Add TempDir option to UnixSocketConfig (#282)

* Add TempDir option to UnixSocketConfig. Allows clients to specify
  a folder where plugin-specific Unix socket directories should be
  created. Still defaults to $TMPDIR (if set) or /tmp.
* Improve UnixSocketConfig field names and comments
* Document exported Unix socket environment variables

client.go

@@ -252,16 +252,15 @@ type UnixSocketConfig struct {
 	// client process must be a member of this group or chown will fail.
 	Group string
 
-	// The directory to create Unix sockets in. Internally managed by go-plugin
-	// and deleted when the plugin is killed.
-	directory string
-}
-
-func unixSocketConfigFromEnv() UnixSocketConfig {
-	return UnixSocketConfig{
-		Group:     os.Getenv(EnvUnixSocketGroup),
-		directory: os.Getenv(EnvUnixSocketDir),
-	}
+	// TempDir specifies the base directory to use when creating a plugin-specific
+	// temporary directory. It is expected to already exist and be writable. If
+	// not set, defaults to the directory chosen by os.MkdirTemp.
+	TempDir string
+
+	// The directory to create Unix sockets in. Internally created and managed
+	// by go-plugin and deleted when the plugin is killed. Will be created
+	// inside TempDir if specified.
+	socketDir string
 }
 
 // ReattachConfig is used to configure a client to reattach to an
@@ -467,7 +466,7 @@ func (c *Client) Kill() {
 	c.l.Lock()
 	runner := c.runner
 	addr := c.address
-	hostSocketDir := c.unixSocketCfg.directory
+	hostSocketDir := c.unixSocketCfg.socketDir
 	c.l.Unlock()
 
 	// If there is no runner or ID, there is nothing to kill.
@@ -652,7 +651,7 @@ func (c *Client) Start() (addr net.Addr, err error) {
 	}
 
 	if c.config.UnixSocketConfig != nil {
-		c.unixSocketCfg.Group = c.config.UnixSocketConfig.Group
+		c.unixSocketCfg = *c.config.UnixSocketConfig
 	}
 
 	if c.unixSocketCfg.Group != "" {
@@ -662,22 +661,22 @@ func (c *Client) Start() (addr net.Addr, err error) {
 	var runner runner.Runner
 	switch {
 	case c.config.RunnerFunc != nil:
-		c.unixSocketCfg.directory, err = os.MkdirTemp("", "plugin-dir")
+		c.unixSocketCfg.socketDir, err = os.MkdirTemp(c.unixSocketCfg.TempDir, "plugin-dir")
 		if err != nil {
 			return nil, err
 		}
 		// os.MkdirTemp creates folders with 0o700, so if we have a group
 		// configured we need to make it group-writable.
 		if c.unixSocketCfg.Group != "" {
-			err = setGroupWritable(c.unixSocketCfg.directory, c.unixSocketCfg.Group, 0o770)
+			err = setGroupWritable(c.unixSocketCfg.socketDir, c.unixSocketCfg.Group, 0o770)
 			if err != nil {
 				return nil, err
 			}
 		}
-		cmd.Env = append(cmd.Env, fmt.Sprintf("%s=%s", EnvUnixSocketDir, c.unixSocketCfg.directory))
-		c.logger.Trace("created temporary directory for unix sockets", "dir", c.unixSocketCfg.directory)
+		cmd.Env = append(cmd.Env, fmt.Sprintf("%s=%s", EnvUnixSocketDir, c.unixSocketCfg.socketDir))
+		c.logger.Trace("created temporary directory for unix sockets", "dir", c.unixSocketCfg.socketDir)
 
-		runner, err = c.config.RunnerFunc(c.logger, cmd, c.unixSocketCfg.directory)
+		runner, err = c.config.RunnerFunc(c.logger, cmd, c.unixSocketCfg.socketDir)
 		if err != nil {
 			return nil, err
 		}

client_unix_test.go

@@ -11,6 +11,7 @@ import (
 	"os"
 	"os/exec"
 	"os/user"
+	"path/filepath"
 	"runtime"
 	"syscall"
 	"testing"
@@ -29,6 +30,11 @@ func TestSetGroup(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	baseTempDir := t.TempDir()
+	baseTempDir, err = filepath.EvalSymlinks(baseTempDir)
+	if err != nil {
+		t.Fatal(err)
+	}
 	for name, tc := range map[string]struct {
 		group string
 	}{
@@ -41,14 +47,18 @@ func TestSetGroup(t *testing.T) {
 				HandshakeConfig: testHandshake,
 				Plugins:         testPluginMap,
 				UnixSocketConfig: &UnixSocketConfig{
-					Group: tc.group,
+					Group:   tc.group,
+					TempDir: baseTempDir,
 				},
 				RunnerFunc: func(l hclog.Logger, cmd *exec.Cmd, tmpDir string) (runner.Runner, error) {
 					// Run tests inside the RunnerFunc to ensure we don't race
 					// with the code that deletes tmpDir when the client fails
 					// to start properly.
 
 					// Test that it creates a directory with the proper owners and permissions.
+					if filepath.Dir(tmpDir) != baseTempDir {
+						t.Errorf("Expected base TempDir to be %s, but tmpDir was %s", baseTempDir, tmpDir)
+					}
 					info, err := os.Lstat(tmpDir)
 					if err != nil {
 						t.Fatal(err)

constants.go

@@ -4,6 +4,11 @@
 package plugin
 
 const (
-	EnvUnixSocketDir   = "PLUGIN_UNIX_SOCKET_DIR"
+	// EnvUnixSocketDir specifies the directory that _plugins_ should create unix
+	// sockets in. Does not affect client behavior.
+	EnvUnixSocketDir = "PLUGIN_UNIX_SOCKET_DIR"
+
+	// EnvUnixSocketGroup specifies the owning, writable group to set for Unix
+	// sockets created by _plugins_. Does not affect client behavior.
 	EnvUnixSocketGroup = "PLUGIN_UNIX_SOCKET_GROUP"
 )

server.go

@@ -134,6 +134,13 @@ type ServeTestConfig struct {
 	SyncStdio bool
 }
 
+func unixSocketConfigFromEnv() UnixSocketConfig {
+	return UnixSocketConfig{
+		Group:     os.Getenv(EnvUnixSocketGroup),
+		socketDir: os.Getenv(EnvUnixSocketDir),
+	}
+}
+
 // protocolVersion determines the protocol version and plugin set to be used by
 // the server. In the event that there is no suitable version, the last version
 // in the config is returned leaving the client to report the incompatibility.
@@ -547,7 +554,7 @@ func serverListener_tcp() (net.Listener, error) {
 }
 
 func serverListener_unix(unixSocketCfg UnixSocketConfig) (net.Listener, error) {
-	tf, err := os.CreateTemp(unixSocketCfg.directory, "plugin")
+	tf, err := os.CreateTemp(unixSocketCfg.socketDir, "plugin")
 	if err != nil {
 		return nil, err
 	}