Add an option to print hovered files to a FIFO (#548)

* Add an option to print hovered files to a FIFO

This adds an env variable, `NNN_FIFO`, that can be set to a path that
`nnn` will open/create as a FIFO, and where every hovered file's path is
printed. This allows creating external perview/quick open plugins, ...

It can be compiled out with the make variable `O_NOFIFO`.

* Check filename ptr instead of full path (for FIFO)

* Add documentation to use NNN_FIFO in plugins

* Fix path sent to FIFO in empty dirs

4 files changed, 109 insertions, 1 deletions

diff --git a/Makefile b/Makefile
index 3c0e08a..004c524 100644
--- a/Makefile
+++ b/Makefile
@@ -57,6 +57,10 @@ ifeq ($(O_NOBATCH),1)
 	CPPFLAGS += -DNOBATCH
 endif
 
+ifeq ($(O_NOFIFO),1)
+	CPPFLAGS += -DNOFIFO
+endif
+
 ifeq ($(shell $(PKG_CONFIG) ncursesw && echo 1),1)
 	CFLAGS_CURSES ?= $(shell $(PKG_CONFIG) --cflags ncursesw)
 	LDLIBS_CURSES ?= $(shell $(PKG_CONFIG) --libs   ncursesw)
diff --git a/nnn.1 b/nnn.1
index 5048bb0..95e93a6 100644
--- a/nnn.1
+++ b/nnn.1
@@ -423,6 +423,13 @@ separated by \fI;\fR:
 .Pp
 \fBNNN_SEL:\fR absolute path to custom selection file.
 .Pp
+\fBNNN_FIFO:\fR path of a named pipe to write current file path:
+.Bd -literal
+    export NNN_FIFO='/tmp/nnn.fifo'
+
+    NOTE: If the FIFO file doesn't exist it will be created, but it will never be removed.
+.Ed
+.Pp
 \fBnnn:\fR this is a special variable set to the hovered entry before executing
 a command from the command prompt or spawning a shell.
 .Pp
diff --git a/plugins/README.md b/plugins/README.md
index e8f8958..40386bb 100644
--- a/plugins/README.md
+++ b/plugins/README.md
@@ -190,7 +190,7 @@ Usage examples can be found in the Examples section below.
 There are many plugins provided by `nnn` which can be used as examples. Here are a few simple selected examples.
 
 - Show the git log of changes to the particular file along with the code for a quick and easy review.
-   ```sh
+    ```sh
     #!/usr/bin/env sh
     git log -p -- "$1"
     ```
@@ -220,6 +220,40 @@ There are many plugins provided by `nnn` which can be used as examples. Here are
     printf "%s" "0c$dir" > "$NNN_PIPE"
     ```
 
+#### Get notified on file hover
+
+If `NNN_FIFO` is set, `nnn` will open it and write every hovered files.
+This can be used in plugins, e.g. to implement file previews.
+
+If a `NNN_FIFO` is set globally, each `nnn` instance will write to it, and a process reading from the pipe will get hovered path from every instance, interleaved.
+
+If you want to prevent this and be sure to have a private pipe to one `nnn` instance, you can unlink (remove) the FIFO file.
+If you opened the FIFO before and you have read from it (so that `nnn` have it opened too), you can still read from it while you don't close it.
+But new `nnn` instances will recreate a new FIFO not linked to the previous one.
+
+Don't forget to fork in the background to avoid blocking `nnn`.
+
+Example (send every hovered file to X selection):
+
+```sh
+#!/usr/bin/env sh
+if [ -z "$NNN_FIFO" ] ; then
+	exit 1
+fi
+
+while read FILE ; do
+	if [ -n "$NNN_FIFO" ] ; then
+		# If you want to remove the FIFO,
+		# don't do it before first read,
+		# nnn won't have it opened yet
+		rm "$NNN_FIFO"
+		NNN_FIFO=
+	fi
+	printf "%s" "$FILE" | xsel
+done < "$NNN_FIFO" &
+disown
+```
+
 ## Contributing plugins
 
 1. Add informative sections like _Description_, _Notes_, _Dependencies_, _Shell_, _Author_ etc. in the plugin.
diff --git a/src/nnn.c b/src/nnn.c
index 19adf93..e646393 100644
--- a/src/nnn.c
+++ b/src/nnn.c
@@ -330,6 +330,9 @@ static context g_ctx[CTX_MAX] __attribute__ ((aligned));
 
 static int ndents, cur, last, curscroll, last_curscroll, total_dents = ENTRY_INCR;
 static int nselected;
+#ifndef NOFIFO
+static int fifofd = -1;
+#endif
 static uint idletimeout, selbufpos, lastappendpos, selbuflen;
 static ushort xlines, xcols;
 static ushort idle;
@@ -350,6 +353,9 @@ static char *prefixpath;
 static char *plugindir;
 static char *sessiondir;
 static char *pnamebuf, *pselbuf;
+#ifndef NOFIFO
+static char *fifopath;
+#endif
 static ull *ihashbmp;
 static struct entry *dents;
 static blkcnt_t ent_blocks;
@@ -4667,6 +4673,40 @@ static void populate(char *path, char *lastname)
 	last_curscroll = -1;
 }
 
+#ifndef NOFIFO
+static void notify_fifo()
+{
+	if (fifofd == -1) {
+		fifofd = open(fifopath, O_WRONLY|O_NONBLOCK);
+		if (fifofd == -1) {
+			if (errno != ENXIO)
+				/* Unexpected error, the FIFO file might have been removed */
+				/* We give up FIFO notification */
+				fifopath = NULL;
+			return;
+		}
+	}
+
+	static char *name = NULL;
+
+	if (dents[cur].name == name)
+		return;
+
+	name = dents[cur].name;
+
+	char path[PATH_MAX];
+	size_t len = mkpath(g_ctx[cfg.curctx].c_path, ndents ? name : "", path);
+
+	path[len - 1] = '\n';
+
+	ssize_t ret = write(fifofd, path, len);
+
+	if (ret != (ssize_t)len && !(ret == -1 && (errno == EAGAIN || errno == EPIPE))) {
+		DPRINTF_S(strerror(errno));
+	}
+}
+#endif
+
 static void move_cursor(int target, int ignore_scrolloff)
 {
 	int onscreen = xlines - 4; /* Leave top 2 and bottom 2 lines */
@@ -4693,6 +4733,11 @@ static void move_cursor(int target, int ignore_scrolloff)
 	}
 	curscroll = MIN(curscroll, MIN(cur, ndents - onscreen));
 	curscroll = MAX(curscroll, MAX(cur - (onscreen - 1), 0));
+
+#ifndef NOFIFO
+	if (fifopath)
+		notify_fifo();
+#endif
 }
 
 static void handle_screen_move(enum action sel)
@@ -7022,6 +7067,19 @@ int main(int argc, char *argv[])
 
 	DPRINTF_S(getenv("PWD"));
 
+#ifndef NOFIFO
+	/* Create fifo */
+	fifopath = getenv("NNN_FIFO");
+	if (fifopath) {
+		if (mkfifo(fifopath, 0600) != 0 && !(errno == EEXIST && access(fifopath, W_OK) == 0)) {
+			xerror();
+			return _FAILURE;
+		}
+
+		signal(SIGPIPE, SIG_IGN);
+	}
+#endif
+
 #ifdef LINUX_INOTIFY
 	/* Initialize inotify */
 	inotify_fd = inotify_init1(IN_NONBLOCK);
@@ -7145,5 +7203,10 @@ int main(int argc, char *argv[])
 	haiku_close_nm(haiku_hnd);
 #endif
 
+#ifndef NOFIFO
+	if (fifofd != -1)
+		close(fifofd);
+#endif
+
 	return opt;
 }