=%s,cache=off,if=" DRIVE_IF, filename);
+  } else {
+    fd = open (filename, O_RDONLY);
+    if (fd >= 0) {
+      close (fd);
+      snprintf (buf, len, "file=%s,if=" DRIVE_IF, filename);
+    } else {
+      perrorf (g, "%s", filename);
+      return -1;
+    }
   }
 
-  /* cache=off improves reliability in the event of a host crash. */
-  snprintf (buf, len, "file=%s,cache=off,if=%s", filename, DRIVE_IF);
-
   return guestfs_config (g, "-drive", buf);
 }