From e8397ecded1208ba85d4df126dd3b4efdfe56190 Mon Sep 17 00:00:00 2001
From: Ruben Fiszel <ruben@windmill.dev>
Date: Tue, 15 Sep 2026 17:33:38 +0200
Subject: [PATCH] fix: key a cached workflow-as-code task on its step key, not
 its name

---
 backend/tests/fixtures/wac_flow_script.sql  |  2 +-
 backend/windmill-common/src/wac.rs          |  8 +++-----
 backend/windmill-worker/src/bun_executor.rs |  1 -
 backend/windmill-worker/src/common.rs       | 21 +++++++++++----------
 4 files changed, 15 insertions(+), 17 deletions(-)

diff --git a/backend/tests/fixtures/wac_flow_script.sql b/backend/tests/fixtures/wac_flow_script.sql
index da50a6d35d..d284a500c0 100644
--- a/backend/tests/fixtures/wac_flow_script.sql
+++ b/backend/tests/fixtures/wac_flow_script.sql
@@ -66,7 +66,7 @@ const double = task(async (n: number) => {
 }, { cache_ttl: 60 });
 
 export const main = workflow(async (n: number) => {
-  const r = await step("pick", async () => Math.random());
+  const r = await step("pick", async () => Math.floor(Math.random() * 1e9));
   const d = await double(r);
   return { fresh: d === r * 2 };
 });'
diff --git a/backend/windmill-common/src/wac.rs b/backend/windmill-common/src/wac.rs
index 7a42ba413d..c994dcc4c9 100644
--- a/backend/windmill-common/src/wac.rs
+++ b/backend/windmill-common/src/wac.rs
@@ -40,11 +40,9 @@ pub struct WacCheckpoint {
     #[serde(skip_serializing_if = "Option::is_none")]
     #[serde(default)]
     pub _executing_key: Option<String>,
-    /// With `_executing_key`: the task the child runs and the arguments it was
-    /// called with, the identity its cached result is keyed on.
-    #[serde(skip_serializing_if = "Option::is_none")]
-    #[serde(default)]
-    pub _executing_task: Option<String>,
+    /// With `_executing_key`: the arguments the task was called with. A cached
+    /// result is keyed on both; the key alone is a name and a position, which two
+    /// tasks can share, and the arguments alone say nothing about which task ran.
     #[serde(skip_serializing_if = "Option::is_none")]
     #[serde(default)]
     pub _executing_args: Option<serde_json::Map<String, Value>>,
diff --git a/backend/windmill-worker/src/bun_executor.rs b/backend/windmill-worker/src/bun_executor.rs
index a0d25f6025..8380ab0ef2 100644
--- a/backend/windmill-worker/src/bun_executor.rs
+++ b/backend/windmill-worker/src/bun_executor.rs
@@ -3237,7 +3237,6 @@ pub async fn handle_wac_v2_output(
                         let child_checkpoint_json = serde_json::json!({
                             "completed_steps": &checkpoint.completed_steps,
                             "_executing_key": &step.key,
-                            "_executing_task": &step.name,
                             "_executing_args": &step.args,
                         });
                         sqlx::query(
diff --git a/backend/windmill-worker/src/common.rs b/backend/windmill-worker/src/common.rs
index 714652a484..b96416ad2a 100644
--- a/backend/windmill-worker/src/common.rs
+++ b/backend/windmill-worker/src/common.rs
@@ -1575,15 +1575,16 @@ pub async fn cached_result_path(
         }
     }
     // A workflow-as-code task child carries its parent's arguments, which say
-    // nothing about the task's own inputs; its result is keyed on the task it
-    // runs and the arguments that task was called with.
-    let (task, args) = match wac_task_identity(db, job).await? {
-        Some((task, args)) => (Some(task), Some(Json(args))),
+    // nothing about the task's own inputs; its result is keyed on the step it
+    // runs (a task's name and position, which is what tells two tasks of one
+    // name apart) and the arguments that task was called with.
+    let (step, args) = match wac_task_identity(db, job).await? {
+        Some((step, args)) => (Some(step), Some(Json(args))),
         None => (None, job.args.clone()),
     };
-    if let Some(task) = task {
-        hasher.update(b"wac_task:");
-        hasher.update(task.as_bytes());
+    if let Some(step) = step {
+        hasher.update(b"wac_step:");
+        hasher.update(step.as_bytes());
     }
     hash_args(
         db,
@@ -1598,7 +1599,7 @@ pub async fn cached_result_path(
     Ok(format!("g/results/{:064x}", hasher.finalize()))
 }
 
-/// The task name and call arguments a workflow-as-code parent seeded in this
+/// The step key and call arguments a workflow-as-code parent seeded in this
 /// child's checkpoint at push time; `None` for any job that is not such a child.
 async fn wac_task_identity(
     db: &DB,
@@ -1609,7 +1610,7 @@ async fn wac_task_identity(
     }
     let identity: Option<(Option<String>, Option<Json<HashMap<String, Box<RawValue>>>>)> =
         sqlx::query_as(
-            "SELECT workflow_as_code_status->'_checkpoint'->>'_executing_task', \
+            "SELECT workflow_as_code_status->'_checkpoint'->>'_executing_key', \
                     workflow_as_code_status->'_checkpoint'->'_executing_args' \
              FROM v2_job_status WHERE id = $1",
         )
@@ -1617,7 +1618,7 @@ async fn wac_task_identity(
         .fetch_optional(db)
         .await?;
     Ok(match identity {
-        Some((Some(task), Some(Json(args)))) => Some((task, args)),
+        Some((Some(step), Some(Json(args)))) => Some((step, args)),
         _ => None,
     })
 }