Allow accepting remote connections

Close #3498

  # FZF_API_KEY is required for a non-localhost listen address
  FZF_API_KEY=xxx fzf --listen 0.0.0.0:6266

5 files changed, 72 insertions, 40 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a45122cc..e561df6a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,12 @@ CHANGELOG
       fzf --preview='fzf-preview.sh {}'
       ```
 - (Experimental) Sixel and Kitty image support now also available on Windows
+- HTTP server can be configured to accept remote connections
+  ```sh
+  # FZF_API_KEY is required for a non-localhost listen address
+  export FZF_API_KEY="$(head -c 32 /dev/urandom | base64)"
+  fzf --listen 0.0.0.0:6266
+  ```
 - Bug fixes
 
 0.43.0
diff --git a/man/man1/fzf.1 b/man/man1/fzf.1
index 309be90e..66bc678b 100644
--- a/man/man1/fzf.1
+++ b/man/man1/fzf.1
@@ -793,14 +793,14 @@ ncurses finder only after the input stream is complete.
 e.g. \fBfzf --multi | fzf --sync\fR
 .RE
 .TP
-.B "--listen[=HTTP_PORT]"
-Start HTTP server on the given port. It allows external processes to send
-actions to perform via POST method. If the port number is omitted or given as
-0, fzf will choose the port automatically and export it as \fBFZF_PORT\fR
-environment variable to the child processes started via \fBexecute\fR and
-\fBexecute-silent\fR actions. 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.
+.B "--listen[=[ADDR:]PORT]"
+Start HTTP server and listen on the given address. It allows external processes
+to send actions to perform via POST method. 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. 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.
+\fBFZF_API_KEY\fR is required for a non-localhost listen address.
 
 e.g.
      \fB# Start HTTP server on port 6266
@@ -812,8 +812,12 @@ e.g.
      # Send action to the server
      curl -XPOST localhost:6266 -d 'reload(seq 100)+change-prompt(hundred> )'
 
-     # Start HTTP server on port 6266 and send an authenticated action
+     # Start HTTP server on port 6266 with remote connections allowed
+     # * Listening on non-localhost address requires using an API key
      export FZF_API_KEY="$(head -c 32 /dev/urandom | base64)"
+     fzf --listen 0.0.0.0:6266
+
+     # Send an authenticated action
      curl -XPOST localhost:6266 -H "x-api-key: $FZF_API_KEY" -d 'change-query(yo)'
 
      # Choose port automatically and export it as $FZF_PORT to the child process
diff --git a/src/options.go b/src/options.go
index b4f74e95..4176292f 100644
--- a/src/options.go
+++ b/src/options.go
@@ -118,7 +118,7 @@ const usage = `usage: fzf [options]
     --read0                Read input delimited by ASCII NUL characters
     --print0               Print output delimited by ASCII NUL characters
     --sync                 Synchronous search for multi-staged filtering
-    --listen[=HTTP_PORT]   Start HTTP server to receive actions (POST /)
+    --listen[=[ADDR:]PORT] Start HTTP server to receive actions (POST /)
     --version              Display version information and exit
 
   Environment variables
@@ -334,7 +334,7 @@ type Options struct {
 	PreviewLabel labelOpts
 	Unicode      bool
 	Tabstop      int
-	ListenPort   *int
+	ListenAddr   *string
 	ClearOnExit  bool
 	Version      bool
 }
@@ -1833,10 +1833,13 @@ func parseOptions(opts *Options, allArgs []string) {
 		case "--tabstop":
 			opts.Tabstop = nextInt(allArgs, &i, "tab stop required")
 		case "--listen":
-			port := optionalNumeric(allArgs, &i, 0)
-			opts.ListenPort = &port
+			given, addr := optionalNextString(allArgs, &i)
+			if !given {
+				addr = defaultListenAddr
+			}
+			opts.ListenAddr = &addr
 		case "--no-listen":
-			opts.ListenPort = nil
+			opts.ListenAddr = nil
 		case "--clear":
 			opts.ClearOnExit = true
 		case "--no-clear":
@@ -1927,8 +1930,7 @@ func parseOptions(opts *Options, allArgs []string) {
 			} else if match, value := optString(arg, "--tabstop="); match {
 				opts.Tabstop = atoi(value)
 			} else if match, value := optString(arg, "--listen="); match {
-				port := atoi(value)
-				opts.ListenPort = &port
+				opts.ListenAddr = &value
 			} else if match, value := optString(arg, "--hscroll-off="); match {
 				opts.HscrollOff = atoi(value)
 			} else if match, value := optString(arg, "--scroll-off="); match {
@@ -1958,10 +1960,6 @@ func parseOptions(opts *Options, allArgs []string) {
 		errorExit("tab stop must be a positive integer")
 	}
 
-	if opts.ListenPort != nil && (*opts.ListenPort < 0 || *opts.ListenPort > 65535) {
-		errorExit("invalid listen port")
-	}
-
 	if len(opts.JumpLabels) == 0 {
 		errorExit("empty jump labels")
 	}
diff --git a/src/server.go b/src/server.go
index 8fc605ac..a89c5e08 100644
--- a/src/server.go
+++ b/src/server.go
@@ -26,12 +26,13 @@ type getParams struct {
 }
 
 const (
-	crlf             = "\r\n"
-	httpOk           = "HTTP/1.1 200 OK" + crlf
-	httpBadRequest   = "HTTP/1.1 400 Bad Request" + crlf
-	httpUnauthorized = "HTTP/1.1 401 Unauthorized" + crlf
-	httpReadTimeout  = 10 * time.Second
-	maxContentLength = 1024 * 1024
+	crlf              = "\r\n"
+	httpOk            = "HTTP/1.1 200 OK" + crlf
+	httpBadRequest    = "HTTP/1.1 400 Bad Request" + crlf
+	httpUnauthorized  = "HTTP/1.1 401 Unauthorized" + crlf
+	httpReadTimeout   = 10 * time.Second
+	maxContentLength  = 1024 * 1024
+	defaultListenAddr = "localhost:0"
 )
 
 type httpServer struct {
@@ -40,30 +41,52 @@ type httpServer struct {
 	responseChannel chan string
 }
 
-func startHttpServer(port int, actionChannel chan []*action, responseChannel chan string) (error, int) {
-	if port < 0 {
-		return nil, port
+func parseListenAddress(address string) (error, string, int) {
+	parts := strings.SplitN(address, ":", 3)
+	if len(parts) == 1 {
+		parts = []string{"localhost", parts[0]}
 	}
+	if len(parts) != 2 {
+		return fmt.Errorf("invalid listen address: %s", address), "", 0
+	}
+	portStr := parts[len(parts)-1]
+	port, err := strconv.Atoi(portStr)
+	if err != nil || port < 0 || port > 65535 {
+		return fmt.Errorf("invalid listen port: %s", portStr), "", 0
+	}
+	if len(parts[0]) == 0 {
+		parts[0] = "localhost"
+	}
+	return nil, parts[0], port
+}
 
-	listener, err := net.Listen("tcp", fmt.Sprintf("localhost:%d", port))
+func startHttpServer(address string, actionChannel chan []*action, responseChannel chan string) (error, int) {
+	err, host, port := parseListenAddress(address)
+	if err != nil {
+		return err, port
+	}
+
+	apiKey := os.Getenv("FZF_API_KEY")
+	if host != "localhost" && host != "127.0.0.1" && len(apiKey) == 0 {
+		return fmt.Errorf("FZF_API_KEY is required for remote access"), port
+	}
+	listener, err := net.Listen("tcp", fmt.Sprintf("%s:%d", host, port))
 	if err != nil {
-		return fmt.Errorf("port not available: %d", port), port
+		return fmt.Errorf("failed to listen on %s", address), port
 	}
 	if port == 0 {
 		addr := listener.Addr().String()
-		parts := strings.SplitN(addr, ":", 2)
+		parts := strings.Split(addr, ":")
 		if len(parts) < 2 {
 			return fmt.Errorf("cannot extract port: %s", addr), port
 		}
-		var err error
-		port, err = strconv.Atoi(parts[1])
-		if err != nil {
+		if port, err := strconv.Atoi(parts[len(parts)-1]); err != nil {
 			return err, port
 		}
 	}
 
 	server := httpServer{
-		apiKey:          []byte(os.Getenv("FZF_API_KEY")),
+		apiKey:          []byte(apiKey),
 		actionChannel:   actionChannel,
 		responseChannel: responseChannel,
 	}
diff --git a/src/terminal.go b/src/terminal.go
index e4ff304c..326c07cb 100644
--- a/src/terminal.go
+++ b/src/terminal.go
@@ -235,6 +235,7 @@ type Terminal struct {
 	margin             [4]sizeSpec
 	padding            [4]sizeSpec
 	unicode            bool
+	listenAddr         *string
 	listenPort         *int
 	borderShape        tui.BorderShape
 	cleanExit          bool
@@ -586,7 +587,7 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 	}
 	var previewBox *util.EventBox
 	// We need to start previewer if HTTP server is enabled even when --preview option is not specified
-	if len(opts.Preview.command) > 0 || hasPreviewAction(opts) || opts.ListenPort != nil {
+	if len(opts.Preview.command) > 0 || hasPreviewAction(opts) || opts.ListenAddr != nil {
 		previewBox = util.NewEventBox()
 	}
 	var renderer tui.Renderer
@@ -659,7 +660,7 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 		margin:             opts.Margin,
 		padding:            opts.Padding,
 		unicode:            opts.Unicode,
-		listenPort:         opts.ListenPort,
+		listenAddr:         opts.ListenAddr,
 		borderShape:        opts.BorderShape,
 		borderWidth:        1,
 		borderLabel:        nil,
@@ -748,8 +749,8 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 
 	_, t.hasLoadActions = t.keymap[tui.Load.AsEvent()]
 
-	if t.listenPort != nil {
-		err, port := startHttpServer(*t.listenPort, t.serverInputChan, t.serverOutputChan)
+	if t.listenAddr != nil {
+		err, port := startHttpServer(*t.listenAddr, t.serverInputChan, t.serverOutputChan)
 		if err != nil {
 			errorExit(err.Error())
 		}