s3: add s3:ExistingObjectTag condition support for bucket policies (#7677)

* s3: add s3:ExistingObjectTag condition support in policy engine

Add support for s3:ExistingObjectTag/<tag-key> condition keys in bucket
policies, allowing access control based on object tags.

Changes:
- Add ObjectEntry field to PolicyEvaluationArgs (entry.Extended metadata)
- Update EvaluateConditions to handle s3:ExistingObjectTag/<key> format
- Extract tag value from entry metadata using X-Amz-Tagging-<key> prefix

This enables policies like:
{
  "Condition": {
    "StringEquals": {
      "s3:ExistingObjectTag/status": ["public"]
    }
  }
}

Fixes: https://github.com/seaweedfs/seaweedfs/issues/7447

* s3: update EvaluatePolicy to accept object entry for tag conditions

Update BucketPolicyEngine.EvaluatePolicy to accept objectEntry parameter
(entry.Extended metadata) for evaluating tag-based policy conditions.

Changes:
- Add objectEntry parameter to EvaluatePolicy method
- Update callers in auth_credentials.go and s3api_bucket_handlers.go
- Pass nil for objectEntry in auth layer (entry fetched later in handlers)

For tag-based conditions to work, handlers should call EvaluatePolicy
with the object's entry.Extended after fetching the entry from filer.

* s3: add tests for s3:ExistingObjectTag policy conditions

Add comprehensive tests for object tag-based policy conditions:

- TestExistingObjectTagCondition: Basic tag matching scenarios
  - Matching/non-matching tag values
  - Missing tags, no tags, empty tags
  - Multiple tags with one matching

- TestExistingObjectTagConditionMultipleTags: Multiple tag conditions
  - Both tags match
  - Only one tag matches

- TestExistingObjectTagDenyPolicy: Deny policies with tag conditions
  - Default allow without tag
  - Deny when specific tag present

* s3: document s3:ExistingObjectTag support and feature status

Update policy engine documentation:

- Add s3:ExistingObjectTag/<tag-key> to supported condition keys
- Add 'Object Tag-Based Access Control' section with examples
- Add 'Feature Status' section with implemented and planned features

Planned features for future implementation:
- s3:RequestObjectTag/<key>
- s3:RequestObjectTagKeys
- s3:x-amz-server-side-encryption
- Cross-account access

* Implement tag-based policy re-check in handlers

- Add checkPolicyWithEntry helper to S3ApiServer for handlers to re-check
  policy after fetching object entry (for s3:ExistingObjectTag conditions)
- Add HasPolicyForBucket method to policy engine for efficient check
- Integrate policy re-check in GetObjectHandler after entry is fetched
- Integrate policy re-check in HeadObjectHandler after entry is fetched
- Update auth_credentials.go comments to explain two-phase evaluation
- Update documentation with supported operations for tag-based conditions

This implements 'Approach 1' where handlers re-check the policy with
the object entry after fetching it, allowing tag-based conditions to
be properly evaluated.

* Add integration tests for s3:ExistingObjectTag conditions

- Add TestCheckPolicyWithEntry: tests checkPolicyWithEntry helper with various
  tag scenarios (matching tags, non-matching tags, empty entry, nil entry)
- Add TestCheckPolicyWithEntryNoPolicyForBucket: tests early return when no policy
- Add TestCheckPolicyWithEntryNilPolicyEngine: tests nil engine handling
- Add TestCheckPolicyWithEntryDenyPolicy: tests deny policies with tag conditions
- Add TestHasPolicyForBucket: tests HasPolicyForBucket method

These tests cover the Phase 2 policy evaluation with object entry metadata,
ensuring tag-based conditions are properly evaluated.

* Address code review nitpicks

- Remove unused extractObjectTags placeholder function (engine.go)
- Add clarifying comment about s3:ExistingObjectTag/<key> evaluation
- Consolidate duplicate tag-based examples in README
- Factor out tagsToEntry helper to package level in tests

* Address code review feedback

- Fix unsafe type assertions in GetObjectHandler and HeadObjectHandler
  when getting identity from context (properly handle type assertion failure)
- Extract getConditionContextValue helper to eliminate duplicated logic
  between EvaluateConditions and EvaluateConditionsLegacy
- Ensure consistent handling of missing condition keys (always return
  empty slice)

* Fix GetObjectHandler to match HeadObjectHandler pattern

Add safety check for nil objectEntryForSSE before tag-based policy
evaluation, ensuring tag-based conditions are always evaluated rather
than silently skipped if entry is unexpectedly nil.

Addresses review comment from Copilot.

* Fix HeadObject action name in docs for consistency

Change 'HeadObject' to 's3:HeadObject' to match other action names.

* Extract recheckPolicyWithObjectEntry helper to reduce duplication

Move the repeated identity extraction and policy re-check logic from
GetObjectHandler and HeadObjectHandler into a shared helper method.

* Add validation for empty tag key in s3:ExistingObjectTag condition

Prevent potential issues with malformed policies containing
s3:ExistingObjectTag/ (empty tag key after slash).

weed/s3api/s3api_bucket_policy_engine.go

@@ -87,12 +87,27 @@ func (bpe *BucketPolicyEngine) DeleteBucketPolicy(bucket string) error {
 	return bpe.engine.DeleteBucketPolicy(bucket)
 }
 
+// HasPolicyForBucket checks if a bucket has a policy configured
+func (bpe *BucketPolicyEngine) HasPolicyForBucket(bucket string) bool {
+	return bpe.engine.HasPolicyForBucket(bucket)
+}
+
 // EvaluatePolicy evaluates whether an action is allowed by bucket policy
-// Returns: (allowed bool, evaluated bool, error)
-// - allowed: whether the policy allows the action
-// - evaluated: whether a policy was found and evaluated (false = no policy exists)
-// - error: any error during evaluation
-func (bpe *BucketPolicyEngine) EvaluatePolicy(bucket, object, action, principal string) (allowed bool, evaluated bool, err error) {
+//
+// Parameters:
+//   - bucket: the bucket name
+//   - object: the object key (can be empty for bucket-level operations)
+//   - action: the action being performed (e.g., "Read", "Write")
+//   - principal: the principal ARN or identifier
+//   - r: the HTTP request (optional, used for condition evaluation and action resolution)
+//   - objectEntry: the object's metadata from entry.Extended (can be nil at auth time,
+//     should be passed when available for tag-based conditions like s3:ExistingObjectTag)
+//
+// Returns:
+//   - allowed: whether the policy allows the action
+//   - evaluated: whether a policy was found and evaluated (false = no policy exists)
+//   - error: any error during evaluation
+func (bpe *BucketPolicyEngine) EvaluatePolicy(bucket, object, action, principal string, r *http.Request, objectEntry map[string][]byte) (allowed bool, evaluated bool, err error) {
 	// Validate required parameters
 	if bucket == "" {
 		return false, false, fmt.Errorf("bucket cannot be empty")
@@ -101,19 +116,22 @@ func (bpe *BucketPolicyEngine) EvaluatePolicy(bucket, object, action, principal
 		return false, false, fmt.Errorf("action cannot be empty")
 	}
 
-	// Convert action to S3 action format using base mapping (no HTTP context available)
-	s3Action := mapBaseActionToS3Format(action)
+	// Convert action to S3 action format
+	// ResolveS3Action handles nil request internally (falls back to mapBaseActionToS3Format)
+	s3Action := ResolveS3Action(r, action, bucket, object)
 
 	// Build resource ARN
 	resource := buildResourceARN(bucket, object)
 
-	glog.V(4).Infof("EvaluatePolicy: bucket=%s, resource=%s, action=%s, principal=%s", bucket, resource, s3Action, principal)
+	glog.V(4).Infof("EvaluatePolicy: bucket=%s, resource=%s, action=%s, principal=%s",
+		bucket, resource, s3Action, principal)
 
 	// Evaluate using the policy engine
 	args := &policy_engine.PolicyEvaluationArgs{
-		Action:    s3Action,
-		Resource:  resource,
-		Principal: principal,
+		Action:      s3Action,
+		Resource:    resource,
+		Principal:   principal,
+		ObjectEntry: objectEntry,
 	}
 
 	result := bpe.engine.EvaluatePolicy(bucket, args)
@@ -133,53 +151,3 @@ func (bpe *BucketPolicyEngine) EvaluatePolicy(bucket, object, action, principal
 		return false, false, fmt.Errorf("unknown policy result: %v", result)
 	}
 }
-
-// EvaluatePolicyWithContext evaluates whether an action is allowed by bucket policy using HTTP request context
-// This version uses the HTTP request to determine the actual S3 action more accurately
-func (bpe *BucketPolicyEngine) EvaluatePolicyWithContext(bucket, object, action, principal string, r *http.Request) (allowed bool, evaluated bool, err error) {
-	// Validate required parameters
-	if bucket == "" {
-		return false, false, fmt.Errorf("bucket cannot be empty")
-	}
-	if action == "" {
-		return false, false, fmt.Errorf("action cannot be empty")
-	}
-
-	// Convert action to S3 action format using request context
-	// ResolveS3Action handles nil request internally (falls back to mapBaseActionToS3Format)
-	s3Action := ResolveS3Action(r, action, bucket, object)
-
-	// Build resource ARN
-	resource := buildResourceARN(bucket, object)
-
-	glog.V(4).Infof("EvaluatePolicyWithContext: bucket=%s, resource=%s, action=%s (from %s), principal=%s",
-		bucket, resource, s3Action, action, principal)
-
-	// Evaluate using the policy engine
-	args := &policy_engine.PolicyEvaluationArgs{
-		Action:    s3Action,
-		Resource:  resource,
-		Principal: principal,
-	}
-
-	result := bpe.engine.EvaluatePolicy(bucket, args)
-
-	switch result {
-	case policy_engine.PolicyResultAllow:
-		glog.V(3).Infof("EvaluatePolicyWithContext: ALLOW - bucket=%s, action=%s, principal=%s", bucket, s3Action, principal)
-		return true, true, nil
-	case policy_engine.PolicyResultDeny:
-		glog.V(3).Infof("EvaluatePolicyWithContext: DENY - bucket=%s, action=%s, principal=%s", bucket, s3Action, principal)
-		return false, true, nil
-	case policy_engine.PolicyResultIndeterminate:
-		// No policy exists for this bucket
-		glog.V(4).Infof("EvaluatePolicyWithContext: INDETERMINATE (no policy) - bucket=%s", bucket)
-		return false, false, nil
-	default:
-		return false, false, fmt.Errorf("unknown policy result: %v", result)
-	}
-}
-
-// NOTE: The convertActionToS3Format wrapper has been removed for simplicity.
-// EvaluatePolicy and EvaluatePolicyWithContext now call ResolveS3Action or
-// mapBaseActionToS3Format directly, making the control flow more explicit.