Immutable SearchPaths

[pinch] / pinch.py

diff --git a/pinch.py b/pinch.py
index f0f0422047a0f434b9c07c618a9ac5dea45f84ea..e1d8d3bbe859922421f2b34545e5069d1125d2da 100644 (file)
--- a/pinch.py
+++ b/pinch.py
@@ -1,11 +1,16 @@
+import argparse
+import configparser

 import filecmp
 import functools
 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 shutil
 import subprocess

+import sys

 import tempfile
 import types
 import urllib.parse
 import tempfile
 import types
 import urllib.parse


@@ -16,29 +21,28 @@ from typing import (
     Dict,
     Iterable,
     List,
     Dict,
     Iterable,
     List,

+    Mapping,
+    NamedTuple,

     NewType,
     NewType,

-    Sequence,
+    Optional,
+    Set,

     Tuple,
     Tuple,

+    Type,
+    TypeVar,
+    Union,

 )
 
 )
 

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

 
 
 
 

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

 
 
 
 

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

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


@@ -51,7 +55,7 @@ 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


@@ -61,9 +65,9 @@ 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()


@@ -76,10 +80,98 @@ class Verification:
         self.result(True)
 
 
         self.result(True)
 
 

-def compare(a: str,
-            b: str) -> Tuple[Sequence[str],
-                             Sequence[str],
-                             Sequence[str]]:
+Digest16 = NewType('Digest16', str)
+Digest32 = NewType('Digest32', str)
+
+
+class ChannelTableEntry(types.SimpleNamespace):
+    absolute_url: str
+    digest: Digest16
+    file: str
+    size: int
+    url: str
+
+
+class AliasPin(NamedTuple):
+    pass
+
+
+class GitPin(NamedTuple):
+    git_revision: str
+    release_name: str
+
+
+class ChannelPin(NamedTuple):
+    git_revision: str
+    release_name: str
+    tarball_url: str
+    tarball_sha256: str
+
+
+Pin = Union[AliasPin, GitPin, ChannelPin]
+
+
+class AliasSearchPath(NamedTuple):
+    alias_of: str
+
+    # pylint: disable=no-self-use
+    def pin(self, _: Verification, __: Optional[Pin]) -> AliasPin:
+        return AliasPin()
+
+
+class GitSearchPath(NamedTuple):
+    git_ref: str
+    git_repo: str
+
+    def pin(self, v: Verification, old_pin: Optional[Pin]) -> GitPin:
+        if old_pin is not None:
+            assert isinstance(old_pin, GitPin)
+        old_revision = old_pin.git_revision if old_pin is not None else None
+
+        new_revision = git_fetch(v, self, None, old_revision)
+        return GitPin(release_name=git_revision_name(v, self, new_revision),
+                      git_revision=new_revision)
+
+    def fetch(self, v: Verification, pin: Pin) -> str:
+        assert isinstance(pin, GitPin)
+        ensure_git_rev_available(v, self, pin, None)
+        return git_get_tarball(v, self, pin)
+
+
+class ChannelSearchPath(NamedTuple):
+    channel_url: str
+    git_ref: str
+    git_repo: str
+
+    def pin(self, v: Verification, old_pin: Optional[Pin]) -> ChannelPin:
+        if old_pin is not None:
+            assert isinstance(old_pin, ChannelPin)
+        old_revision = old_pin.git_revision if old_pin is not None else None
+
+        channel_html, forwarded_url = fetch_channel(v, self)
+        table, new_gitpin = parse_channel(v, channel_html)
+        fetch_resources(v, new_gitpin, forwarded_url, table)
+        ensure_git_rev_available(v, self, new_gitpin, old_revision)
+        check_channel_contents(v, self, table, new_gitpin)
+        return ChannelPin(
+            release_name=new_gitpin.release_name,
+            tarball_url=table['nixexprs.tar.xz'].absolute_url,
+            tarball_sha256=table['nixexprs.tar.xz'].digest,
+            git_revision=new_gitpin.git_revision)
+
+    # pylint: disable=no-self-use
+    def fetch(self, v: Verification, pin: Pin) -> str:
+        assert isinstance(pin, ChannelPin)
+
+        return fetch_with_nix_prefetch_url(
+            v, pin.tarball_url, Digest16(pin.tarball_sha256))
+
+
+SearchPath = Union[AliasSearchPath, GitSearchPath, ChannelSearchPath]
+TarrableSearchPath = Union[GitSearchPath, ChannelSearchPath]
+
+
+def compare(a: str, b: str) -> Tuple[List[str], List[str], List[str]]:

 
     def throw(error: OSError) -> None:
         raise error
 
     def throw(error: OSError) -> None:
         raise error


@@ -107,22 +199,21 @@ def compare(a: str,
     return filecmp.cmpfiles(a, b, files, shallow=False)
 
 
     return filecmp.cmpfiles(a, b, files, shallow=False)
 
 

-def fetch(v: Verification, channel_url: str) -> Info:
-    info = Info()
-    info.url = channel_url
+def fetch_channel(
+        v: Verification, channel: ChannelSearchPath) -> Tuple[str, str]:

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

-    request = urllib.request.urlopen(
-        'https://channels.nixos.org/nixos-20.03', timeout=10)
-    info.channel_html = request.read()
-    info.forwarded_url = request.geturl()
-    v.result(request.status == 200)
-    v.check('Got forwarded', info.url != info.forwarded_url)
-    return info
+    request = urllib.request.urlopen(channel.channel_url, timeout=10)
+    channel_html = request.read()
+    forwarded_url = request.geturl()
+    v.result(request.status == 200)  # type: ignore  # (for old mypy)
+    v.check('Got forwarded', channel.channel_url != forwarded_url)
+    return channel_html, forwarded_url

 
 
 
 

-def parse_channel(v: Verification, info: Info) -> None:
+def parse_channel(v: Verification, channel_html: str) \
+        -> Tuple[Dict[str, ChannelTableEntry], GitPin]:

     v.status('Parsing channel description as XML')
     v.status('Parsing channel description as XML')

-    d = xml.dom.minidom.parseString(info.channel_html)
+    d = xml.dom.minidom.parseString(channel_html)

     v.ok()
 
     v.status('Extracting release name:')
     v.ok()
 
     v.status('Extracting release name:')


@@ -131,25 +222,29 @@ def parse_channel(v: Verification, info: Info) -> None:
     h1_name = d.getElementsByTagName('h1')[0].firstChild.nodeValue.split()[2]
     v.status(title_name)
     v.result(title_name == h1_name)
     h1_name = d.getElementsByTagName('h1')[0].firstChild.nodeValue.split()[2]
     v.status(title_name)
     v.result(title_name == h1_name)

-    info.release_name = title_name

 
     v.status('Extracting git commit:')
     git_commit_node = d.getElementsByTagName('tt')[0]
 
     v.status('Extracting git commit:')
     git_commit_node = d.getElementsByTagName('tt')[0]

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

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

-    info.table = {}
+    table: Dict[str, ChannelTableEntry] = {}

     for row in d.getElementsByTagName('tr')[1:]:
         name = row.childNodes[0].firstChild.firstChild.nodeValue
         url = row.childNodes[0].firstChild.getAttribute('href')
         size = int(row.childNodes[1].firstChild.nodeValue)
         digest = Digest16(row.childNodes[2].firstChild.firstChild.nodeValue)
     for row in d.getElementsByTagName('tr')[1:]:
         name = row.childNodes[0].firstChild.firstChild.nodeValue
         url = row.childNodes[0].firstChild.getAttribute('href')
         size = int(row.childNodes[1].firstChild.nodeValue)
         digest = Digest16(row.childNodes[2].firstChild.firstChild.nodeValue)

-        info.table[name] = InfoTableEntry(url=url, digest=digest, size=size)
+        table[name] = ChannelTableEntry(url=url, digest=digest, size=size)

     v.ok()
     v.ok()

+    return table, GitPin(release_name=title_name, git_revision=git_revision)
+
+
+def digest_string(s: bytes) -> Digest16:
+    return Digest16(hashlib.sha256(s).hexdigest())

 
 
 def digest_file(filename: str) -> Digest16:
 
 
 def digest_file(filename: str) -> Digest16:


@@ -164,7 +259,7 @@ 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())
 


@@ -172,7 +267,7 @@ 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())
 


@@ -183,7 +278,7 @@ 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 == ''


@@ -192,36 +287,424 @@ 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, info: Info) -> None:
+def fetch_resources(
+        v: Verification,
+        pin: GitPin,
+        forwarded_url: str,
+        table: Dict[str, ChannelTableEntry]) -> None:

     for resource in ['git-revision', 'nixexprs.tar.xz']:
     for resource in ['git-revision', 'nixexprs.tar.xz']:

-        fields = info.table[resource]
-        url = urllib.parse.urljoin(info.forwarded_url, fields.url)
-        fields.file = fetch_with_nix_prefetch_url(v, url, fields.digest)
+        fields = table[resource]
+        fields.absolute_url = urllib.parse.urljoin(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.status('Verifying git commit on main page matches git commit in table')

+    v.result(open(table['git-revision'].file).read(999) == pin.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, pin: GitPin) -> str:
+    return os.path.join(
+        xdg.XDG_CACHE_HOME,
+        'pinch/git-tarball',
+        '%s-%s-%s' %
+        (digest_string(channel.git_repo.encode()),
+         pin.git_revision,
+         pin.release_name))
+
+
+def verify_git_ancestry(
+        v: Verification,
+        channel: TarrableSearchPath,
+        new_revision: str,
+        old_revision: Optional[str]) -> 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',
+                              new_revision,
+                              channel.git_ref])
+    v.result(process.returncode == 0)
+
+    if old_revision is not None:
+        v.status(
+            'Verifying rev is an ancestor of previous rev %s' %
+            old_revision)
+        process = subprocess.run(['git',
+                                  '-C',
+                                  cachedir,
+                                  'merge-base',
+                                  '--is-ancestor',
+                                  old_revision,
+                                  new_revision])
+        v.result(process.returncode == 0)
+
+
+def git_fetch(
+        v: Verification,
+        channel: TarrableSearchPath,
+        desired_revision: Optional[str],
+        old_revision: Optional[str]) -> str:
+    # 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.
+    # :(
+
+    cachedir = git_cachedir(channel.git_repo)
+    if not os.path.exists(cachedir):
+        v.status("Initializing git repo")
+        process = subprocess.run(
+            ['git', 'init', '--bare', cachedir])
+        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 desired_revision is not None:
+        v.status('Verifying that fetch retrieved this rev')
+        process = subprocess.run(
+            ['git', '-C', cachedir, 'cat-file', '-e', desired_revision])
+        v.result(process.returncode == 0)
+
+    new_revision = open(
+        os.path.join(
+            cachedir,
+            'refs',
+            'heads',
+            channel.git_ref)).read(999).strip()
+
+    verify_git_ancestry(v, channel, new_revision, old_revision)
+
+    return new_revision
+
+
+def ensure_git_rev_available(
+        v: Verification,
+        channel: TarrableSearchPath,
+        pin: GitPin,
+        old_revision: Optional[str]) -> 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', pin.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, pin.git_revision, old_revision)
+            return
+    git_fetch(v, channel, pin.git_revision, old_revision)
+
+
+def compare_tarball_and_git(
+        v: Verification,
+        pin: GitPin,
+        channel_contents: str,
+        git_contents: str) -> None:
+    v.status('Comparing channel tarball with git checkout')
+    match, mismatch, errors = compare(os.path.join(
+        channel_contents, pin.release_name), git_contents)
+    v.ok()
+    v.check('%d files match' % len(match), len(match) > 0)
+    v.check('%d files differ' % len(mismatch), len(mismatch) == 0)
+    expected_errors = [
+        '.git-revision',
+        '.version-suffix',
+        'nixpkgs',
+        'programs.sqlite',
+        'svn-revision']
+    benign_errors = []
+    for ee in expected_errors:
+        if ee in errors:
+            errors.remove(ee)
+            benign_errors.append(ee)
+    v.check(
+        '%d unexpected incomparable files' %
+        len(errors),
+        len(errors) == 0)
+    v.check(
+        '(%d of %d expected incomparable files)' %
+        (len(benign_errors),
+         len(expected_errors)),
+        len(benign_errors) == len(expected_errors))
+
+
+def extract_tarball(
+        v: Verification,
+        table: Dict[str, ChannelTableEntry],
+        dest: str) -> None:
+    v.status('Extracting tarball %s' % table['nixexprs.tar.xz'].file)
+    shutil.unpack_archive(table['nixexprs.tar.xz'].file, dest)
+    v.ok()
+
+
+def git_checkout(
+        v: Verification,
+        channel: TarrableSearchPath,
+        pin: GitPin,
+        dest: str) -> None:
+    v.status('Checking out corresponding git revision')
+    git = subprocess.Popen(['git',
+                            '-C',
+                            git_cachedir(channel.git_repo),
+                            'archive',
+                            pin.git_revision],
+                           stdout=subprocess.PIPE)
+    tar = subprocess.Popen(
+        ['tar', 'x', '-C', dest, '-f', '-'], stdin=git.stdout)
+    if git.stdout:
+        git.stdout.close()
+    tar.wait()
+    git.wait()
+    v.result(git.returncode == 0 and tar.returncode == 0)
+
+
+def git_get_tarball(
+        v: Verification,
+        channel: TarrableSearchPath,
+        pin: GitPin) -> str:
+    cache_file = tarball_cache_file(channel, pin)
+    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, pin.release_name + '.tar.xz')
+        with open(output_filename, 'w') as output_file:
+            v.status(
+                'Generating tarball for git revision %s' %
+                pin.git_revision)
+            git = subprocess.Popen(['git',
+                                    '-C',
+                                    git_cachedir(channel.git_repo),
+                                    'archive',
+                                    '--prefix=%s/' % pin.release_name,
+                                    pin.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,
+        pin: GitPin,
+        channel_contents: str) -> None:
+    v.status('Verifying git commit in channel tarball')

     v.result(
         open(
     v.result(
         open(

-            info.table['git-revision'].file).read(999) == info.git_commit)
+            os.path.join(
+                channel_contents,
+                pin.release_name,
+                '.git-revision')).read(999) == pin.git_revision)
+
+    v.status(
+        'Verifying version-suffix is a suffix of release name %s:' %
+        pin.release_name)
+    version_suffix = open(
+        os.path.join(
+            channel_contents,
+            pin.release_name,
+            '.version-suffix')).read(999)
+    v.status(version_suffix)
+    v.result(pin.release_name.endswith(version_suffix))
+
+
+def check_channel_contents(
+        v: Verification,
+        channel: TarrableSearchPath,
+        table: Dict[str, ChannelTableEntry],
+        pin: GitPin) -> None:
+    with tempfile.TemporaryDirectory() as channel_contents, \
+            tempfile.TemporaryDirectory() as git_contents:

 
 

+        extract_tarball(v, table, channel_contents)
+        check_channel_metadata(v, pin, channel_contents)

 
 

-def check_channel_contents(v: Verification, info: Info) -> None:
-    with tempfile.TemporaryDirectory() as d:
-        v.status('Extracting %s' % info.table['nixexprs.tar.xz'].file)
-        shutil.unpack_archive(info.table['nixexprs.tar.xz'].file, d)
-        v.ok()
-        v.status('Removing temporary directory')
+        git_checkout(v, channel, pin, git_contents)
+
+        compare_tarball_and_git(v, pin, channel_contents, git_contents)
+
+        v.status('Removing temporary directories')

     v.ok()
 
 
     v.ok()
 
 

-def main() -> None:
+def git_revision_name(
+        v: Verification,
+        channel: TarrableSearchPath,
+        git_revision: str) -> 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',
+                              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())
+
+
+K = TypeVar('K')
+V = TypeVar('V')
+
+
+def filter_dict(d: Dict[K, V], fields: Set[K]
+                ) -> Tuple[Dict[K, V], Dict[K, V]]:
+    selected: Dict[K, V] = {}
+    remaining: Dict[K, V] = {}
+    for k, v in d.items():
+        if k in fields:
+            selected[k] = v
+        else:
+            remaining[k] = v
+    return selected, remaining
+
+
+def read_search_path(
+        conf: configparser.SectionProxy) -> Tuple[SearchPath, Optional[Pin]]:
+    mapping: Mapping[str, Tuple[Type[SearchPath], Type[Pin]]] = {
+        'alias': (AliasSearchPath, AliasPin),
+        'channel': (ChannelSearchPath, ChannelPin),
+        'git': (GitSearchPath, GitPin),
+    }
+    SP, P = mapping[conf['type']]
+    _, all_fields = filter_dict(dict(conf.items()), set(['type']))
+    pin_fields, remaining_fields = filter_dict(all_fields, set(P._fields))
+    # Error suppression works around https://github.com/python/mypy/issues/9007
+    pin_present = pin_fields != {} or P._fields == ()
+    pin = P(**pin_fields) if pin_present else None  # type:ignore[call-arg]
+    return SP(**remaining_fields), pin
+
+
+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 pinCommand(args: argparse.Namespace) -> None:

     v = Verification()
     v = Verification()

-    info = fetch(v, 'https://channels.nixos.org/nixos-20.03')
-    parse_channel(v, info)
-    fetch_resources(v, info)
-    check_channel_contents(v, info)
-    print(info)
+    config = read_config(args.channels_file)
+    for section in config.sections():
+        if args.channels and section not in args.channels:
+            continue
+
+        sp, old_pin = read_search_path(config[section])
+
+        config[section].update(sp.pin(v, old_pin)._asdict())

 
 

+    with open(args.channels_file, 'w') as configfile:
+        config.write(configfile)

 
 

-main()
+
+def updateCommand(args: argparse.Namespace) -> None:
+    v = Verification()
+    exprs: Dict[str, str] = {}
+    config = read_config_files(args.channels_file)
+    for section in config:
+        sp, pin = read_search_path(config[section])
+        if pin is None:
+            raise Exception(
+                'Cannot update unpinned channel "%s" (Run "pin" before "update")' %
+                section)
+        if isinstance(sp, AliasSearchPath):
+            assert 'git_repo' not in config[section]
+            continue
+        tarball = sp.fetch(v, pin)
+        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=pinCommand)
+    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=updateCommand)
+    args = parser.parse_args()
+    args.func(args)
+
+
+if __name__ == '__main__':
+    main()