PHP not waiting for exec() to finish when I want it to

Question

I am running a web interface for a data logging tool on a Raspberry Pi. PHP5.

The user presses a button which calls a PHP script to run a process (using exec) and then return the resultant file. For whatever reason, the exec() function does not wait to complete, and the script fails as the file it's looking for hasn't been created yet.

When I replace the script with a simple script such as print, wait 5 seconds, print - then it does wait for exec() to finish. The only difference is in the python script that I call. Does anyone have any insight as to what might be causing this?

PHP Script:

<?php

    $file = "/var/tmp/logs/FaultLog.csv";

    exec("python /data/scripts/test.py", $output, $return);

    if (!file_exists($file)) {
        die ("Error: Fault log could not be created.");
    }

    $type = filetype($file);
    // Send file headers
    header("Content-type: $type");
    header("Content-Disposition: attachment;filename=FaultLog.csv");
    header("Content-Transfer-Encoding: binary"); 
    header('Pragma: no-cache'); 
    header('Expires: 0');
    // Send the file contents.
    set_time_limit(0); 
    readfile($file);
?>

Python Script that works:

import time
print "hello"
time.sleep(5)
print "goodbye"

Python Script that doesn't work. It's long, but I decided to include it all just in case.

#!/usr/bin/env python

import sqlite3, subprocess, re, os

faultID = hours = minutes = 0

csv_path = "/var/tmp/logs/"
db_path = "/data/databases/FaultDictionary.db"

def parse_msg (line):

        global faultID, hours, minutes

        if (" 12 41 01 " in line):                        #single message
                #parse CAN data
                try:
                        data = line.split("  ")[4][12:17].replace(" ","")
                        faultID = int(data[2:] + data[:2], 16)
                except:
                        return "Error: Unable to parse data. Data: " + line

        elif (" 12 41 02 " in line):                      #single message
                #parse CAN data
                try:
                        data = line.split("  ")[4][12:17].replace(" ","")
                        hours = int(data[2:] + data[:2], 16)
                except:
                        return "Error: Unable to parse data. Data: " + line

        elif (" 12 41 03 " in line):                      #single message
                #parse CAN data
                try:
                        minutes = int(line.split("  ")[4][12:14], 16)
                except:
                        return "Error: Unable to parse data. Data: " + line

        elif ("581   [8]  80" in line):                           #crashed
                return "Error: Crashed motor controller. Please try again."
        else:
                return "Error: Unexpected message format, cannot decode reply from motor controller. Data: " + line

        return ""

conn = sqlite3.connect(db_path)
curs = conn.cursor()

#note time
p = subprocess.Popen("date +\"%Y-%m-%d %H:%M\"", stdout=subprocess.PIPE, shell=True)
(output, err) = p.communicate()
current_date = output

#Create needed folder structure
if not (os.path.exists(csv_path)):
        os.makedirs(csv_path)

csv_path += "FaultLog.csv"

with open(csv_path, 'w+') as file:
        file.write("Date:," + current_date[:10] + "\n")
        file.write("Time:," + current_date[11:] + "\n\n")

        #SEVCON Fault Retreival
        file.write('Hours Ago [h], Minutes & Seconds Ago, Fault ID, Fault Name, Fault Cause, Fault Remedy\n')

        #send password to gain access
        p = subprocess.Popen("cansend can0 601#2B005002DF4BEFFA", cwd="/data/can-utils/", stdout=subprocess.PIPE, shell=True)
        p = subprocess.Popen("candump -t A -n 1 -T 100 can0,581:7ff", cwd="/data/can-utils/", stdout=subprocess.PIPE, shell=True)
        (output, err) = p.communicate()

        for num in range(0,40):
                pass

                #select event through 4111
                p = subprocess.Popen("cansend can0 601#2B114100" + hex(num)[2:].zfill(2) + "00EFFA", cwd="/data/can-utils/", stdout=subprocess.PIPE, shell=True)
                p = subprocess.Popen("candump -t A -n 1 -T 100 can0,581:7ff", cwd="/data/can-utils/", stdout=subprocess.PIPE, shell=True)
                (output, err) = p.communicate()

                #send request for event ID through 4112
                p = subprocess.Popen("(sleep 0.1; cansend can0 601#4012410100000000; cansend can0 601#4012410200000000; cansend can0 601#4012410300000000;) &", cwd="/data/can-utils/", stdout=subprocess.PIPE, shell=True)
                p = subprocess.Popen("candump -t A -n 3 -T 500 can0,581:7ff", cwd="/data/can-utils/", stdout=subprocess.PIPE, shell=True)
                (output, err) = p.communicate()

                message = ""
                if len(output) > 0:  # got the response message

                        lines = output.strip().split("\n")

                        for line in lines:
                                message = parse_msg(line)

                else:
                        message = "Error: Did not receive reply from motor controller."

                if message == "":      
                        #query database for fault description and remedy
                        curs.execute("SELECT * FROM FaultDictionary WHERE faultID = '" + str(faultID) + "'")   #faultID, message, description, action
                        for row in curs: #only ever one, since unique
                                #record to csv
                                file.write(str(hours) + "," + str(minutes) + "," + str(row[0]) + "," + str(row[1]) + "," + str(row[2]) + "," + str(row[3]) + "\n")
                                #print str(hours) + "," + str(minutes) + "," + str(row[0]) + "," + str(row[1]) + "," + str(row[2]) + "," + str(row[3]) + "\n"
                else:
                        print message
                        break

        #BMS Fault Retrieval
        #file.write('\n\nFault ID\n')

if not message == "":
        os.remove(csv_path)

conn.close()

tldr; PHP exec() function calling python script. Exec() is supposed to wait for whatever you're calling to complete. Does so with some scripts, but not others.

Answer 1

Exec is a synchronous blocking call when there is no & passed to the argument to force the program / execution to occur in the background.

This leads me to believe that the owner of the Apache instance handling the request does not have permission to execute the Python script.

You should adjust the permissions of the script or the Apache user accordingly.