Add Unix domain socket support for --listen

Close #4541

5 files changed, 86 insertions, 25 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1b3a4aa8..91b0450f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -124,6 +124,24 @@ This version includes a few minor updates to fzf's classic visual style:
 - Markers no longer use background colors.
 - The `--color base16` theme (alias: `16`) has been updated for better compatibility with both dark and light themes.
 
+### `--listen` now supports Unix domain sockets
+
+If an argument to `--listen` ends with `.sock`, fzf will listen on a Unix
+domain socket at the specified path.
+
+```sh
+fzf --listen /tmp/fzf.sock --no-tmux
+
+# GET
+curl --unix-socket /tmp/fzf.sock http
+
+# POST
+curl --unix-socket /tmp/fzf.sock http -d up
+```
+
+Note that any existing file at the given path will be removed before creating
+the socket, so avoid using an important file path.
+
 ### Added options
 
 #### `--gutter CHAR`
diff --git a/man/man1/fzf.1 b/man/man1/fzf.1
index 95f33fb6..b5135101 100644
--- a/man/man1/fzf.1
+++ b/man/man1/fzf.1
@@ -1133,19 +1133,25 @@ On Windows, the default value is \fBcmd /s/c\fR when \fB$SHELL\fR is not
 set.
 
 .TP
-.B "\-\-listen[=[ADDR:]PORT]" "\-\-listen\-unsafe[=[ADDR:]PORT]"
-Start HTTP server and listen on the given address. It allows external processes
-to send actions to perform via POST method.
+.B "\-\-listen[=SOCKET_PATH|[ADDR:]PORT]" "\-\-listen\-unsafe[=[ADDR:]PORT]"
+Start HTTP server and listen on the given address or Unix socket. It allows
+external processes to send actions to perform via POST method and query the
+program state via GET method. For the argument to be recognized as a socket
+path, it must have \fB.sock\fR extension.
 
 - If the port number is omitted or given as 0, fzf will automatically choose
-a port and export it as \fBFZF_PORT\fR environment variable to the child processes
+a port and export it as \fBFZF_PORT\fR environment variable to the child processes.
+
+- If a Unix socket path is given, fzf will create a Unix domain socket at the
+  given path. The existing file will be removed. The path to the socket file
+  is exported as \fBFZF_SOCK\fR environment variable.
 
 - If \fBFZF_API_KEY\fR environment variable is set, the server would require
-sending an API key with the same value in the \fBx\-api\-key\fR HTTP header
+  sending an API key with the same value in the \fBx\-api\-key\fR HTTP header.
 
-- \fBFZF_API_KEY\fR is required for a non-localhost listen address
+- \fBFZF_API_KEY\fR is required for a non-localhost listen address.
 
-- To allow remote process execution, use \fB\-\-listen\-unsafe\fR
+- To allow remote process execution, use \fB\-\-listen\-unsafe\fR.
 
 e.g.
      \fB# Start HTTP server on port 6266
@@ -1184,6 +1190,18 @@ e.g.
      '
      \fR
 
+Here is an example script that uses a Unix socket instead of a TCP port.
+
+    \fB
+    fzf --listen=/tmp/fzf.sock
+
+    # GET
+    curl --unix-socket /tmp/fzf.sock http
+
+    # POST
+    curl --unix-socket /tmp/fzf.sock http -d up
+    \fR
+
 .SS DIRECTORY TRAVERSAL
 .TP
 .B "\-\-walker=[file][,dir][,follow][,hidden]"
@@ -1373,6 +1391,8 @@ fzf exports the following environment variables to its child processes.
 .br
 .BR FZF_PORT "            Port number when \-\-listen option is used"
 .br
+.BR FZF_SOCK "            Unix socket path when \-\-listen option is used"
+.br
 .BR FZF_PREVIEW_TOP "     Top position of the preview window"
 .br
 .BR FZF_PREVIEW_LEFT "    Left position of the preview window"
diff --git a/src/options.go b/src/options.go
index a0b6f514..c1f75585 100644
--- a/src/options.go
+++ b/src/options.go
@@ -206,7 +206,9 @@ Usage: fzf [options]
 
   ADVANCED
     --with-shell=STR         Shell command and flags to start child processes with
-    --listen[=[ADDR:]PORT]   Start HTTP server to receive actions (POST /)
+    --listen[=SOCKET_PATH]   Start HTTP server to receive actions via Unix domain socket
+                             (Path should end with .sock)
+    --listen[=[ADDR:]PORT]   Start HTTP server to receive actions via TCP
                              (To allow remote process execution, use --listen-unsafe)
 
   DIRECTORY TRAVERSAL        (Only used when $FZF_DEFAULT_COMMAND is not set)
diff --git a/src/server.go b/src/server.go
index 5757e160..9f1200da 100644
--- a/src/server.go
+++ b/src/server.go
@@ -46,15 +46,20 @@ type httpServer struct {
 type listenAddress struct {
 	host string
 	port int
+	sock string
 }
 
 func (addr listenAddress) IsLocal() bool {
-	return addr.host == "localhost" || addr.host == "127.0.0.1"
+	return addr.host == "localhost" || addr.host == "127.0.0.1" || len(addr.sock) > 0
 }
 
-var defaultListenAddr = listenAddress{"localhost", 0}
+var defaultListenAddr = listenAddress{"localhost", 0, ""}
 
 func parseListenAddress(address string) (listenAddress, error) {
+	if strings.HasSuffix(address, ".sock") {
+		return listenAddress{"", 0, address}, nil
+	}
+
 	parts := strings.SplitN(address, ":", 3)
 	if len(parts) == 1 {
 		parts = []string{"localhost", parts[0]}
@@ -70,7 +75,7 @@ func parseListenAddress(address string) (listenAddress, error) {
 	if len(parts[0]) == 0 {
 		parts[0] = "localhost"
 	}
-	return listenAddress{parts[0], port}, nil
+	return listenAddress{parts[0], port, ""}, nil
 }
 
 func startHttpServer(address listenAddress, actionChannel chan []*action, getHandler func(getParams) string) (net.Listener, int, error) {
@@ -80,21 +85,32 @@ func startHttpServer(address listenAddress, actionChannel chan []*action, getHan
 	if !address.IsLocal() && len(apiKey) == 0 {
 		return nil, port, errors.New("FZF_API_KEY is required to allow remote access")
 	}
-	addrStr := fmt.Sprintf("%s:%d", host, port)
-	listener, err := net.Listen("tcp", addrStr)
-	if err != nil {
-		return nil, port, fmt.Errorf("failed to listen on %s", addrStr)
-	}
-	if port == 0 {
-		addr := listener.Addr().String()
-		parts := strings.Split(addr, ":")
-		if len(parts) < 2 {
-			return nil, port, fmt.Errorf("cannot extract port: %s", addr)
+
+	var listener net.Listener
+	var err error
+	if len(address.sock) > 0 {
+		os.Remove(address.sock)
+		listener, err = net.Listen("unix", address.sock)
+		if err != nil {
+			return nil, 0, fmt.Errorf("failed to listen on %s", address.sock)
 		}
-		var err error
-		port, err = strconv.Atoi(parts[len(parts)-1])
+	} else {
+		addrStr := fmt.Sprintf("%s:%d", host, port)
+		listener, err = net.Listen("tcp", addrStr)
 		if err != nil {
-			return nil, port, err
+			return nil, port, fmt.Errorf("failed to listen on %s", addrStr)
+		}
+		if port == 0 {
+			addr := listener.Addr().String()
+			parts := strings.Split(addr, ":")
+			if len(parts) < 2 {
+				return nil, port, fmt.Errorf("cannot extract port: %s", addr)
+			}
+			var err error
+			port, err = strconv.Atoi(parts[len(parts)-1])
+			if err != nil {
+				return nil, port, err
+			}
 		}
 	}
 
diff --git a/src/terminal.go b/src/terminal.go
index 9a014d14..cf95623c 100644
--- a/src/terminal.go
+++ b/src/terminal.go
@@ -1268,7 +1268,9 @@ func NewTerminal(opts *Options, eventBox *util.EventBox, executor *util.Executor
 			return nil, err
 		}
 		t.listener = listener
-		t.listenPort = &port
+		if port > 0 {
+			t.listenPort = &port
+		}
 	}
 
 	if t.hasStartActions {
@@ -1292,6 +1294,9 @@ func (t *Terminal) environForPreview() []string {
 
 func (t *Terminal) environImpl(forPreview bool) []string {
 	env := os.Environ()
+	if t.listenAddr != nil && len(t.listenAddr.sock) > 0 {
+		env = append(env, "FZF_SOCK="+t.listenAddr.sock)
+	}
 	if t.listenPort != nil {
 		env = append(env, fmt.Sprintf("FZF_PORT=%d", *t.listenPort))
 	}