Add support for GCP IAM impersonation

go.mod

@@ -16,6 +16,7 @@ require (
 	gocloud.dev v0.34.0
 	golang.org/x/net v0.26.0
 	golang.org/x/oauth2 v0.10.0
+	google.golang.org/api v0.134.0
 )
 
 require (
@@ -91,7 +92,6 @@ require (
 	golang.org/x/sys v0.21.0 // indirect
 	golang.org/x/text v0.16.0 // indirect
 	golang.org/x/time v0.3.0 // indirect
-	google.golang.org/api v0.134.0 // indirect
 	google.golang.org/appengine v1.6.7 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20230731193218-e0aa005b6bdf // indirect
 	google.golang.org/grpc v1.57.0 // indirect

postgresql/config.go

@@ -12,9 +12,12 @@ import (
 
 	"github.com/blang/semver"
 	_ "github.com/lib/pq" // PostgreSQL db
+	"gocloud.dev/gcp"
+	"gocloud.dev/gcp/cloudsql"
 	"gocloud.dev/postgres"
 	_ "gocloud.dev/postgres/awspostgres"
-	_ "gocloud.dev/postgres/gcppostgres"
+	"gocloud.dev/postgres/gcppostgres"
+	"google.golang.org/api/impersonate"
 )
 
 type featureName uint
@@ -157,21 +160,22 @@ type ClientCertificateConfig struct {
 
 // Config - provider config
 type Config struct {
-	Scheme            string
-	Host              string
-	Port              int
-	Username          string
-	Password          string
-	DatabaseUsername  string
-	Superuser         bool
-	SSLMode           string
-	ApplicationName   string
-	Timeout           int
-	ConnectTimeoutSec int
-	MaxConns          int
-	ExpectedVersion   semver.Version
-	SSLClientCert     *ClientCertificateConfig
-	SSLRootCertPath   string
+	Scheme                          string
+	Host                            string
+	Port                            int
+	Username                        string
+	Password                        string
+	DatabaseUsername                string
+	Superuser                       bool
+	SSLMode                         string
+	ApplicationName                 string
+	Timeout                         int
+	ConnectTimeoutSec               int
+	MaxConns                        int
+	ExpectedVersion                 semver.Version
+	SSLClientCert                   *ClientCertificateConfig
+	SSLRootCertPath                 string
+	GCPIAMImpersonateServiceAccount string
 }
 
 // Client struct holding connection string
@@ -280,6 +284,25 @@ func (c *Client) Connect() (*DBConnection, error) {
 		var err error
 		if c.config.Scheme == "postgres" {
 			db, err = sql.Open(proxyDriverName, dsn)
+		} else if c.config.Scheme == "gcppostgres" && c.config.GCPIAMImpersonateServiceAccount != "" {
+			ts, err := impersonate.CredentialsTokenSource(context.Background(), impersonate.CredentialsConfig{
+				TargetPrincipal: c.config.GCPIAMImpersonateServiceAccount,
+				Scopes:          []string{"https://www.googleapis.com/auth/sqlservice.admin"},
+			})
+			if err != nil {
+				return nil, fmt.Errorf("Error creating token source with service account impersonation of %s: %w", c.config.GCPIAMImpersonateServiceAccount, err)
+			}
+			client, err := gcp.NewHTTPClient(gcp.DefaultTransport(), ts)
+			if err != nil {
+				return nil, fmt.Errorf("Error creating HTTP client with service account impersonation of %s: %w", c.config.GCPIAMImpersonateServiceAccount, err)
+			}
+			certSource := cloudsql.NewCertSourceWithIAM(client, ts)
+			opener := gcppostgres.URLOpener{CertSource: certSource}
+			dbURL, err := url.Parse(dsn)
+			if err != nil {
+				return nil, fmt.Errorf("Error parsing connection string: %w", err)
+			}
+			db, err = opener.OpenPostgresURL(context.Background(), dbURL)
 		} else {
 			db, err = postgres.Open(context.Background(), dsn)
 		}

postgresql/provider.go

@@ -3,9 +3,10 @@ package postgresql
 import (
 	"context"
 	"fmt"
+	"os"
+
 	"github.com/Azure/azure-sdk-for-go/sdk/azcore/policy"
 	"github.com/Azure/azure-sdk-for-go/sdk/azidentity"
-	"os"
 
 	"github.com/blang/semver"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema"
@@ -103,6 +104,13 @@ func Provider() *schema.Provider {
 				Description: "MS Azure tenant ID (see: https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/data-sources/client_config.html)",
 			},
 
+			"gcp_iam_impersonate_service_account": {
+				Type:        schema.TypeString,
+				Optional:    true,
+				Default:     "",
+				Description: "Service account to impersonate when using GCP IAM authentication.",
+			},
+
 			// Conection username can be different than database username with user name mapas (e.g.: in Azure)
 			// See https://www.postgresql.org/docs/current/auth-username-maps.html
 			"database_username": {
@@ -323,19 +331,20 @@ func providerConfigure(d *schema.ResourceData) (interface{}, error) {
 	}
 
 	config := Config{
-		Scheme:            d.Get("scheme").(string),
-		Host:              host,
-		Port:              port,
-		Username:          username,
-		Password:          password,
-		DatabaseUsername:  d.Get("database_username").(string),
-		Superuser:         d.Get("superuser").(bool),
-		SSLMode:           sslMode,
-		ApplicationName:   "Terraform provider",
-		ConnectTimeoutSec: d.Get("connect_timeout").(int),
-		MaxConns:          d.Get("max_connections").(int),
-		ExpectedVersion:   version,
-		SSLRootCertPath:   d.Get("sslrootcert").(string),
+		Scheme:                          d.Get("scheme").(string),
+		Host:                            host,
+		Port:                            port,
+		Username:                        username,
+		Password:                        password,
+		DatabaseUsername:                d.Get("database_username").(string),
+		Superuser:                       d.Get("superuser").(bool),
+		SSLMode:                         sslMode,
+		ApplicationName:                 "Terraform provider",
+		ConnectTimeoutSec:               d.Get("connect_timeout").(int),
+		MaxConns:                        d.Get("max_connections").(int),
+		ExpectedVersion:                 version,
+		SSLRootCertPath:                 d.Get("sslrootcert").(string),
+		GCPIAMImpersonateServiceAccount: d.Get("gcp_iam_impersonate_service_account").(string),
 	}
 
 	if value, ok := d.GetOk("clientcert"); ok {

website/docs/index.html.markdown

@@ -214,7 +214,28 @@ To enable GoCloud for GCP SQL, set `scheme` to `gcppostgres` and `host` to the c
 For GCP, GoCloud also requires the `GOOGLE_APPLICATION_CREDENTIALS` environment variable to be set to the service account credentials file.
 These credentials can be created here: https://console.cloud.google.com/iam-admin/serviceaccounts
 
-See also: https://cloud.google.com/docs/authentication/production
+In addition, the provider supports service account impersonation with the `gcp_iam_impersonate_service_account` option. You must ensure:
+
+- the IAM database user has sufficient permissions to connect to the database, e.g., `roles/cloudsql.instanceUser`
+- The principle (IAM user or IAM service accont) behind the `GOOGLE_APPLICATION_CREDENTIALS` has sufficient permissions to impersonate the provided service account. Learn more from [roles for service account authentication](https://cloud.google.com/iam/docs/service-account-permissions).
+
+```hcl
+provider "postgresql" {
+  scheme   = "gcppostgres"
+  host     = "test-project/europe-west3/test-instance"
+  port     = 5432
+
+  username                            = "service_account_id@$project_id.iam"
+  gcp_iam_impersonate_service_account = "service_account_id@$project_id.iam.gserviceaccount.com"
+
+  superuser = false
+}
+```
+
+See also: 
+
+- https://cloud.google.com/docs/authentication/production
+- https://cloud.google.com/sql/docs/postgres/iam-logins
 
 ---
 **Note**