Add symlink SearchPath type

[pinch] / pinch.py

diff --git a/pinch.py b/pinch.py
index 5bb4e955d4f996ad7513533a579abba98d6e58e0..37d37a7c137727bed59251a0b0b1619f19cbeab7 100644 (file)
--- a/pinch.py
+++ b/pinch.py
@@ -7,8 +7,11 @@ import hashlib
 import operator
 import os
 import os.path
 import operator
 import os
 import os.path

+import shlex

 import shutil
 import subprocess
 import shutil
 import subprocess

+import sys
+import tarfile

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


@@ -16,35 +19,32 @@ import urllib.request
 import xml.dom.minidom
 
 from typing import (
 import xml.dom.minidom
 
 from typing import (

+    Callable,

     Dict,
     Iterable,
     List,
     Dict,
     Iterable,
     List,

+    Mapping,
+    NamedTuple,

     NewType,
     NewType,

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

 
 
 
 

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

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


@@ -57,7 +57,7 @@ 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


@@ -67,9 +67,9 @@ 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()


@@ -82,6 +82,130 @@ 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 AliasPin(NamedTuple):
+    pass
+
+
+class SymlinkPin(NamedTuple):
+    @property
+    def release_name(self) -> str:
+        return 'link'
+
+
+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, SymlinkPin, GitPin, ChannelPin]
+
+
+def copy_to_nix_store(v: Verification, filename: str) -> str:
+    v.status('Putting tarball in Nix store')
+    process = subprocess.run(
+        ['nix-store', '--add', filename], stdout=subprocess.PIPE)
+    v.result(process.returncode == 0)
+    return process.stdout.decode().strip()
+
+
+class AliasSearchPath(NamedTuple):
+    alias_of: str
+
+    # pylint: disable=no-self-use
+    def pin(self, _: Verification, __: Optional[Pin]) -> AliasPin:
+        return AliasPin()
+
+
+class SymlinkSearchPath(NamedTuple):
+    path: str
+
+    # pylint: disable=no-self-use
+    def pin(self, _: Verification, __: Optional[Pin]) -> SymlinkPin:
+        return SymlinkPin()
+
+    def fetch(self, v: Verification, _: Pin) -> str:
+        with tempfile.TemporaryDirectory() as td:
+            archive_filename = os.path.join(td, 'link.tar.gz')
+            os.symlink(self.path, os.path.join(td, 'link'))
+            with tarfile.open(archive_filename, mode='x:gz') as t:
+                t.add(os.path.join(td, 'link'), arcname='link')
+            return copy_to_nix_store(v, archive_filename)
+
+
+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,
+                   SymlinkSearchPath,
+                   GitSearchPath,
+                   ChannelSearchPath]
+TarrableSearchPath = Union[GitSearchPath, ChannelSearchPath]
+
+

 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:


@@ -110,18 +234,21 @@ 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_channel(
+        v: Verification, channel: ChannelSearchPath) -> Tuple[str, str]:

     v.status('Fetching channel')
     request = urllib.request.urlopen(channel.channel_url, timeout=10)
     v.status('Fetching channel')
     request = urllib.request.urlopen(channel.channel_url, timeout=10)

-    channel.channel_html = request.read()
-    channel.forwarded_url = request.geturl()
-    v.result(request.status == 200)
-    v.check('Got forwarded', channel.channel_url != channel.forwarded_url)
+    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, channel: Channel) -> 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(channel.channel_html)
+    d = xml.dom.minidom.parseString(channel_html)

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


@@ -130,26 +257,25 @@ def parse_channel(v: Verification, channel: Channel) -> 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)

-    channel.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]

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

-    channel.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)

-        channel.table[name] = ChannelTableEntry(
-            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:
 
 
 def digest_string(s: bytes) -> Digest16:


@@ -168,7 +294,7 @@ 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())
 


@@ -176,7 +302,7 @@ 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())
 


@@ -187,7 +313,7 @@ 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 == ''


@@ -196,31 +322,45 @@ 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,
+        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 = channel.table[resource]
-        fields.absolute_url = urllib.parse.urljoin(
-            channel.forwarded_url, fields.url)
+        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')
         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(
-            channel.table['git-revision'].file).read(999) == channel.git_revision)
+    v.result(open(table['git-revision'].file).read(999) == pin.git_revision)

 
 
 def git_cachedir(git_repo: str) -> str:
 
 
 def git_cachedir(git_repo: str) -> str:

-    # TODO: Consider using pyxdg to find this path.
-    return os.path.expanduser(
-        '~/.cache/nix-pin-channel/git/%s' %
-        digest_string(
-            git_repo.encode()))
+    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: Channel) -> None:
+
+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',
     cachedir = git_cachedir(channel.git_repo)
     v.status('Verifying rev is an ancestor of ref')
     process = subprocess.run(['git',


@@ -228,25 +368,29 @@ def verify_git_ancestry(v: Verification, channel: Channel) -> None:
                               cachedir,
                               'merge-base',
                               '--is-ancestor',
                               cachedir,
                               'merge-base',
                               '--is-ancestor',

-                              channel.git_revision,
+                              new_revision,

                               channel.git_ref])
     v.result(process.returncode == 0)
 
                               channel.git_ref])
     v.result(process.returncode == 0)
 

-    if hasattr(channel, 'old_git_revision'):
+    if old_revision is not None:

         v.status(
             'Verifying rev is an ancestor of previous rev %s' %
         v.status(
             'Verifying rev is an ancestor of previous rev %s' %

-            channel.old_git_revision)
+            old_revision)

         process = subprocess.run(['git',
                                   '-C',
                                   cachedir,
                                   'merge-base',
                                   '--is-ancestor',
         process = subprocess.run(['git',
                                   '-C',
                                   cachedir,
                                   'merge-base',
                                   '--is-ancestor',

-                                  channel.old_git_revision,
-                                  channel.git_revision])
+                                  old_revision,
+                                  new_revision])

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

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


@@ -272,47 +416,53 @@ def git_fetch(v: Verification, channel: Channel) -> None:
                                          channel.git_ref)])
     v.result(process.returncode == 0)
 
                                          channel.git_ref)])
     v.result(process.returncode == 0)
 

-    if hasattr(channel, 'git_revision'):
+    if desired_revision is not None:

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

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

 
 

-    verify_git_ancestry(v, channel)
+    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: Channel) -> None:
+
+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(
     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])
+            ['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:
         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)
+            verify_git_ancestry(v, channel, pin.git_revision, old_revision)

             return
             return

-    git_fetch(v, channel)
+    git_fetch(v, channel, pin.git_revision, old_revision)

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

-        channel: Channel,
+        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: str,
         git_contents: str) -> None:
     v.status('Comparing channel tarball with git checkout')
     match, mismatch, errors = compare(os.path.join(

-        channel_contents, channel.release_name), git_contents)
+        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)
     v.ok()
     v.check('%d files match' % len(match), len(match) > 0)
     v.check('%d files differ' % len(mismatch), len(mismatch) == 0)


@@ -338,177 +488,254 @@ 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:
-    v.status('Extracting tarball %s' %
-             channel.table['nixexprs.tar.xz'].file)
-    shutil.unpack_archive(
-        channel.table['nixexprs.tar.xz'].file,
-        dest)
+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()
 
 
     v.ok()
 
 

-def git_checkout(v: Verification, channel: Channel, dest: str) -> None:
+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',
     v.status('Checking out corresponding git revision')
     git = subprocess.Popen(['git',
                             '-C',
                             git_cachedir(channel.git_repo),
                             'archive',

-                            channel.git_revision],
+                            pin.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: Channel) -> str:
+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(
     with tempfile.TemporaryDirectory() as output_dir:
         output_filename = os.path.join(

-            output_dir, channel.release_name + '.tar.xz')
+            output_dir, pin.release_name + '.tar.xz')

         with open(output_filename, 'w') as output_file:
             v.status(
                 'Generating tarball for git revision %s' %
         with open(output_filename, 'w') as output_file:
             v.status(
                 'Generating tarball for git revision %s' %

-                channel.git_revision)
+                pin.git_revision)

             git = subprocess.Popen(['git',
                                     '-C',
                                     git_cachedir(channel.git_repo),
                                     'archive',
             git = subprocess.Popen(['git',
                                     '-C',
                                     git_cachedir(channel.git_repo),
                                     'archive',

-                                    '--prefix=%s/' % channel.release_name,
-                                    channel.git_revision],
+                                    '--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)
 
                                    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], capture_output=True)
-        v.result(process.returncode == 0)
-        return process.stdout.decode().strip()
+        store_tarball = copy_to_nix_store(v, output_filename)
+
+        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,
+        pin: GitPin,

         channel_contents: str) -> None:
     v.status('Verifying git commit in channel tarball')
     v.result(
         open(
             os.path.join(
                 channel_contents,
         channel_contents: str) -> None:
     v.status('Verifying git commit in channel tarball')
     v.result(
         open(
             os.path.join(
                 channel_contents,

-                channel.release_name,
-                '.git-revision')).read(999) == channel.git_revision)
+                pin.release_name,
+                '.git-revision')).read(999) == pin.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:' %

-        channel.release_name)
+        pin.release_name)

     version_suffix = open(
         os.path.join(
             channel_contents,
     version_suffix = open(
         os.path.join(
             channel_contents,

-            channel.release_name,
+            pin.release_name,

             '.version-suffix')).read(999)
     v.status(version_suffix)
             '.version-suffix')).read(999)
     v.status(version_suffix)

-    v.result(channel.release_name.endswith(version_suffix))
+    v.result(pin.release_name.endswith(version_suffix))

 
 
 
 

-def check_channel_contents(v: Verification, channel: Channel) -> None:
+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:
 
     with tempfile.TemporaryDirectory() as channel_contents, \
             tempfile.TemporaryDirectory() as git_contents:
 

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

 
 

-        git_checkout(v, channel, git_contents)
+        git_checkout(v, channel, pin, git_contents)

 
 

-        compare_tarball_and_git(v, channel, channel_contents, git_contents)
+        compare_tarball_and_git(v, pin, channel_contents, git_contents)

 
         v.status('Removing temporary directories')
     v.ok()
 
 
 
         v.status('Removing temporary directories')
     v.ok()
 
 

-def pin_channel(v: Verification, channel: Channel) -> None:
-    fetch(v, channel)
-    parse_channel(v, channel)
-    fetch_resources(v, channel)
-    ensure_git_rev_available(v, channel)
-    check_channel_contents(v, channel)
-
-
-def git_revision_name(v: Verification, channel: Channel) -> str:
+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),
     v.status('Getting commit date')
     process = subprocess.run(['git',
                               '-C',
                               git_cachedir(channel.git_repo),

-                              'lo',
+                              'log',

                               '-n1',
                               '--format=%ct-%h',
                               '--abbrev=11',
                               '-n1',
                               '--format=%ct-%h',
                               '--abbrev=11',

-                              channel.git_revision],
-                             capture_output=True)
-    v.result(process.returncode == 0 and process.stdout != '')
+                              '--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())
 
 
     return '%s-%s' % (os.path.basename(channel.git_repo),
                       process.stdout.decode().strip())
 
 

-def pin(args: argparse.Namespace) -> None:
-    v = Verification()
+K = TypeVar('K')
+V = TypeVar('V')
+
+
+def partition_dict(pred: Callable[[K, V], bool],
+                   d: Dict[K, V]) -> Tuple[Dict[K, V], Dict[K, V]]:
+    selected: Dict[K, V] = {}
+    remaining: Dict[K, V] = {}
+    for k, v in d.items():
+        if pred(k, v):
+            selected[k] = v
+        else:
+            remaining[k] = v
+    return selected, remaining
+
+
+def filter_dict(d: Dict[K, V], fields: Set[K]
+                ) -> Tuple[Dict[K, V], Dict[K, V]]:
+    return partition_dict(lambda k, v: k in fields, d)
+
+
+def read_config_section(
+        conf: configparser.SectionProxy) -> Tuple[SearchPath, Optional[Pin]]:
+    mapping: Mapping[str, Tuple[Type[SearchPath], Type[Pin]]] = {
+        'alias': (AliasSearchPath, AliasPin),
+        'channel': (ChannelSearchPath, ChannelPin),
+        'git': (GitSearchPath, GitPin),
+        'symlink': (SymlinkSearchPath, SymlinkPin),
+    }
+    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_pinned_config_section(
+        section: str, conf: configparser.SectionProxy) -> Tuple[SearchPath, Pin]:
+    sp, pin = read_config_section(conf)
+    if pin is None:
+        raise Exception(
+            'Cannot update unpinned channel "%s" (Run "pin" before "update")' %
+            section)
+    return sp, pin
+
+
+def read_config(filename: str) -> configparser.ConfigParser:

     config = configparser.ConfigParser()
     config = configparser.ConfigParser()

-    config.read_file(open(args.channels_file), args.channels_file)
+    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()
+    config = read_config(args.channels_file)

     for section in config.sections():
     for section in config.sections():

+        if args.channels and section not in args.channels:
+            continue

 
 

-        channel = Channel(**dict(config[section].items()))
-        if hasattr(channel, 'git_revision'):
-            channel.old_git_revision = channel.git_revision
-            del channel.git_revision
+        sp, old_pin = read_config_section(config[section])

 
 

-        if 'channel_url' in config[section]:
-            pin_channel(v, channel)
-            config[section]['release_name'] = channel.release_name
-            config[section]['tarball_url'] = channel.table['nixexprs.tar.xz'].absolute_url
-            config[section]['tarball_sha256'] = channel.table['nixexprs.tar.xz'].digest
-        else:
-            git_fetch(v, channel)
-            config[section]['release_name'] = git_revision_name(v, channel)
-        config[section]['git_revision'] = channel.git_revision
+        config[section].update(sp.pin(v, old_pin)._asdict())

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

-def update(args: argparse.Namespace) -> None:
+def updateCommand(args: argparse.Namespace) -> None:

     v = Verification()
     v = Verification()

-    config = configparser.ConfigParser()
-    config.read_file(open(args.channels_file), args.channels_file)
-    exprs = []
-    for section in config.sections():
-        if 'channel_url' in config[section]:
-            tarball = fetch_with_nix_prefetch_url(
-                v, config[section]['tarball_url'], Digest16(
-                    config[section]['tarball_sha256']))
-        else:
-            channel = Channel(**dict(config[section].items()))
-            ensure_git_rev_available(v, channel)
-            tarball = git_get_tarball(v, channel)
-        exprs.append(
-            'f: f { name = "%s"; channelName = "%s"; src = builtins.storePath "%s"; }' %
-            (config[section]['release_name'], section, tarball))
-    v.status('Installing channels with nix-env')
-    process = subprocess.run(
-        [
-            'nix-env',
-            '--profile',
-            '/nix/var/nix/profiles/per-user/%s/channels' %
-            getpass.getuser(),
-            '--show-trace',
-            '--file',
-            '<nix/unpack-channel.nix>',
-            '--install',
-            '--from-expression'] +
-        exprs)
-    v.result(process.returncode == 0)
+    exprs: Dict[str, str] = {}
+    config = {
+        section: read_pinned_config_section(section, conf) for section,
+        conf in read_config_files(
+            args.channels_file).items()}
+    alias, nonalias = partition_dict(
+        lambda k, v: isinstance(v[0], AliasSearchPath), config)
+
+    for section, (sp, pin) in nonalias.items():
+        assert not isinstance(sp, AliasSearchPath)  # mypy can't see through
+        assert not isinstance(pin, AliasPin)        # partition_dict()
+        tarball = sp.fetch(v, pin)
+        exprs[section] = (
+            'f: f { name = "%s"; channelName = "%%s"; src = builtins.storePath "%s"; }' %
+            (pin.release_name, tarball))
+
+    for section, (sp, pin) in alias.items():
+        assert isinstance(sp, AliasSearchPath)  # For mypy
+        exprs[section] = exprs[sp.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:
 
 
 def main() -> None:


@@ -516,12 +743,15 @@ def main() -> None:
     subparsers = parser.add_subparsers(dest='mode', required=True)
     parser_pin = subparsers.add_parser('pin')
     parser_pin.add_argument('channels_file', type=str)
     subparsers = parser.add_subparsers(dest='mode', required=True)
     parser_pin = subparsers.add_parser('pin')
     parser_pin.add_argument('channels_file', type=str)

-    parser_pin.set_defaults(func=pin)
+    parser_pin.add_argument('channels', type=str, nargs='*')
+    parser_pin.set_defaults(func=pinCommand)

     parser_update = subparsers.add_parser('update')
     parser_update = subparsers.add_parser('update')

-    parser_update.add_argument('channels_file', type=str)
-    parser_update.set_defaults(func=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)
 
 
     args = parser.parse_args()
     args.func(args)
 
 

-main()
+if __name__ == '__main__':
+    main()