Add a fake backend for component tests by lforchini · Pull Request #78 · nutanix/io-thread-controller

lforchini · 2026-10-01T14:37:00Z

The harness needs a VM the daemon can discover and scale without
libvirt. JSON files carry the snapshot, and set_thread_count records
each target so tests can assert behavior.

Signed-off-by: Leonardo Forchini leonardo.forchini@nutanix.com

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The harness needs a VM the daemon can discover and scale without libvirt. JSON files carry the snapshot, and set_thread_count records each target so tests can assert behavior. Signed-off-by: Leonardo Forchini <leonardo.forchini@nutanix.com>

Add a Python component test interface for the FakeBackend. It is able to read the JSON files used by the backend and provides utilities to modify the 'VM's as required by tests. AI-Generated: Most of it Signed-off-by: Leonardo Forchini <leonardo.forchini@nutanix.com>

tmakatos

I haven't looked at the methods of the FakeBacked class yet. How do you plan to create the tasks on /proc?

tmakatos · 2026-11-13T11:34:58Z

+
+//! File-backed backend for component tests.
+//!
+//! Instead of requiring a running QEMU instance, this backend manages


This reads like we still use QEMU except that we're not running it. I assume you mean that we use a backend that doesn't require QEMU at all. The fact that it's file-based is secondary.

tmakatos · 2026-11-13T11:39:12Z

+
+//! File-backed backend for component tests.
+//!
+//! Instead of requiring a running QEMU instance, this backend manages


Can you also add a paragraph how this backend behaves? E.g. whether you pre-configure some performance counter values that it will return when queried by the controller etc. I feel like test backend is a more suitable name, and it could be used as a n example backend as well, could you document this in the README?

tmakatos · 2026-11-13T11:41:11Z

+//!
+//! Instead of requiring a running QEMU instance, this backend manages
+//! state through JSON files in `state_dir`. This allows for the external
+//! Python component testing harness to manipulate "VM"s as necessary.


You don't explain what happens after the harness has modified the files, does the backend blindly re-read everything when asked by the controller?

tmakatos · 2026-11-13T11:42:34Z

+            continue;
+        };
+        let Some(vm) = read_vm(&path, id) else {
+            continue;


Why ignore the failure?

tmakatos · 2026-11-13T11:42:57Z

+    for entry in entries.flatten() {
+        let path = entry.path();
+        if path.extension().and_then(|ext| ext.to_str()) != Some("json") {
+            continue;


Are we expecting irrelevant files which we should ignore?

tmakatos · 2026-11-13T11:43:33Z

+#[async_trait]
+impl InstanceClient for FakeClient {
+    async fn set_thread_count(&self, count: u32) -> Result<(), BackendClientError> {
+        let _guard = LockGuard::acquire(&lock_path_for(&self.vm_path))?;


Why not lock the VM json file? Or better the entire VM directory? This way we wouldn't need the read/write methods in impl VmFile?

tmakatos · 2026-11-13T11:46:41Z

+from harness.common import physical
+
+DEFAULT_VM = "vm-a"
+STATE_DIR = "/var/fake"


Shouldn't we use a directory in /tmp?

tmakatos · 2026-11-13T11:47:07Z

+
+DEFAULT_VM = "vm-a"
+STATE_DIR = "/var/fake"
+OWNERSHIP_FILE = "/run/io-thread-controller/vm-ownership.json"


tmakatos · 2026-11-13T11:47:41Z

+
+
+class FakeBackend:
+    """JSON VM directory the ``fake`` backend discovers."""


This class represents a directory?

tmakatos · 2026-11-13T11:53:57Z

+        self, threads: int, util: float, io: int = 0, vcpu: int = 16
+    ) -> None:
+        """Set the default VM's threads, utilisation, vCPU cap, and read I/O."""
+        self.set_threads(threads)


Not sure we need those function separate if they're used only once, given how simple they are. Can you open code other such functions, if any?

lforchini added this pull request to stack #83 October 1, 2026 14:37

lforchini force-pushed the test/fake-backend branch from 0cbcd10 to daacaa4 Compare October 1, 2026 14:48

lforchini removed this pull request from stack #83 October 1, 2026 14:50

lforchini added this pull request to stack #84 October 1, 2026 14:50

lforchini force-pushed the test/fake-backend branch from daacaa4 to 2cc2ddc Compare October 1, 2026 15:00

lforchini force-pushed the test/fake-backend branch from 2cc2ddc to 1bbb149 Compare October 5, 2026 08:10

lforchini removed this pull request from stack #84 October 5, 2026 08:12

lforchini force-pushed the test/fake-backend branch from 1bbb149 to de819c5 Compare October 5, 2026 08:15

lforchini changed the base branch from test/refactor to test/remove-to-be-component October 5, 2026 08:16

lforchini added this pull request to stack #87 October 5, 2026 08:16

lforchini marked this pull request as ready for review October 5, 2026 08:28

lforchini requested review from ntnx-matej-genci and tmakatos as code owners October 5, 2026 08:28

lforchini force-pushed the test/fake-backend branch from de819c5 to ff8cf94 Compare October 5, 2026 08:49

lforchini force-pushed the test/fake-backend branch 2 times, most recently from 413624e to 707c7bd Compare October 8, 2026 07:30

lforchini removed this pull request from stack #87 October 8, 2026 15:22

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lforchini added this pull request to stack #88 October 8, 2026 15:23

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lforchini force-pushed the test/fake-backend branch from a5b3533 to 9708bb1 Compare October 13, 2026 12:43

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lforchini removed this pull request from stack #152 October 23, 2026 08:44

lforchini changed the base branch from test/remove-to-be-component to create-comptest-utils October 23, 2026 08:44

lforchini added this pull request to stack #157 October 23, 2026 08:45

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lforchini added this pull request to stack #176 November 5, 2026 15:03

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lforchini added this pull request to stack #178 November 5, 2026 15:10

lforchini added 2 commits November 13, 2026 09:50

Add a fake backend for component tests

785d715

The harness needs a VM the daemon can discover and scale without libvirt. JSON files carry the snapshot, and set_thread_count records each target so tests can assert behavior. Signed-off-by: Leonardo Forchini <leonardo.forchini@nutanix.com>

tmakatos reviewed Nov 13, 2026

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Add a fake backend for component tests - #78

Add a fake backend for component tests#78
lforchini wants to merge 2 commits into
create-comptest-utilsfrom
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		class FakeBackend:
		"""JSON VM directory the ``fake`` backend discovers."""

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