feat(cache): detect stdin data and disable cache update

- Forward stdin from parent to child process concurrently with output draining
- Track whether stdin had data during execution
- Disable cache update when stdin had data (output may depend on input)
- Add CacheNotUpdatedReason::StdinDataExists variant
- Add e2e test for stdin passthrough functionality

This ensures tasks that read from stdin don't get incorrectly cached,
since their output may depend on the input data which we don't fingerprint.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: branchseer

crates/vite_task/src/session/cache/display.rs

@@ -304,10 +304,13 @@ pub fn format_cache_update_status(status: &CacheUpdateStatus) -> Option<String>
             CacheNotUpdatedReason::CacheHit => None,
             CacheNotUpdatedReason::CacheDisabled => None,
             CacheNotUpdatedReason::BuiltInCommand => None,
-            // This needs to be shown - task failed so cache wasn't updated
+            // These need to be shown - they explain why cache wasn't updated
             CacheNotUpdatedReason::NonZeroExitStatus => {
                 Some("→ Cache not updated: task failed".to_string())
             }
+            CacheNotUpdatedReason::StdinDataExists => {
+                Some("→ Cache not updated: stdin had data".to_string())
+            }
         },
     }
 }

crates/vite_task/src/session/event.rs

@@ -29,6 +29,8 @@ pub enum CacheNotUpdatedReason {
     NonZeroExitStatus,
     /// Built-in command doesn't support caching
     BuiltInCommand,
+    /// Stdin had data - output may depend on input, unsafe to cache
+    StdinDataExists,
 }
 
 #[derive(Debug)]

crates/vite_task/src/session/execute/mod.rs

@@ -287,11 +287,18 @@ impl ExecutionContext<'_> {
         };
 
         // 5. Update cache if successful and determine cache update status
+        // Priority: NonZeroExitStatus > StdinDataExists > Success
         let cache_update_status = if let Some((track_result, cache_metadata)) =
             track_result_with_cache_metadata
         {
-            if result.exit_status.success() {
-                // Execution succeeded, attempt cache update
+            if !result.exit_status.success() {
+                // Priority 1: Non-zero exit status - don't update cache
+                CacheUpdateStatus::NotUpdated(CacheNotUpdatedReason::NonZeroExitStatus)
+            } else if result.stdin_had_data {
+                // Priority 2: Stdin had data - output may depend on input, unsafe to cache
+                CacheUpdateStatus::NotUpdated(CacheNotUpdatedReason::StdinDataExists)
+            } else {
+                // Success: attempt cache update
                 let fingerprint_ignores =
                     cache_metadata.spawn_fingerprint.fingerprint_ignores().map(|v| v.as_slice());
                 match PostRunFingerprint::create(
@@ -326,9 +333,6 @@ impl ExecutionContext<'_> {
                         return Err(ExecutionAborted);
                     }
                 }
-            } else {
-                // Execution failed with non-zero exit status, don't update cache
-                CacheUpdateStatus::NotUpdated(CacheNotUpdatedReason::NonZeroExitStatus)
             }
         } else {
             // Caching was disabled for this task