Optimize VFS::touch and fix defects and race conditions.

test/src/unit-vfs.cc

@@ -614,6 +614,12 @@ TEST_CASE("VFS: test ls_with_sizes", "[vfs][ls-with-sizes]") {
   // Directories don't get a size
   REQUIRE(children[1].file_size() == 0);
 
+  // Touch does not overwrite an existing file.
+  require_tiledb_ok(vfs_ls.touch(URI(subdir_file)));
+  uint64_t size;
+  require_tiledb_ok(vfs_ls.file_size(URI(subdir_file), &size));
+  REQUIRE(size == 6);
+
   // Clean up
   require_tiledb_ok(vfs_ls.remove_dir(URI(path)));
 }

tiledb/api/c_api/vfs/vfs_api_external.h

@@ -640,7 +640,9 @@ TILEDB_EXPORT capi_return_t tiledb_vfs_fh_is_closed(
     tiledb_ctx_t* ctx, tiledb_vfs_fh_t* fh, int32_t* is_closed) TILEDB_NOEXCEPT;
 
 /**
- * Touches a file, i.e., creates a new empty file.
+ * Touches a file, i.e., creates a new empty file if it does not already exist.
+ *
+ * The access timestamps of the file are not guaranteed to change.
  *
  * **Example:**
  *

tiledb/sm/cpp_api/vfs.h

@@ -519,7 +519,8 @@ class VFS {
         ctx.ptr().get(), vfs_.get(), old_uri.c_str(), new_uri.c_str()));
   }
 
-  /** Touches a file with the input URI, i.e., creates a new empty file. */
+  /** Touches a file with the input URI, i.e., creates a new empty file if it
+   * does not already exist. */
   void touch(const std::string& uri) const {
     auto& ctx = ctx_.get();
     ctx.handle_error(

tiledb/sm/filesystem/azure.cc

@@ -860,21 +860,24 @@ Status Azure::touch(const URI& uri) const {
         "Cannot create file; URI is a directory: " + uri.to_string())));
   }
 
-  bool is_blob;
-  RETURN_NOT_OK(this->is_blob(uri, &is_blob));
-  if (is_blob) {
-    return Status::Ok();
-  }
-
   std::string container_name;
   std::string blob_path;
   RETURN_NOT_OK(parse_azure_uri(uri, &container_name, &blob_path));
 
   try {
+    ::Azure::Core::IO::MemoryBodyStream stream(nullptr, 0);
+    ::Azure::Storage::Blobs::UploadBlockBlobOptions options;
+    // Set condition that the object does not exist.
+    options.AccessConditions.IfNoneMatch = ::Azure::ETag::Any();
+
     c.GetBlobContainerClient(container_name)
         .GetBlockBlobClient(blob_path)
-        .UploadFrom(nullptr, 0);
+        .Upload(stream, options);
   } catch (const ::Azure::Storage::StorageException& e) {
+    if (e.StatusCode == ::Azure::Core::Http::HttpStatusCode::Conflict) {
+      // Blob already exists.
+      return Status::Ok();
+    }
     return LOG_STATUS(Status_AzureError(
         "Touch blob failed on: " + uri.to_string() + "; " + e.Message));
   }

tiledb/sm/filesystem/gcs.cc

@@ -769,10 +769,20 @@ Status GCS::touch(const URI& uri) const {
   RETURN_NOT_OK(parse_gcs_uri(uri, &bucket_name, &object_path));
 
   google::cloud::StatusOr<google::cloud::storage::ObjectMetadata>
-      object_metadata = client_->InsertObject(bucket_name, object_path, "");
+      object_metadata = client_->InsertObject(
+          bucket_name,
+          object_path,
+          "",
+          // An if-generation-match value of zero makes the request succeed only
+          // if the object does not exist.
+          // https://cloud.google.com/storage/docs/request-preconditions#special-case
+          google::cloud::storage::IfGenerationMatch(0));
 
   if (!object_metadata.ok()) {
-    const google::cloud::Status status = object_metadata.status();
+    const google::cloud::Status& status = object_metadata.status();
+    if (status.code() == google::cloud::StatusCode::kFailedPrecondition) {
+      return Status::Ok();
+    }
 
     return LOG_STATUS(Status_GCSError(std::string(
         "Touch object failed on: " + uri.to_string() + " (" + status.message() +

tiledb/sm/filesystem/win.cc

@@ -159,10 +159,6 @@ Status Win::create_dir(const std::string& path) const {
 }
 
 Status Win::touch(const std::string& filename) const {
-  if (is_file(filename)) {
-    return Status::Ok();
-  }
-
   HANDLE file_h = CreateFile(
       filename.c_str(),
       GENERIC_WRITE,
@@ -178,6 +174,10 @@ Status Win::touch(const std::string& filename) const {
   tiledb::common::ScopedExecutor onexit1(closefileonexit);
   if (file_h == INVALID_HANDLE_VALUE) {
     auto gle = GetLastError();
+    if (gle == ERROR_FILE_EXISTS) {
+      // Do not fail if the file already exists.
+      return Status::Ok();
+    }
     return LOG_STATUS(Status_IOError(
         std::string("Failed to create file '") + filename + " (" +
         get_last_error_msg(gle, "CreateFile") + ")'"));