Omit port number in `--listen` for automatic port assignment

Close #3200

6 files changed, 77 insertions, 24 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 48c2c966..ba10cd1e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,20 @@
 CHANGELOG
 =========
 
+0.39.0
+------
+- If you use `--listen` option without a port number fzf will automatically
+  allocate an available port and export it as `$FZF_PORT` environment
+  variable.
+  ```sh
+  # Automatic port assignment
+  fzf --listen --bind 'start:execute-silent:echo $FZF_PORT > /tmp/fzf-port'
+
+  # Say hello
+  curl "localhost:$(cat /tmp/fzf-port)" -d 'preview:echo Hello, fzf is listening on $FZF_PORT.'
+  ```
+- Bug fixes
+
 0.38.0
 ------
 - New actions
diff --git a/man/man1/fzf.1 b/man/man1/fzf.1
index 6d6d49bd..a538a91c 100644
--- a/man/man1/fzf.1
+++ b/man/man1/fzf.1
@@ -738,9 +738,12 @@ ncurses finder only after the input stream is complete.
 e.g. \fBfzf --multi | fzf --sync\fR
 .RE
 .TP
-.B "--listen=HTTP_PORT"
+.B "--listen[=HTTP_PORT]"
 Start HTTP server on the given port. It allows external processes to send
-actions to perform via POST method.
+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.
 
 e.g.
      \fB# Start HTTP server on port 6266
@@ -748,6 +751,9 @@ e.g.
 
      # Send action to the server
      curl -XPOST localhost:6266 -d 'reload(seq 100)+change-prompt(hundred> )'
+
+     # Choose port automatically and export it as $FZF_PORT to the child process
+     fzf --listen --bind 'start:execute-silent:echo $FZF_PORT > /tmp/fzf-port'
      \fR
 .TP
 .B "--version"
diff --git a/src/options.go b/src/options.go
index 9be29107..f2de1d75 100644
--- a/src/options.go
+++ b/src/options.go
@@ -116,7 +116,7 @@ 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[=HTTP_PORT]   Start HTTP server to receive actions (POST /)
     --version              Display version information and exit
 
   Environment variables
@@ -316,7 +316,7 @@ type Options struct {
 	PreviewLabel labelOpts
 	Unicode      bool
 	Tabstop      int
-	ListenPort   int
+	ListenPort   *int
 	ClearOnExit  bool
 	Version      bool
 }
@@ -1756,9 +1756,10 @@ func parseOptions(opts *Options, allArgs []string) {
 		case "--tabstop":
 			opts.Tabstop = nextInt(allArgs, &i, "tab stop required")
 		case "--listen":
-			opts.ListenPort = nextInt(allArgs, &i, "listen port required")
+			port := optionalNumeric(allArgs, &i, 0)
+			opts.ListenPort = &port
 		case "--no-listen":
-			opts.ListenPort = 0
+			opts.ListenPort = nil
 		case "--clear":
 			opts.ClearOnExit = true
 		case "--no-clear":
@@ -1849,7 +1850,8 @@ 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 {
-				opts.ListenPort = atoi(value)
+				port := atoi(value)
+				opts.ListenPort = &port
 			} else if match, value := optString(arg, "--hscroll-off="); match {
 				opts.HscrollOff = atoi(value)
 			} else if match, value := optString(arg, "--scroll-off="); match {
@@ -1879,7 +1881,7 @@ func parseOptions(opts *Options, allArgs []string) {
 		errorExit("tab stop must be a positive integer")
 	}
 
-	if opts.ListenPort < 0 || opts.ListenPort > 65535 {
+	if opts.ListenPort != nil && (*opts.ListenPort < 0 || *opts.ListenPort > 65535) {
 		errorExit("invalid listen port")
 	}
 
diff --git a/src/server.go b/src/server.go
index 2912b1a4..89a7938c 100644
--- a/src/server.go
+++ b/src/server.go
@@ -19,14 +19,26 @@ const (
 	maxContentLength = 1024 * 1024
 )
 
-func startHttpServer(port int, channel chan []*action) error {
-	if port == 0 {
-		return nil
+func startHttpServer(port int, channel chan []*action) (error, int) {
+	if port < 0 {
+		return nil, port
 	}
 
 	listener, err := net.Listen("tcp", fmt.Sprintf("localhost:%d", port))
 	if err != nil {
-		return fmt.Errorf("port not available: %d", port)
+		return fmt.Errorf("port not available: %d", port), port
+	}
+	if port == 0 {
+		addr := listener.Addr().String()
+		parts := strings.SplitN(addr, ":", 2)
+		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 {
+			return err, port
+		}
 	}
 
 	go func() {
@@ -45,7 +57,7 @@ func startHttpServer(port int, channel chan []*action) error {
 		listener.Close()
 	}()
 
-	return nil
+	return nil, port
 }
 
 // Here we are writing a simplistic HTTP server without using net/http
diff --git a/src/terminal.go b/src/terminal.go
index 0310a9f0..dfc21a3b 100644
--- a/src/terminal.go
+++ b/src/terminal.go
@@ -203,7 +203,7 @@ type Terminal struct {
 	padding            [4]sizeSpec
 	strong             tui.Attr
 	unicode            bool
-	listenPort         int
+	listenPort         *int
 	borderShape        tui.BorderShape
 	cleanExit          bool
 	paused             bool
@@ -538,7 +538,7 @@ 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 > 0 {
+	if len(opts.Preview.command) > 0 || hasPreviewAction(opts) || opts.ListenPort != nil {
 		previewBox = util.NewEventBox()
 	}
 	strongAttr := tui.Bold
@@ -694,13 +694,25 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 
 	_, t.hasLoadActions = t.keymap[tui.Load.AsEvent()]
 
-	if err := startHttpServer(t.listenPort, t.serverChan); err != nil {
-		errorExit(err.Error())
+	if t.listenPort != nil {
+		err, port := startHttpServer(*t.listenPort, t.serverChan)
+		if err != nil {
+			errorExit(err.Error())
+		}
+		t.listenPort = &port
 	}
 
 	return &t
 }
 
+func (t *Terminal) environ() []string {
+	env := os.Environ()
+	if t.listenPort != nil {
+		env = append(env, fmt.Sprintf("FZF_PORT=%d", *t.listenPort))
+	}
+	return env
+}
+
 func borderLines(shape tui.BorderShape) int {
 	switch shape {
 	case tui.BorderHorizontal, tui.BorderRounded, tui.BorderSharp, tui.BorderBold, tui.BorderDouble:
@@ -2248,6 +2260,7 @@ func (t *Terminal) executeCommand(template string, forcePlus bool, background bo
 	}
 	command := t.replacePlaceholder(template, forcePlus, string(t.input), list)
 	cmd := util.ExecCommand(command, false)
+	cmd.Env = t.environ()
 	t.executing.Set(true)
 	if !background {
 		cmd.Stdin = tui.TtyIn()
@@ -2494,17 +2507,17 @@ func (t *Terminal) Loop() {
 					_, query := t.Input()
 					command := t.replacePlaceholder(commandTemplate, false, string(query), items)
 					cmd := util.ExecCommand(command, true)
+					env := t.environ()
 					if pwindow != nil {
 						height := pwindow.Height()
-						env := os.Environ()
 						lines := fmt.Sprintf("LINES=%d", height)
 						columns := fmt.Sprintf("COLUMNS=%d", pwindow.Width())
 						env = append(env, lines)
 						env = append(env, "FZF_PREVIEW_"+lines)
 						env = append(env, columns)
 						env = append(env, "FZF_PREVIEW_"+columns)
-						cmd.Env = env
 					}
+					cmd.Env = env
 
 					out, _ := cmd.StdoutPipe()
 					cmd.Stderr = cmd.Stdout
diff --git a/test/test_go.rb b/test/test_go.rb
index 07a7c195..de81f6e5 100755
--- a/test/test_go.rb
+++ b/test/test_go.rb
@@ -2629,11 +2629,17 @@ class TestGoFZF < TestBase
   end
 
   def test_listen
-    tmux.send_keys 'seq 10 | fzf --listen 6266', :Enter
-    tmux.until { |lines| assert_equal 10, lines.item_count }
-    Net::HTTP.post(URI('http://localhost:6266'), 'change-query(yo)+reload(seq 100)+change-prompt:hundred> ')
-    tmux.until { |lines| assert_equal 100, lines.item_count }
-    tmux.until { |lines| assert_equal 'hundred> yo', lines[-1] }
+    { '--listen 6266' => lambda { URI('http://localhost:6266') },
+      "--listen --sync --bind 'start:execute-silent:echo $FZF_PORT > /tmp/fzf-port'" =>
+        lambda { URI("http://localhost:#{File.read('/tmp/fzf-port').chomp}") } }.each do |opts, fn|
+      tmux.send_keys "seq 10 | fzf #{opts}", :Enter
+      tmux.until { |lines| assert_equal 10, lines.item_count }
+      Net::HTTP.post(fn.call, 'change-query(yo)+reload(seq 100)+change-prompt:hundred> ')
+      tmux.until { |lines| assert_equal 100, lines.item_count }
+      tmux.until { |lines| assert_equal 'hundred> yo', lines[-1] }
+      teardown
+      setup
+    end
   end
 
   def test_toggle_alternative_preview_window