[Libguestfs] [PATCH nbdkit] python: Simulate slow extents

[Nir Soffer]

In some situations getting image extents can be slow. Add configuration
value to simulate slow extents. This can be useful for testing and
optimizing nbd clients.

Example usage - simulating 1 seconds delay:

qemu-img create -f raw test.img 10g

./nbdkit -f -v python ./plugins/python/examples/file.py \
    file=test.img extents_delay=1 2>&1 | grep 'python: extents'
nbdkit: python.1: debug: python: extents count=2147483136 offset=0 req_one=1
nbdkit: python.0: debug: python: extents count=2147483136 offset=2147483136 req_one=1
nbdkit: python.2: debug: python: extents count=2147483136 offset=4294966272 req_one=1
nbdkit: python.3: debug: python: extents count=2147483136 offset=6442449408 req_one=1
nbdkit: python.5: debug: python: extents count=2147483136 offset=8589932544 req_one=1
nbdkit: python.6: debug: python: extents count=2560 offset=10737415680 req_one=1
nbdkit: python.4: debug: python: extents count=2147483136 offset=0 req_one=1
nbdkit: python.9: debug: python: extents count=2147483136 offset=2147483136 req_one=1
nbdkit: python.14: debug: python: extents count=2147483136 offset=4294966272 req_one=1
nbdkit: python.11: debug: python: extents count=2147483136 offset=6442449408 req_one=1
nbdkit: python.9: debug: python: extents count=2147483136 offset=8589932544 req_one=1
nbdkit: python.9: debug: python: extents count=2560 offset=10737415680 req_one=1

In another shell:

$ time qemu-img convert -p -f raw -O raw nbd://localhost/ test.img
(100.00/100%)

real	0m18.029s
user	0m2.314s
sys	0m3.581s

Without extent delay this takes about 5.6 seconds.

Signed-off-by: Nir Soffer <nsoffer at redhat.com>
---
 plugins/python/examples/file.py | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/plugins/python/examples/file.py b/plugins/python/examples/file.py
index 8501b8eb..e607b338 100644
--- a/plugins/python/examples/file.py
+++ b/plugins/python/examples/file.py
@@ -6,6 +6,11 @@
 #
 #   ./nbdkit -f -v python ./plugins/python/examples/file.py file=test.img
 #
+# To simulate slow extents operation, specify a delay:
+#
+#   ./nbdkit -f -v python ./plugins/python/examples/file.py file=test.img \
+#       extents_delay=70
+#
 # Or run it after installing nbdkit like this:
 #
 #   nbdkit -f -v python ./plugins/python/examples/file.py file=test.img
@@ -14,6 +19,7 @@
 # the foreground and print debugging, which is useful when testing.
 
 import os
+import time
 
 import nbdkit
 
@@ -25,13 +31,18 @@ API_VERSION = 2
 # The file we want to serve.
 filename = None
 
+# Simulate slow extents.
+extents_delay = 0.0
+
 
 # Parse the file parameter which contains the name of the file that we
 # want to serve.
 def config(key, value):
-    global filename
+    global filename, extents_delay
     if key == "file":
         filename = os.path.abspath(value)
+    elif key == "extents_delay":
+        extents_delay = float(value)
     else:
         raise RuntimeError("unknown parameter: " + key)
 
@@ -64,6 +75,15 @@ def get_size(h):
     return sb.st_size
 
 
+def can_extents(h):
+    return True
+
+
+def extents(h, count, offset, flags):
+    time.sleep(extents_delay)
+    yield (offset, count, 0)
+
+
 def pread(h, buf, offset, flags):
     n = os.preadv(h['fd'], [buf], offset)
     if n != len(buf):
-- 
2.26.2