feat: add support for running on gcs

Usage.md

@@ -14,6 +14,10 @@ qv ./datasets/tpc-h-parquet/1/customer -s
 
 ## View data on GCS.
 
+### Configuration
+
+QV expects the environment variable 'GOOGLE_APPLICATION_CREDENTIALS' to exist and point to a file which contains google credentials.
+
 ```bash
 qv gs://datafusion-delta-testing/data/delta/COVID-19_NYT
 ```

src/main.rs

@@ -77,11 +77,21 @@ async fn main() -> Result<()> {
             .map_err(|e| DataFusionError::Execution(format!("Failed to parse url, {e}")))?;
         let gcs = build_gcs(&gcs_url).await?;
         let gcs_arc = Arc::new(gcs);
-        ctx.runtime_env().register_object_store(&gcs_url, gcs_arc);
+        ctx.runtime_env().register_object_store(&gcs_url, gcs_arc.clone());
 
         deltalake::gcp::register_handlers(None);
 
-        data_path
+        // add trailing slash to folder
+        if !data_path.ends_with('/') {
+            let path = Path::parse(gcs_url.path())?;
+            if gcs_arc.head(&path).await.is_err() {
+                format!("{data_path}/")
+            } else {
+                data_path
+            }
+        } else {
+            data_path
+        }
     } else {
         data_path
     };
@@ -396,7 +406,7 @@ async fn build_s3(url: &Url, sdk_config: &SdkConfig) -> Result<AmazonS3> {
 
 async fn build_gcs(gcs_url: &Url) -> Result<GoogleCloudStorage> {
     let google_application_credentials = env::var("GOOGLE_APPLICATION_CREDENTIALS")
-        .map_err(|e| DataFusionError::External(Box::new(e)))?;
+        .map_err(|_| DataFusionError::Execution(format!("Could not find GOOGLE_APPLICATION_CREDENTIALS environment variable")))?;
 
     let bucket_name = gcs_url.host_str().unwrap();