Implement write-file command

USAGE.md

@@ -19,6 +19,9 @@ Currently, the agent has the following modes:
 `wait-signal`
 : Waits for a specific signal or signals before it exits.
 
+`write-file`
+: Write standard input into the given file
+
 `license`
 : This mode will print the license of the agent and exit.
 
@@ -58,6 +61,13 @@ the value of `\0` or a newline (`\n`) is received on `stdin`. This byte will
 not be passed to the program and if any other byte is received the agent
 will exit to prevent data stream corruption.
 
+## Writing files
+
+The backend APIs do not support adding custom files on the fly in containers.
+So the `write-file` mode was created. The mode takes just one argument, the
+name and path of the file to write. The contents of the file will be read from
+standard input
+
 ## Reporting PID
 
 Since most container implementations don't report the PID of the process

main.go

@@ -56,6 +56,8 @@ func main() {
 		})
 	case "wait-signal":
 		waitSignal(os.Stdout, args[2:], os.Exit)
+	case "write-file":
+		writeFile(args[2:], os.Stdin, os.Stderr, os.Exit)
 	case "license":
 		license(args[2:])
 	default:

write.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+	"io"
+	"os"
+	"log"
+)
+
+func writeFile(args []string, stdin io.Reader, stderr io.Writer, exit exitFunc) {
+	l := log.New(stderr, "", 0)
+	if len(args) < 1 {
+		exit(1)
+	}
+
+	file, err := os.OpenFile(args[0], os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
+	if err != nil {
+		l.Printf("Error opening file: %+v", err)
+		exit(1)
+	}
+
+	err = file.Chmod(0600)
+	if err != nil {
+		l.Printf("Error chmod'ing file: %+v", err)
+		exit(1)
+	}
+
+	buf := make([]byte, 8192)
+	for {
+		nBytes, err := stdin.Read(buf)
+		if err != nil {
+			exit(0)
+		}
+
+		wBytes, err := file.Write(buf[0:nBytes])
+		if err != nil {
+			exit(1)
+		}
+		if wBytes != nBytes {
+			// Write should never fail without error
+			exit(1)
+		}
+	}
+}