chore: adjust pagination for postgres database and flavor listing (#20)

* feat: implement pagination for database listing

* fix: change database_id attribute type from string to int64

* refactor: rename getDatabase to getDatabaseById for clarity

* fix: improve error handling for database not found scenario

* feat: add validation for database_id and name attributes; implement separate functions for fetching databases by ID and name

* feat: implement database client interface and update database fetching functions

* refactor: rename matcher to filter for clarity and update pagination logic

* feat: implement flavors retrieval with pagination and filtering support

* refactor: rename flavor import for consistency and clarity

* feat: add support for InstanceStatePending in wait handler logic

* refactor: simplify GetFlavorsRequest and GetFlavorsRequestExecute by removing pagination parameters

* refactor: improve readability of test cases by formatting function signatures and restructuring test runs

* refactor: remove pagination parameters from GetFlavorsRequest in test case

* refactor: simplify function signatures and improve readability in datasource and resource files

* refactor: add descriptions for user-related attributes in datasource schema

* refactor: enhance user resource schema with additional attributes and improve logging

* refactor: delete unused file

* refactor: standardize formatting and improve function naming for user resource management

* refactor: remove skip from TestMapFields and update roles initialization in resource tests

* fix: golangci lint issues

* fix: golangci lint issues again

* fix: golangci lint issues again

stackit/internal/services/postgresflexalpha/database/functions.go

@@ -0,0 +1,81 @@
+package postgresflexalpha
+
+import (
+	"context"
+	"fmt"
+
+	postgresflex "github.com/mhenselin/terraform-provider-stackitprivatepreview/pkg/postgresflexalpha"
+)
+
+// databaseClientReader represents the contract to listing databases from postgresflex.APIClient.
+type databaseClientReader interface {
+	ListDatabasesRequest(
+		ctx context.Context,
+		projectId string,
+		region string,
+		instanceId string,
+	) postgresflex.ApiListDatabasesRequestRequest
+}
+
+// getDatabaseById gets a database by its ID.
+func getDatabaseById(
+	ctx context.Context,
+	client databaseClientReader,
+	projectId, region, instanceId string,
+	databaseId int64,
+) (*postgresflex.ListDatabase, error) {
+	filter := func(db postgresflex.ListDatabase) bool {
+		return db.Id != nil && *db.Id == databaseId
+	}
+	return getDatabase(ctx, client, projectId, region, instanceId, filter)
+}
+
+// getDatabaseByName gets a database by its name.
+func getDatabaseByName(
+	ctx context.Context,
+	client databaseClientReader,
+	projectId, region, instanceId, databaseName string,
+) (*postgresflex.ListDatabase, error) {
+	filter := func(db postgresflex.ListDatabase) bool {
+		return db.Name != nil && *db.Name == databaseName
+	}
+	return getDatabase(ctx, client, projectId, region, instanceId, filter)
+}
+
+// getDatabase is a helper function to retrieve a database using a filter function.
+// Hint: The API does not have a GetDatabase endpoint, only ListDatabases
+func getDatabase(
+	ctx context.Context,
+	client databaseClientReader,
+	projectId, region, instanceId string,
+	filter func(db postgresflex.ListDatabase) bool,
+) (*postgresflex.ListDatabase, error) {
+	if projectId == "" || region == "" || instanceId == "" {
+		return nil, fmt.Errorf("all parameters (project, region, instance) are required")
+	}
+
+	const pageSize = 25
+
+	for page := int64(1); ; page++ {
+		res, err := client.ListDatabasesRequest(ctx, projectId, region, instanceId).
+			Page(page).Size(pageSize).Sort(postgresflex.DATABASESORT_INDEX_ASC).Execute()
+		if err != nil {
+			return nil, fmt.Errorf("requesting database list (page %d): %w", page, err)
+		}
+
+		// If the API returns no databases, we have reached the end of the list.
+		if res.Databases == nil || len(*res.Databases) == 0 {
+			break
+		}
+
+		// Iterate over databases to find a match
+		for _, db := range *res.Databases {
+			if filter(db) {
+				foundDb := db
+				return &foundDb, nil
+			}
+		}
+	}
+
+	return nil, fmt.Errorf("database not found for instance %s", instanceId)
+}