[Bug]: Preserve region fallback for SQL dynamic S3

pkg/fileservice/aws_sdk_v2.go

@@ -25,6 +25,7 @@ import (
 	"math"
 	nethttp "net/http"
 	"net/url"
+	"os"
 	gotrace "runtime/trace"
 	"slices"
 	"strings"
@@ -1179,8 +1180,18 @@ func (a *AwsSDKv2) mapError(err error, path string) error {
 // explicitAWSConfig never consults process environment or shared configuration.
 // S3 and STS share this mode so role refresh cannot rediscover another principal.
 func explicitAWSConfig(o ObjectStorageArguments, provider aws.CredentialsProvider, client *nethttp.Client) aws.Config {
+	region := o.Region
+	if region == "" {
+		// SQL dynamic providers must not discover credentials from the server
+		// environment, but the region is non-secret configuration. Preserve the
+		// AWS SDK's environment-region fallback for existing S3-compatible URLs.
+		region = os.Getenv("AWS_REGION")
+		if region == "" {
+			region = os.Getenv("AWS_DEFAULT_REGION")
+		}
+	}
 	return aws.Config{
-		Region: o.Region, Credentials: provider, HTTPClient: client,
+		Region: region, Credentials: provider, HTTPClient: client,
 		Logger: logutil.GetS3Logger(), Retryer: func() aws.Retryer { return newAWSRetryer() },
 		RequestChecksumCalculation: aws.RequestChecksumCalculationWhenRequired,
 		ResponseChecksumValidation: aws.ResponseChecksumValidationWhenRequired,

pkg/fileservice/sql_file_access_test.go

@@ -173,6 +173,50 @@ func TestSQLFileCredentialsDoNotDiscoverServerIdentity(t *testing.T) {
 	require.Equal(t, 4, calls)
 }
 
+func TestSQLFileS3UsesEnvironmentRegionWhenNotSpecified(t *testing.T) {
+	t.Setenv("AWS_REGION", "us-east-2")
+	t.Setenv("AWS_DEFAULT_REGION", "us-west-1")
+	t.Setenv("AWS_ACCESS_KEY_ID", "ambient-key")
+	t.Setenv("AWS_SECRET_ACCESS_KEY", "ambient-secret")
+	old := httpRoundTripper
+	t.Cleanup(func() { httpRoundTripper = old })
+	calls := 0
+	httpRoundTripper = sqlCredentialTransport(func(r *http.Request) (*http.Response, error) {
+		calls++
+		assert.Equal(t, http.MethodHead, r.Method)
+		authorization := r.Header.Get("Authorization")
+		assert.Contains(t, authorization, "Credential=caller-key/")
+		assert.Contains(t, authorization, "/us-east-2/s3/aws4_request")
+		assert.NotContains(t, authorization, "ambient")
+		return &http.Response{
+			StatusCode: http.StatusOK,
+			Header:     http.Header{"Content-Length": []string{"0"}, "Last-Modified": []string{"Mon, 02 Jan 2006 15:04:05 GMT"}, "Etag": []string{`"owned"`}},
+			Body:       io.NopCloser(strings.NewReader("")),
+			Request:    r,
+		}, nil
+	})
+
+	ctx := WithSQLFileAccess(t.Context(), "tenant", "public")
+	spec := "s3-opts,endpoint=https://owned.invalid,bucket=owned-bucket-123456,key=caller-key,secret=caller-secret:object"
+	fs, path, err := GetForETL(ctx, nil, spec)
+	require.NoError(t, err)
+	defer fs.Close(ctx)
+	_, err = fs.StatFile(ctx, path)
+	require.NoError(t, err)
+	require.Equal(t, 1, calls)
+}
+
+func TestExplicitAWSConfigRegionFallback(t *testing.T) {
+	t.Setenv("AWS_REGION", "us-east-2")
+	t.Setenv("AWS_DEFAULT_REGION", "us-west-1")
+	require.Equal(t, "us-east-2", explicitAWSConfig(ObjectStorageArguments{}, nil, nil).Region)
+
+	t.Setenv("AWS_REGION", "")
+	require.Equal(t, "us-west-1", explicitAWSConfig(ObjectStorageArguments{}, nil, nil).Region)
+
+	require.Equal(t, "eu-west-1", explicitAWSConfig(ObjectStorageArguments{Region: "eu-west-1"}, nil, nil).Region)
+}
+
 func TestSQLFileRoleFailureCannotFallBack(t *testing.T) {
 	old := httpRoundTripper
 	t.Cleanup(func() { httpRoundTripper = old })