examples: add vtail.v, deprecate File.read_bytes_into_newline/1 in fa…

…vor of File.read_bytes_with_newline/1 (#21182)

examples/vtail.v

@@ -0,0 +1,27 @@
+import os
+import time
+
+if os.args.len == 1 {
+	eprintln('A small `tail -f file` like program, written in V.')
+	eprintln('Usage: `v run examples/vtail.v your_long_file.log`')
+	exit(0)
+}
+
+tfile := os.args[1] or { panic('pass 1 file path as argument') }
+mut f := os.open_file(tfile, 'r') or { panic('file ${tfile} does not exist') }
+f.seek(0, .end)!
+mut read_pos := f.tell()!
+mut buf := []u8{len: 10 * 1024}
+for {
+	bytes := f.read_bytes_with_newline(mut buf)!
+	if bytes == 0 && f.eof() {
+		// The end of the file has been reached, so wait a bit, and retry from the same position:
+		f.close()
+		time.sleep(500 * time.millisecond)
+		f = os.open_file(tfile, 'r')!
+		f.seek(read_pos, .start)!
+		continue
+	}
+	read_pos += bytes
+	print(unsafe { (&u8(buf.data)).vstring_with_len(bytes) })
+}

vlib/os/file.c.v

@@ -450,10 +450,18 @@ pub fn (f &File) read_bytes_at(size int, pos u64) []u8 {
 	return arr[0..nreadbytes]
 }
 
-// read_bytes_into_newline reads from the beginning of the file into the provided buffer.
-// Each consecutive call on the same file continues reading where it previously ended.
-// A read call is either stopped, if the buffer is full, a newline was read or EOF.
+// read_bytes_into_newline reads from the current position of the file into the provided buffer.
+@[deprecated: 'use read_bytes_with_newline instead']
+@[deprecated_after: '2024-05-04']
 pub fn (f &File) read_bytes_into_newline(mut buf []u8) !int {
+	return f.read_bytes_with_newline(mut buf)
+}
+
+// read_bytes_with_newline reads from the current position of the file into the provided buffer.
+// Each consecutive call on the same file, continues reading, from where it previously ended.
+// A read call is either stopped, if the buffer is full, a newline was read or EOF.
+// On EOF, the method returns 0. The methods will also return any IO error encountered.
+pub fn (f &File) read_bytes_with_newline(mut buf []u8) !int {
 	if buf.len == 0 {
 		return error(@FN + ': `buf.len` == 0')
 	}