Introduce types {Git,Channel}SearchPath

[pinch] / pinch.py

diff --git a/pinch.py b/pinch.py
index 1dbd6679c068ced7cdfdb0162abccd32aa3751ab..b76c8da36db18e4b608fad64c5e4bbd422b8c948 100644 (file)
--- a/pinch.py
+++ b/pinch.py
@@ -1,10 +1,14 @@
+from abc import ABC, abstractmethod
+import argparse

 import configparser
 import filecmp
 import functools
 import configparser
 import filecmp
 import functools

+import getpass

 import hashlib
 import operator
 import os
 import os.path
 import hashlib
 import operator
 import os
 import os.path

+import shlex

 import shutil
 import subprocess
 import sys
 import shutil
 import subprocess
 import sys


@@ -22,27 +26,17 @@ from typing import (
     Tuple,
 )
 
     Tuple,
 )
 

-Digest16 = NewType('Digest16', str)
-Digest32 = NewType('Digest32', str)
+# Use xdg module when it's less painful to have as a dependency

 
 
 
 

-class ChannelTableEntry(types.SimpleNamespace):
-    digest: Digest16
-    file: str
-    size: int
-    url: str
+class XDG(types.SimpleNamespace):
+    XDG_CACHE_HOME: str

 
 
 
 

-class Channel(types.SimpleNamespace):
-    channel_html: bytes
-    forwarded_url: str
-    git_cachedir: str
-    git_ref: str
-    git_repo: str
-    git_revision: str
-    release_name: str
-    table: Dict[str, ChannelTableEntry]
-    url: str
+xdg = XDG(
+    XDG_CACHE_HOME=os.getenv(
+        'XDG_CACHE_HOME',
+        os.path.expanduser('~/.cache')))

 
 
 class VerificationError(Exception):
 
 
 class VerificationError(Exception):


@@ -55,7 +49,7 @@ class Verification:
         self.line_length = 0
 
     def status(self, s: str) -> None:
         self.line_length = 0
 
     def status(self, s: str) -> None:

-        print(s, end=' ', flush=True)
+        print(s, end=' ', file=sys.stderr, flush=True)

         self.line_length += 1 + len(s)  # Unicode??
 
     @staticmethod
         self.line_length += 1 + len(s)  # Unicode??
 
     @staticmethod


@@ -65,9 +59,9 @@ class Verification:
     def result(self, r: bool) -> None:
         message, color = {True: ('OK ', 92), False: ('FAIL', 91)}[r]
         length = len(message)
     def result(self, r: bool) -> None:
         message, color = {True: ('OK ', 92), False: ('FAIL', 91)}[r]
         length = len(message)

-        cols = shutil.get_terminal_size().columns
+        cols = shutil.get_terminal_size().columns or 80

         pad = (cols - (self.line_length + length)) % cols
         pad = (cols - (self.line_length + length)) % cols

-        print(' ' * pad + self._color(message, color))
+        print(' ' * pad + self._color(message, color), file=sys.stderr)

         self.line_length = 0
         if not r:
             raise VerificationError()
         self.line_length = 0
         if not r:
             raise VerificationError()


@@ -80,6 +74,82 @@ class Verification:
         self.result(True)
 
 
         self.result(True)
 
 

+Digest16 = NewType('Digest16', str)
+Digest32 = NewType('Digest32', str)
+
+
+class ChannelTableEntry(types.SimpleNamespace):
+    absolute_url: str
+    digest: Digest16
+    file: str
+    size: int
+    url: str
+
+
+class SearchPath(types.SimpleNamespace, ABC):
+    release_name: str
+
+    @abstractmethod
+    def pin(self, v: Verification, conf: configparser.SectionProxy) -> None:
+        pass
+
+
+class AliasSearchPath(SearchPath):
+    alias_of: str
+
+    def pin(self, v: Verification, conf: configparser.SectionProxy) -> None:
+        assert not hasattr(self, 'git_repo')
+
+
+class TarrableSearchPath(SearchPath):
+    channel_html: bytes
+    channel_url: str
+    forwarded_url: str
+    git_ref: str
+    git_repo: str
+    git_revision: str
+    old_git_revision: str
+    table: Dict[str, ChannelTableEntry]
+
+    def pin(self, v: Verification, conf: configparser.SectionProxy) -> None:
+        if hasattr(self, 'git_revision'):
+            self.old_git_revision = self.git_revision
+            del self.git_revision
+
+        if 'channel_url' in conf:
+            pin_channel(v, self)
+            conf['release_name'] = self.release_name
+            conf['tarball_url'] = self.table['nixexprs.tar.xz'].absolute_url
+            conf['tarball_sha256'] = self.table['nixexprs.tar.xz'].digest
+        else:
+            git_fetch(v, self)
+            conf['release_name'] = git_revision_name(v, self)
+        conf['git_revision'] = self.git_revision
+
+    def fetch(self, v: Verification, section: str,
+              conf: configparser.SectionProxy) -> str:
+        if 'git_repo' not in conf or 'release_name' not in conf:
+            raise Exception(
+                'Cannot update unpinned channel "%s" (Run "pin" before "update")' %
+                section)
+
+        if 'channel_url' in conf:
+            return fetch_with_nix_prefetch_url(
+                v, conf['tarball_url'], Digest16(
+                    conf['tarball_sha256']))
+
+        ensure_git_rev_available(v, self)
+        return git_get_tarball(v, self)
+
+
+class GitSearchPath(TarrableSearchPath):
+    pass
+
+
+class ChannelSearchPath(TarrableSearchPath):
+    pass
+
+

 def compare(a: str, b: str) -> Tuple[List[str], List[str], List[str]]:
 
     def throw(error: OSError) -> None:
 def compare(a: str, b: str) -> Tuple[List[str], List[str], List[str]]:
 
     def throw(error: OSError) -> None:


@@ -108,16 +178,16 @@ def compare(a: str, b: str) -> Tuple[List[str], List[str], List[str]]:
     return filecmp.cmpfiles(a, b, files, shallow=False)
 
 
     return filecmp.cmpfiles(a, b, files, shallow=False)
 
 

-def fetch(v: Verification, channel: Channel) -> None:
+def fetch(v: Verification, channel: TarrableSearchPath) -> None:

     v.status('Fetching channel')
     v.status('Fetching channel')

-    request = urllib.request.urlopen(channel.url, timeout=10)
+    request = urllib.request.urlopen(channel.channel_url, timeout=10)

     channel.channel_html = request.read()
     channel.forwarded_url = request.geturl()
     channel.channel_html = request.read()
     channel.forwarded_url = request.geturl()

-    v.result(request.status == 200)
-    v.check('Got forwarded', channel.url != channel.forwarded_url)
+    v.result(request.status == 200)  # type: ignore  # (for old mypy)
+    v.check('Got forwarded', channel.channel_url != channel.forwarded_url)

 
 
 
 

-def parse_channel(v: Verification, channel: Channel) -> None:
+def parse_channel(v: Verification, channel: TarrableSearchPath) -> None:

     v.status('Parsing channel description as XML')
     d = xml.dom.minidom.parseString(channel.channel_html)
     v.ok()
     v.status('Parsing channel description as XML')
     d = xml.dom.minidom.parseString(channel.channel_html)
     v.ok()


@@ -132,8 +202,8 @@ def parse_channel(v: Verification, channel: Channel) -> None:
 
     v.status('Extracting git commit:')
     git_commit_node = d.getElementsByTagName('tt')[0]
 
     v.status('Extracting git commit:')
     git_commit_node = d.getElementsByTagName('tt')[0]

-    channel.git_commit = git_commit_node.firstChild.nodeValue
-    v.status(channel.git_commit)
+    channel.git_revision = git_commit_node.firstChild.nodeValue
+    v.status(channel.git_revision)

     v.ok()
     v.status('Verifying git commit label')
     v.result(git_commit_node.previousSibling.nodeValue == 'Git commit ')
     v.ok()
     v.status('Verifying git commit label')
     v.result(git_commit_node.previousSibling.nodeValue == 'Git commit ')


@@ -166,7 +236,7 @@ def digest_file(filename: str) -> Digest16:
 def to_Digest16(v: Verification, digest32: Digest32) -> Digest16:
     v.status('Converting digest to base16')
     process = subprocess.run(
 def to_Digest16(v: Verification, digest32: Digest32) -> Digest16:
     v.status('Converting digest to base16')
     process = subprocess.run(

-        ['nix', 'to-base16', '--type', 'sha256', digest32], capture_output=True)
+        ['nix', 'to-base16', '--type', 'sha256', digest32], stdout=subprocess.PIPE)

     v.result(process.returncode == 0)
     return Digest16(process.stdout.decode().strip())
 
     v.result(process.returncode == 0)
     return Digest16(process.stdout.decode().strip())
 


@@ -174,7 +244,7 @@ def to_Digest16(v: Verification, digest32: Digest32) -> Digest16:
 def to_Digest32(v: Verification, digest16: Digest16) -> Digest32:
     v.status('Converting digest to base32')
     process = subprocess.run(
 def to_Digest32(v: Verification, digest16: Digest16) -> Digest32:
     v.status('Converting digest to base32')
     process = subprocess.run(

-        ['nix', 'to-base32', '--type', 'sha256', digest16], capture_output=True)
+        ['nix', 'to-base32', '--type', 'sha256', digest16], stdout=subprocess.PIPE)

     v.result(process.returncode == 0)
     return Digest32(process.stdout.decode().strip())
 
     v.result(process.returncode == 0)
     return Digest32(process.stdout.decode().strip())
 


@@ -185,7 +255,7 @@ def fetch_with_nix_prefetch_url(
         digest: Digest16) -> str:
     v.status('Fetching %s' % url)
     process = subprocess.run(
         digest: Digest16) -> str:
     v.status('Fetching %s' % url)
     process = subprocess.run(

-        ['nix-prefetch-url', '--print-path', url, digest], capture_output=True)
+        ['nix-prefetch-url', '--print-path', url, digest], stdout=subprocess.PIPE)

     v.result(process.returncode == 0)
     prefetch_digest, path, empty = process.stdout.decode().split('\n')
     assert empty == ''
     v.result(process.returncode == 0)
     prefetch_digest, path, empty = process.stdout.decode().split('\n')
     assert empty == ''


@@ -194,77 +264,129 @@ def fetch_with_nix_prefetch_url(
     v.status("Verifying file digest")
     file_digest = digest_file(path)
     v.result(file_digest == digest)
     v.status("Verifying file digest")
     file_digest = digest_file(path)
     v.result(file_digest == digest)

-    return path
+    return path  # type: ignore  # (for old mypy)

 
 
 
 

-def fetch_resources(v: Verification, channel: Channel) -> None:
+def fetch_resources(v: Verification, channel: TarrableSearchPath) -> None:

     for resource in ['git-revision', 'nixexprs.tar.xz']:
         fields = channel.table[resource]
     for resource in ['git-revision', 'nixexprs.tar.xz']:
         fields = channel.table[resource]

-        url = urllib.parse.urljoin(channel.forwarded_url, fields.url)
-        fields.file = fetch_with_nix_prefetch_url(v, url, fields.digest)
+        fields.absolute_url = urllib.parse.urljoin(
+            channel.forwarded_url, fields.url)
+        fields.file = fetch_with_nix_prefetch_url(
+            v, fields.absolute_url, fields.digest)

     v.status('Verifying git commit on main page matches git commit in table')
     v.result(
         open(
     v.status('Verifying git commit on main page matches git commit in table')
     v.result(
         open(

-            channel.table['git-revision'].file).read(999) == channel.git_commit)
+            channel.table['git-revision'].file).read(999) == channel.git_revision)
+
+
+def git_cachedir(git_repo: str) -> str:
+    return os.path.join(
+        xdg.XDG_CACHE_HOME,
+        'pinch/git',
+        digest_string(git_repo.encode()))
+
+
+def tarball_cache_file(channel: TarrableSearchPath) -> str:
+    return os.path.join(
+        xdg.XDG_CACHE_HOME,
+        'pinch/git-tarball',
+        '%s-%s-%s' %
+        (digest_string(channel.git_repo.encode()),
+         channel.git_revision,
+         channel.release_name))
+
+
+def verify_git_ancestry(v: Verification, channel: TarrableSearchPath) -> None:
+    cachedir = git_cachedir(channel.git_repo)
+    v.status('Verifying rev is an ancestor of ref')
+    process = subprocess.run(['git',
+                              '-C',
+                              cachedir,
+                              'merge-base',
+                              '--is-ancestor',
+                              channel.git_revision,
+                              channel.git_ref])
+    v.result(process.returncode == 0)
+
+    if hasattr(channel, 'old_git_revision'):
+        v.status(
+            'Verifying rev is an ancestor of previous rev %s' %
+            channel.old_git_revision)
+        process = subprocess.run(['git',
+                                  '-C',
+                                  cachedir,
+                                  'merge-base',
+                                  '--is-ancestor',
+                                  channel.old_git_revision,
+                                  channel.git_revision])
+        v.result(process.returncode == 0)

 
 
 
 

-def git_fetch(v: Verification, channel: Channel) -> None:
+def git_fetch(v: Verification, channel: TarrableSearchPath) -> None:

     # It would be nice if we could share the nix git cache, but as of the time
     # of writing it is transitioning from gitv2 (deprecated) to gitv3 (not ready
     # yet), and trying to straddle them both is too far into nix implementation
     # details for my comfort.  So we re-implement here half of nix.fetchGit.
     # :(
 
     # It would be nice if we could share the nix git cache, but as of the time
     # of writing it is transitioning from gitv2 (deprecated) to gitv3 (not ready
     # yet), and trying to straddle them both is too far into nix implementation
     # details for my comfort.  So we re-implement here half of nix.fetchGit.
     # :(
 

-    # TODO: Consider using pyxdg to find this path.
-    channel.git_cachedir = os.path.expanduser(
-        '~/.cache/nix-pin-channel/git/%s' %
-        digest_string(
-            channel.url.encode()))
-    if not os.path.exists(channel.git_cachedir):
+    cachedir = git_cachedir(channel.git_repo)
+    if not os.path.exists(cachedir):

         v.status("Initializing git repo")
         process = subprocess.run(
         v.status("Initializing git repo")
         process = subprocess.run(

-            ['git', 'init', '--bare', channel.git_cachedir])
+            ['git', 'init', '--bare', cachedir])

         v.result(process.returncode == 0)
 
         v.result(process.returncode == 0)
 

-    v.status('Checking if we already have this rev:')
-    process = subprocess.run(
-        ['git', '-C', channel.git_cachedir, 'cat-file', '-e', channel.git_commit])
-    if process.returncode == 0:
-        v.status('yes')
-    if process.returncode == 1:
-        v.status('no')
-    v.result(process.returncode == 0 or process.returncode == 1)
-    if process.returncode == 1:
-        v.status('Fetching ref "%s"' % channel.git_ref)
-        # We don't use --force here because we want to abort and freak out if forced
-        # updates are happening.
-        process = subprocess.run(['git',
-                                  '-C',
-                                  channel.git_cachedir,
-                                  'fetch',
-                                  channel.git_repo,
-                                  '%s:%s' % (channel.git_ref,
-                                             channel.git_ref)])
-        v.result(process.returncode == 0)
+    v.status('Fetching ref "%s" from %s' % (channel.git_ref, channel.git_repo))
+    # We don't use --force here because we want to abort and freak out if forced
+    # updates are happening.
+    process = subprocess.run(['git',
+                              '-C',
+                              cachedir,
+                              'fetch',
+                              channel.git_repo,
+                              '%s:%s' % (channel.git_ref,
+                                         channel.git_ref)])
+    v.result(process.returncode == 0)
+
+    if hasattr(channel, 'git_revision'):

         v.status('Verifying that fetch retrieved this rev')
         process = subprocess.run(
         v.status('Verifying that fetch retrieved this rev')
         process = subprocess.run(

-            ['git', '-C', channel.git_cachedir, 'cat-file', '-e', channel.git_commit])
+            ['git', '-C', cachedir, 'cat-file', '-e', channel.git_revision])

         v.result(process.returncode == 0)
         v.result(process.returncode == 0)

+    else:
+        channel.git_revision = open(
+            os.path.join(
+                cachedir,
+                'refs',
+                'heads',
+                channel.git_ref)).read(999).strip()

 
 

-    v.status('Verifying rev is an ancestor of ref')
-    process = subprocess.run(['git',
-                              '-C',
-                              channel.git_cachedir,
-                              'merge-base',
-                              '--is-ancestor',
-                              channel.git_commit,
-                              channel.git_ref])
-    v.result(process.returncode == 0)
+    verify_git_ancestry(v, channel)
+
+
+def ensure_git_rev_available(
+        v: Verification,
+        channel: TarrableSearchPath) -> None:
+    cachedir = git_cachedir(channel.git_repo)
+    if os.path.exists(cachedir):
+        v.status('Checking if we already have this rev:')
+        process = subprocess.run(
+            ['git', '-C', cachedir, 'cat-file', '-e', channel.git_revision])
+        if process.returncode == 0:
+            v.status('yes')
+        if process.returncode == 1:
+            v.status('no')
+        v.result(process.returncode == 0 or process.returncode == 1)
+        if process.returncode == 0:
+            verify_git_ancestry(v, channel)
+            return
+    git_fetch(v, channel)

 
 
 def compare_tarball_and_git(
         v: Verification,
 
 
 def compare_tarball_and_git(
         v: Verification,

-        channel: Channel,
+        channel: TarrableSearchPath,

         channel_contents: str,
         git_contents: str) -> None:
     v.status('Comparing channel tarball with git checkout')
         channel_contents: str,
         git_contents: str) -> None:
     v.status('Comparing channel tarball with git checkout')


@@ -295,7 +417,10 @@ def compare_tarball_and_git(
         len(benign_errors) == len(expected_errors))
 
 
         len(benign_errors) == len(expected_errors))
 
 

-def extract_tarball(v: Verification, channel: Channel, dest: str) -> None:
+def extract_tarball(
+        v: Verification,
+        channel: TarrableSearchPath,
+        dest: str) -> None:

     v.status('Extracting tarball %s' %
              channel.table['nixexprs.tar.xz'].file)
     shutil.unpack_archive(
     v.status('Extracting tarball %s' %
              channel.table['nixexprs.tar.xz'].file)
     shutil.unpack_archive(


@@ -304,25 +429,66 @@ def extract_tarball(v: Verification, channel: Channel, dest: str) -> None:
     v.ok()
 
 
     v.ok()
 
 

-def git_checkout(v: Verification, channel: Channel, dest: str) -> None:
+def git_checkout(
+        v: Verification,
+        channel: TarrableSearchPath,
+        dest: str) -> None:

     v.status('Checking out corresponding git revision')
     git = subprocess.Popen(['git',
                             '-C',
     v.status('Checking out corresponding git revision')
     git = subprocess.Popen(['git',
                             '-C',

-                            channel.git_cachedir,
+                            git_cachedir(channel.git_repo),

                             'archive',
                             'archive',

-                            channel.git_commit],
+                            channel.git_revision],

                            stdout=subprocess.PIPE)
     tar = subprocess.Popen(
         ['tar', 'x', '-C', dest, '-f', '-'], stdin=git.stdout)
                            stdout=subprocess.PIPE)
     tar = subprocess.Popen(
         ['tar', 'x', '-C', dest, '-f', '-'], stdin=git.stdout)

-    git.stdout.close()
+    if git.stdout:
+        git.stdout.close()

     tar.wait()
     git.wait()
     v.result(git.returncode == 0 and tar.returncode == 0)
 
 
     tar.wait()
     git.wait()
     v.result(git.returncode == 0 and tar.returncode == 0)
 
 

+def git_get_tarball(v: Verification, channel: TarrableSearchPath) -> str:
+    cache_file = tarball_cache_file(channel)
+    if os.path.exists(cache_file):
+        cached_tarball = open(cache_file).read(9999)
+        if os.path.exists(cached_tarball):
+            return cached_tarball
+
+    with tempfile.TemporaryDirectory() as output_dir:
+        output_filename = os.path.join(
+            output_dir, channel.release_name + '.tar.xz')
+        with open(output_filename, 'w') as output_file:
+            v.status(
+                'Generating tarball for git revision %s' %
+                channel.git_revision)
+            git = subprocess.Popen(['git',
+                                    '-C',
+                                    git_cachedir(channel.git_repo),
+                                    'archive',
+                                    '--prefix=%s/' % channel.release_name,
+                                    channel.git_revision],
+                                   stdout=subprocess.PIPE)
+            xz = subprocess.Popen(['xz'], stdin=git.stdout, stdout=output_file)
+            xz.wait()
+            git.wait()
+            v.result(git.returncode == 0 and xz.returncode == 0)
+
+        v.status('Putting tarball in Nix store')
+        process = subprocess.run(
+            ['nix-store', '--add', output_filename], stdout=subprocess.PIPE)
+        v.result(process.returncode == 0)
+        store_tarball = process.stdout.decode().strip()
+
+        os.makedirs(os.path.dirname(cache_file), exist_ok=True)
+        open(cache_file, 'w').write(store_tarball)
+        return store_tarball  # type: ignore  # (for old mypy)
+
+

 def check_channel_metadata(
         v: Verification,
 def check_channel_metadata(
         v: Verification,

-        channel: Channel,
+        channel: TarrableSearchPath,

         channel_contents: str) -> None:
     v.status('Verifying git commit in channel tarball')
     v.result(
         channel_contents: str) -> None:
     v.status('Verifying git commit in channel tarball')
     v.result(


@@ -330,7 +496,7 @@ def check_channel_metadata(
             os.path.join(
                 channel_contents,
                 channel.release_name,
             os.path.join(
                 channel_contents,
                 channel.release_name,

-                '.git-revision')).read(999) == channel.git_commit)
+                '.git-revision')).read(999) == channel.git_revision)

 
     v.status(
         'Verifying version-suffix is a suffix of release name %s:' %
 
     v.status(
         'Verifying version-suffix is a suffix of release name %s:' %


@@ -344,7 +510,9 @@ def check_channel_metadata(
     v.result(channel.release_name.endswith(version_suffix))
 
 
     v.result(channel.release_name.endswith(version_suffix))
 
 

-def check_channel_contents(v: Verification, channel: Channel) -> None:
+def check_channel_contents(
+        v: Verification,
+        channel: TarrableSearchPath) -> None:

     with tempfile.TemporaryDirectory() as channel_contents, \
             tempfile.TemporaryDirectory() as git_contents:
 
     with tempfile.TemporaryDirectory() as channel_contents, \
             tempfile.TemporaryDirectory() as git_contents:
 


@@ -359,17 +527,122 @@ def check_channel_contents(v: Verification, channel: Channel) -> None:
     v.ok()
 
 
     v.ok()
 
 

-def main(argv: List[str]) -> None:
-    v = Verification()
-    config = configparser.ConfigParser()
-    config.read_file(open(argv[1]), argv[1])
-    channel = Channel(**dict(config['nixos'].items()))
+def pin_channel(v: Verification, channel: TarrableSearchPath) -> None:

     fetch(v, channel)
     parse_channel(v, channel)
     fetch_resources(v, channel)
     fetch(v, channel)
     parse_channel(v, channel)
     fetch_resources(v, channel)

-    git_fetch(v, channel)
+    ensure_git_rev_available(v, channel)

     check_channel_contents(v, channel)
     check_channel_contents(v, channel)

-    print(channel)

 
 
 
 

-main(sys.argv)
+def git_revision_name(v: Verification, channel: TarrableSearchPath) -> str:
+    v.status('Getting commit date')
+    process = subprocess.run(['git',
+                              '-C',
+                              git_cachedir(channel.git_repo),
+                              'log',
+                              '-n1',
+                              '--format=%ct-%h',
+                              '--abbrev=11',
+                              '--no-show-signature',
+                              channel.git_revision],
+                             stdout=subprocess.PIPE)
+    v.result(process.returncode == 0 and process.stdout != b'')
+    return '%s-%s' % (os.path.basename(channel.git_repo),
+                      process.stdout.decode().strip())
+
+
+def read_search_path(conf: configparser.SectionProxy) -> SearchPath:
+    if 'alias_of' in conf:
+        return AliasSearchPath(**dict(conf.items()))
+    if 'channel_url' in conf:
+        return ChannelSearchPath(**dict(conf.items()))
+    return GitSearchPath(**dict(conf.items()))
+
+
+def read_config(filename: str) -> configparser.ConfigParser:
+    config = configparser.ConfigParser()
+    config.read_file(open(filename), filename)
+    return config
+
+
+def read_config_files(
+        filenames: Iterable[str]) -> Dict[str, configparser.SectionProxy]:
+    merged_config: Dict[str, configparser.SectionProxy] = {}
+    for file in filenames:
+        config = read_config(file)
+        for section in config.sections():
+            if section in merged_config:
+                raise Exception('Duplicate channel "%s"' % section)
+            merged_config[section] = config[section]
+    return merged_config
+
+
+def pin(args: argparse.Namespace) -> None:
+    v = Verification()
+    config = read_config(args.channels_file)
+    for section in config.sections():
+        if args.channels and section not in args.channels:
+            continue
+
+        sp = read_search_path(config[section])
+
+        sp.pin(v, config[section])
+
+    with open(args.channels_file, 'w') as configfile:
+        config.write(configfile)
+
+
+def update(args: argparse.Namespace) -> None:
+    v = Verification()
+    exprs: Dict[str, str] = {}
+    config = read_config_files(args.channels_file)
+    for section in config:
+        sp = read_search_path(config[section])
+        if isinstance(sp, AliasSearchPath):
+            assert 'git_repo' not in config[section]
+            continue
+        tarball = sp.fetch(v, section, config[section])
+        exprs[section] = (
+            'f: f { name = "%s"; channelName = "%%s"; src = builtins.storePath "%s"; }' %
+            (config[section]['release_name'], tarball))
+
+    for section in config:
+        if 'alias_of' in config[section]:
+            exprs[section] = exprs[str(config[section]['alias_of'])]
+
+    command = [
+        'nix-env',
+        '--profile',
+        '/nix/var/nix/profiles/per-user/%s/channels' %
+        getpass.getuser(),
+        '--show-trace',
+        '--file',
+        '<nix/unpack-channel.nix>',
+        '--install',
+        '--from-expression'] + [exprs[name] % name for name in sorted(exprs.keys())]
+    if args.dry_run:
+        print(' '.join(map(shlex.quote, command)))
+    else:
+        v.status('Installing channels with nix-env')
+        process = subprocess.run(command)
+        v.result(process.returncode == 0)
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(prog='pinch')
+    subparsers = parser.add_subparsers(dest='mode', required=True)
+    parser_pin = subparsers.add_parser('pin')
+    parser_pin.add_argument('channels_file', type=str)
+    parser_pin.add_argument('channels', type=str, nargs='*')
+    parser_pin.set_defaults(func=pin)
+    parser_update = subparsers.add_parser('update')
+    parser_update.add_argument('--dry-run', action='store_true')
+    parser_update.add_argument('channels_file', type=str, nargs='+')
+    parser_update.set_defaults(func=update)
+    args = parser.parse_args()
+    args.func(args)
+
+
+if __name__ == '__main__':
+    main()