#!/usr/bin/env python3
# Copyright (c) 2016-2017 The Bitcoin Core developers
# Distributed under the MIT software license, see the accompanying
# file COPYING or http://www.opensource.org/licenses/mit-license.php.

# This script will locally construct a merge commit for a pull request on a
# github repository, inspect it, sign it and optionally push it.

# The following temporary branches are created/overwritten and deleted:
# * pull/$PULL/base (the current master we're merging onto)
# * pull/$PULL/head (the current state of the remote pull request)
# * pull/$PULL/merge (github's merge)
# * pull/$PULL/local-merge (our merge)

# In case of a clean merge that is accepted by the user, the local branch with
# name $BRANCH is overwritten with the merged result, and optionally pushed.
import os
from sys import stdin,stdout,stderr
import argparse
import hashlib
import subprocess
import sys
import json
import codecs
from urllib.request import Request, urlopen
from urllib.error import HTTPError

# External tools (can be overridden using environment)
GIT = os.getenv('GIT','git')
BASH = os.getenv('BASH','bash')

# OS specific configuration for terminal attributes
ATTR_RESET = ''
ATTR_PR = ''
COMMIT_FORMAT = '%H %s (%an)%d'
if os.name == 'posix': # if posix, assume we can use basic terminal escapes
    ATTR_RESET = '\033[0m'
    ATTR_PR = '\033[1;36m'
    COMMIT_FORMAT = '%C(bold blue)%H%Creset %s %C(cyan)(%an)%Creset%C(green)%d%Creset'

def git_config_get(option, default=None):
    '''
    Get named configuration option from git repository.
    '''
    try:
        return subprocess.check_output([GIT,'config','--get',option]).rstrip().decode('utf-8')
    except subprocess.CalledProcessError:
        return default

def retrieve_pr_info(repo,pull,ghtoken):
    '''
    Retrieve pull request information from github.
    Return None if no title can be found, or an error happens.
    '''
    try:
        req = Request("https://api.github.com/repos/"+repo+"/pulls/"+pull)
        if ghtoken is not None:
            req.add_header('Authorization', 'token ' + ghtoken)
        result = urlopen(req)
        reader = codecs.getreader('utf-8')
        obj = json.load(reader(result))
        return obj
    except HTTPError as e:
        error_message = e.read()
        print('Warning: unable to retrieve pull information from github: %s' % e)
        print('Detailed error: %s' % error_message)
        return None
    except Exception as e:
        print('Warning: unable to retrieve pull information from github: %s' % e)
        return None

def ask_prompt(text):
    print(text,end=" ",file=stderr)
    stderr.flush()
    reply = stdin.readline().rstrip()
    print("",file=stderr)
    return reply

def get_symlink_files():
    files = sorted(subprocess.check_output([GIT, 'ls-tree', '--full-tree', '-r', 'HEAD']).splitlines())
    ret = []
    for f in files:
        if (int(f.decode('utf-8').split(" ")[0], 8) & 0o170000) == 0o120000:
            ret.append(f.decode('utf-8').split("\t")[1])
    return ret

def tree_sha512sum(commit='HEAD'):
    # request metadata for entire tree, recursively
    files = []
    blob_by_name = {}
    for line in subprocess.check_output([GIT, 'ls-tree', '--full-tree', '-r', commit]).splitlines():
        name_sep = line.index(b'\t')
        metadata = line[:name_sep].split() # perms, 'blob', blobid
        assert(metadata[1] == b'blob')
        name = line[name_sep+1:]
        files.append(name)
        blob_by_name[name] = metadata[2]

    files.sort()
    # open connection to git-cat-file in batch mode to request data for all blobs
    # this is much faster than launching it per file
    p = subprocess.Popen([GIT, 'cat-file', '--batch'], stdout=subprocess.PIPE, stdin=subprocess.PIPE)
    overall = hashlib.sha512()
    for f in files:
        blob = blob_by_name[f]
        # request blob
        p.stdin.write(blob + b'\n')
        p.stdin.flush()
        # read header: blob, "blob", size
        reply = p.stdout.readline().split()
        assert(reply[0] == blob and reply[1] == b'blob')
        size = int(reply[2])
        # hash the blob data
        intern = hashlib.sha512()
        ptr = 0
        while ptr < size:
            bs = min(65536, size - ptr)
            piece = p.stdout.read(bs)
            if len(piece) == bs:
                intern.update(piece)
            else:
                raise IOError('Premature EOF reading git cat-file output')
            ptr += bs
        dig = intern.hexdigest()
        assert(p.stdout.read(1) == b'\n') # ignore LF that follows blob data
        # update overall hash with file hash
        overall.update(dig.encode("utf-8"))
        overall.update("  ".encode("utf-8"))
        overall.update(f)
        overall.update("\n".encode("utf-8"))
    p.stdin.close()
    if p.wait():
        raise IOError('Non-zero return value executing git cat-file')
    return overall.hexdigest()

def print_merge_details(pull, title, branch, base_branch, head_branch):
    print('%s#%s%s %s %sinto %s%s' % (ATTR_RESET+ATTR_PR,pull,ATTR_RESET,title,ATTR_RESET+ATTR_PR,branch,ATTR_RESET))
    subprocess.check_call([GIT,'log','--graph','--topo-order','--pretty=format:'+COMMIT_FORMAT,base_branch+'..'+head_branch])

def parse_arguments():
    epilog = '''
        In addition, you can set the following git configuration variables:
        githubmerge.repository (mandatory),
        user.signingkey (mandatory),
        user.ghtoken (default: none).
        githubmerge.host (default: git@github.com),
        githubmerge.branch (no default),
        githubmerge.testcmd (default: none).
    '''
    parser = argparse.ArgumentParser(description='Utility to merge, sign and push github pull requests',
            epilog=epilog)
    parser.add_argument('pull', metavar='PULL', type=int, nargs=1,
        help='Pull request ID to merge')
    parser.add_argument('branch', metavar='BRANCH', type=str, nargs='?',
        default=None, help='Branch to merge against (default: githubmerge.branch setting, or base branch for pull, or \'master\')')
    return parser.parse_args()

def main():
    # Extract settings from git repo
    repo = git_config_get('githubmerge.repository')
    host = git_config_get('githubmerge.host','git@github.com')
    opt_branch = git_config_get('githubmerge.branch',None)
    testcmd = git_config_get('githubmerge.testcmd')
    ghtoken = git_config_get('user.ghtoken')
    signingkey = git_config_get('user.signingkey')
    if repo is None:
        print("ERROR: No repository configured. Use this command to set:", file=stderr)
        print("git config githubmerge.repository <owner>/<repo>", file=stderr)
        sys.exit(1)
    if signingkey is None:
        print("ERROR: No GPG signing key set. Set one using:",file=stderr)
        print("git config --global user.signingkey <key>",file=stderr)
        sys.exit(1)

    if host.startswith(('https:','http:')):
        host_repo = host+"/"+repo+".git"
    else:
        host_repo = host+":"+repo

    # Extract settings from command line
    args = parse_arguments()
    pull = str(args.pull[0])

    # Receive pull information from github
    info = retrieve_pr_info(repo,pull,ghtoken)
    if info is None:
        sys.exit(1)
    title = info['title'].strip()
    body = info['body'].strip()
    # precedence order for destination branch argument:
    #   - command line argument
    #   - githubmerge.branch setting
    #   - base branch for pull (as retrieved from github)
    #   - 'master'
    branch = args.branch or opt_branch or info['base']['ref'] or 'master'

    # Initialize source branches
    head_branch = 'pull/'+pull+'/head'
    base_branch = 'pull/'+pull+'/base'
    merge_branch = 'pull/'+pull+'/merge'
    local_merge_branch = 'pull/'+pull+'/local-merge'

    devnull = open(os.devnull, 'w', encoding="utf8")
    try:
        subprocess.check_call([GIT,'checkout','-q',branch])
    except subprocess.CalledProcessError:
        print("ERROR: Cannot check out branch %s." % (branch), file=stderr)
        sys.exit(3)
    try:
        subprocess.check_call([GIT,'fetch','-q',host_repo,'+refs/pull/'+pull+'/*:refs/heads/pull/'+pull+'/*',
                                                          '+refs/heads/'+branch+':refs/heads/'+base_branch])
    except subprocess.CalledProcessError:
        print("ERROR: Cannot find pull request #%s or branch %s on %s." % (pull,branch,host_repo), file=stderr)
        sys.exit(3)
    try:
        subprocess.check_call([GIT,'log','-q','-1','refs/heads/'+head_branch], stdout=devnull, stderr=stdout)
    except subprocess.CalledProcessError:
        print("ERROR: Cannot find head of pull request #%s on %s." % (pull,host_repo), file=stderr)
        sys.exit(3)
    try:
        subprocess.check_call([GIT,'log','-q','-1','refs/heads/'+merge_branch], stdout=devnull, stderr=stdout)
    except subprocess.CalledProcessError:
        print("ERROR: Cannot find merge of pull request #%s on %s." % (pull,host_repo), file=stderr)
        sys.exit(3)
    subprocess.check_call([GIT,'checkout','-q',base_branch])
    subprocess.call([GIT,'branch','-q','-D',local_merge_branch], stderr=devnull)
    subprocess.check_call([GIT,'checkout','-q','-b',local_merge_branch])

    try:
        # Go up to the repository's root.
        toplevel = subprocess.check_output([GIT,'rev-parse','--show-toplevel']).strip()
        os.chdir(toplevel)
        # Create unsigned merge commit.
        if title:
            firstline = 'Merge #%s: %s' % (pull,title)
        else:
            firstline = 'Merge #%s' % (pull,)
        message = firstline + '\n\n'
        message += subprocess.check_output([GIT,'log','--no-merges','--topo-order','--pretty=format:%H %s (%an)',base_branch+'..'+head_branch]).decode('utf-8')
        message += '\n\nPull request description:\n\n  ' + body.replace('\n', '\n  ') + '\n'
        try:
            subprocess.check_call([GIT,'merge','-q','--commit','--no-edit','--no-ff','--no-gpg-sign','-m',message.encode('utf-8'),head_branch])
        except subprocess.CalledProcessError:
            print("ERROR: Cannot be merged cleanly.",file=stderr)
            subprocess.check_call([GIT,'merge','--abort'])
            sys.exit(4)
        logmsg = subprocess.check_output([GIT,'log','--pretty=format:%s','-n','1']).decode('utf-8')
        if logmsg.rstrip() != firstline.rstrip():
            print("ERROR: Creating merge failed (already merged?).",file=stderr)
            sys.exit(4)

        symlink_files = get_symlink_files()
        for f in symlink_files:
            print("ERROR: File %s was a symlink" % f)
        if len(symlink_files) > 0:
            sys.exit(4)

        # Put tree SHA512 into the message
        try:
            first_sha512 = tree_sha512sum()
            message += '\n\nTree-SHA512: ' + first_sha512
        except subprocess.CalledProcessError:
            print("ERROR: Unable to compute tree hash")
            sys.exit(4)
        try:
            subprocess.check_call([GIT,'commit','--amend','--no-gpg-sign','-m',message.encode('utf-8')])
        except subprocess.CalledProcessError:
            print("ERROR: Cannot update message.", file=stderr)
            sys.exit(4)

        print_merge_details(pull, title, branch, base_branch, head_branch)
        print()

        # Run test command if configured.
        if testcmd:
            if subprocess.call(testcmd,shell=True):
                print("ERROR: Running %s failed." % testcmd,file=stderr)
                sys.exit(5)

            # Show the created merge.
            diff = subprocess.check_output([GIT,'diff',merge_branch+'..'+local_merge_branch])
            subprocess.check_call([GIT,'diff',base_branch+'..'+local_merge_branch])
            if diff:
                print("WARNING: merge differs from github!",file=stderr)
                reply = ask_prompt("Type 'ignore' to continue.")
                if reply.lower() == 'ignore':
                    print("Difference with github ignored.",file=stderr)
                else:
                    sys.exit(6)
        else:
            # Verify the result manually.
            print("Dropping you on a shell so you can try building/testing the merged source.",file=stderr)
            print("Run 'git diff HEAD~' to show the changes being merged.",file=stderr)
            print("Type 'exit' when done.",file=stderr)
            if os.path.isfile('/etc/debian_version'): # Show pull number on Debian default prompt
                os.putenv('debian_chroot',pull)
            subprocess.call([BASH,'-i'])

        second_sha512 = tree_sha512sum()
        if first_sha512 != second_sha512:
            print("ERROR: Tree hash changed unexpectedly",file=stderr)
            sys.exit(8)

        # Sign the merge commit.
        print_merge_details(pull, title, branch, base_branch, head_branch)
        while True:
            reply = ask_prompt("Type 's' to sign off on the above merge, or 'x' to reject and exit.").lower()
            if reply == 's':
                try:
                    subprocess.check_call([GIT,'commit','-q','--gpg-sign','--amend','--no-edit'])
                    break
                except subprocess.CalledProcessError:
                    print("Error while signing, asking again.",file=stderr)
            elif reply == 'x':
                print("Not signing off on merge, exiting.",file=stderr)
                sys.exit(1)

        # Put the result in branch.
        subprocess.check_call([GIT,'checkout','-q',branch])
        subprocess.check_call([GIT,'reset','-q','--hard',local_merge_branch])
    finally:
        # Clean up temporary branches.
        subprocess.call([GIT,'checkout','-q',branch])
        subprocess.call([GIT,'branch','-q','-D',head_branch],stderr=devnull)
        subprocess.call([GIT,'branch','-q','-D',base_branch],stderr=devnull)
        subprocess.call([GIT,'branch','-q','-D',merge_branch],stderr=devnull)
        subprocess.call([GIT,'branch','-q','-D',local_merge_branch],stderr=devnull)

    # Push the result.
    while True:
        reply = ask_prompt("Type 'push' to push the result to %s, branch %s, or 'x' to exit without pushing." % (host_repo,branch)).lower()
        if reply == 'push':
            subprocess.check_call([GIT,'push',host_repo,'refs/heads/'+branch])
            break
        elif reply == 'x':
            sys.exit(1)

if __name__ == '__main__':
    main()